# Encyclopædia Britannica, Ninth Edition/Biology

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BIOLOGY
THE Biological sciences are those which deal with the phenomena manifested by living matter; and though it is customary and convenient to group apart such of these phenomena as are termed mental, and such of them as are exhibited by men in society, under the heads of Psychology and Sociology, yet it must be allowed that no natural boundary separates the subject matter of the latter sciences from that of Biology. Psychology is inseparably linked with Physiology; and the phases of social life exhibited by animals other than man, which sometimes curiously foreshadow human policy, fall strictly within the province of the biologist.
On the other hand, the biological sciences are sharply marked off from the abiological, or those which treat of the phenomena manifested by not-living matter, in so far as the properties of living matter distinguish it absolutely from all other kinds of things, and as the present state of knowledge furnishes us with no link between the living and the not-living.
These distinctive properties of living matter are—
1. Its chemical composition—containing, as it invariably does, one or more forms of a complex compound of carbon, hydrogen, oxygen, and nitrogen, the so-called protein (which has never yet been obtained except as a product of living bodies) united with a large proportion of water, and forming the chief constituent of a substance which, in its primary unmodified state, is known as protoplasm.
2. Its universal disintegration and waste by oxidation; and its concomitant reintegration by the intus-susception of new matter.
A process of waste resulting from the decomposition of the molecules of the protoplasm, in virtue of which they break up into more highly oxidated products, which cease to form any part of the living body, is a constant concomitant of lira. There is reason to believe that carbonic acid is always one of these waste products, while the others contain the remainder of the carbon, the nitrogen, the hydrogen, and the other elements which may enter into the composition of the protoplasm.
The new matter taken in to make good this constant loss is either a ready-formed protoplasmic material, supplied by some other living being, or it consists of the elements of protoplasm, united together in simpler combinations, which consequently have to be built up into protoplasm by the agency of the living matter itself. In either case, the addition of molecules to those which already existed takes place, not at the surface of the living mass, but by interposition between the existing molecules of the latter. If the processes of disintegration and of reconstruction which characterize life balance one another, the size of the mass of living matter remains stationary, while, if the reconstructive process is the more rapid, the living body grows. But the increase of size which constitutes growth is the result of a process of molecular intus-susception, and therefore differs altogether from the process of growth by accretion, which may be observed in crystals and is effected purely by the external addition of new matter so that, in the well-known aphorism of Linnæus, the word "grow," as applied to stones, signifies a totally different process from what is called "growth" in plants and animals.
3. Its tendency to undergo cyclical changes.
In the ordinary course of nature, all living matter proceeds from pre-existing living matter, a portion of the latter being detached and acquiring an independent existence. The new form takes on the characters of that from which it arose; exhibits the same power of propagating itself by means of an offshoot; and, sooner or later, like its predecessor, ceases to live, and is resolved into more highly oxidated compounds of its elements.
Thus an individual living body is not only constantly changing its substance, but its size and form are undergoing continual modifications, the end of which is the death and decay of that individual; the continuation of the kind being secured by the detachment of portions which tend to run through the same cycle of forms as the parent. No forms of matter which are either not living, or have not been derived from living matter, exhibit these three properties, nor any approach to the remarkable phenomena defined under the second and third heads. But in addition to these distinctive characters, living matter has some ​other peculiarities, the chief of which are the dependence of all its activities upon moisture and upon heat, within a limited range of temperature, and the fact that it usually possesses a certain structure, or organization.
Life conditioned by moisture.As has been said, a large proportion of water enters into the composition of all living matter; a certain amount of drying arrests vital activity, and the complete abstraction of this water is absolutely incompatible with either actual or potential life. But many of the simpler forms of life may undergo desiccation to such an extent as to arrest their vital manifestations and convert them into the semblance of not-living matter, and yet remain potentially alive. That is to say, on being duly moistened they return to life again. And this revivification may take place after months, or even years, of arrested life.
Life conditioned by temperature.The properties of living matter are intimately related to temperature. Not only does exposure to heat sufficient to decompose protein matter destroy life, by demolishing the molecular structure upon which life depends; but all vital activity, all phenomena of nutritive growth, movement, and reproduction are possible only between certain limits of temperature. As the temperature approaches these limits the manifestations of life vanish, though they may be recovered by return to the normal conditions; but if it pass far beyond these limits, death takes place.
This much is clear; but it is not easy to say exactly what the limits of temperature are, as they appear to vary in part with the kind of living matter, and in part with the conditions of moisture which obtain along with the temperature. The conditions of life are so complex in the higher organisms, that the experimental investigation of this question can be satisfactorily attempted only in the lowest and simplest forms. It appears that, in the dry state, these are able to bear far greater extremes both of heat and cold than in the moist condition. Thus Pasteur found that the spores of fungi, when dry, could be exposed without destruction to a temperature of 120°-125° C. (248°-257° Fahr.), while the same spores, when moist, were all killed by exposure to 100° C. (212° Fahr.) On the other hand, Cagniard de la Tour found that dry yeast might be exposed to the extremely low temperature of solid carbonic acid (-60° C. or -76° Fahr.) without being killed. In the moist state he found that it might be frozen and cooled to -5° C. (23° Fahr.), but that it was killed by lower temperatures. However, it is very desirable that these experiments should be repeated, for Cohn's careful observations on Bacteria show that, though they fall into a state of torpidity, and, like yeast, lose all their powers of exciting fermentation at, or near, the freezing point of water, they are not killed by exposure for five hours to a temperature below -10° C. (14° Fahr.), and, for some time, sinking to 18° C. (-0°·4 Fahr.) Specimens of Spirillum volutans, which had been cooled to this extent, began to move about some little time after the ice containing them thawed. But Cohn remarks that Euglenæ, which were frozen along with them, were all killed and disorganised, and that the same fate had befallen the higher Infusoria and Rotifera, with the exception of some encysted Vorticellæ, in which the rhythmical movements of the contractile vesicle showed that life was preserved.
Thus it would appear that the resistance of living matter to cold depends greatly on the special form of that matter, and that the limit of the Euglena, simple organism as it is, is much higher than that of the Bacterium.
Considerations of this kind throw some light upon the apparently anomalous conditions under which many of the lower plants, such as Protococcus and the Diatomaceæ, and some of the lower animals, such as the Radiolaria, are observed to flourish. Protococcus has been found, not only on the snows of great heights in temperate latitudes, but covering extensive areas of ice and snow in the Arctic regions, where it must be exposed to extremely low temperatures, in the latter case for many months together; while the Arctic and Antarctic seas swarm with Diatomaceæ and Radiolaria. It is on the Diatomaceæ, as Hooker has well shown, that all surface life in these regions ultimately depends; and their enormous multitudes prove that their rate of multiplication is adequate to meet the demands made upon them, and is not seriously impeded by the low temperature of the waters, never much above the freezing point, in which they habitually live.
The maximum limit of heat which living matter can resist is no less variable than its minimum limit. Kühne found that marine Amœbæ were killed when the temperature reached 35° C. (95° Fahr,), while this was not the case with fresh-water Amœbæ, which survived a heat of 5°, or even 10°, C. higher. And Actinophrys Eichornii was not killed until the temperature rose to 44° or 45° C. Didymium serpula is killed at 35° C.; while another Myxomycete, Æthalium septicum, succumbs only at 40° C.
Cohn ("Untersuchungen über Bacterien," Beiträge zur Biologie der Pflanzen, Heft 2, 1872) has given the results of a series of experiments conducted with the view of ascertaining the temperature at which Bacteria are destroyed, when living in a fluid of definite chemical composition, and free from all such complications as must arise from the inequalities of physical condition when solid particles other than the Bacteria co-exist with them. The fluid employed contained 0·1 gramme potassium phosphate, 0·1 gr. crystallised magnesium sulphate, 0·1 gr. tribasic calcium phosphate, and 0·2 gr. ammonium tartrate, dissolved in 20 cubic centimetres of distilled water. If to a certain quantity of this "normal fluid" a small proportion of water containing Bacteria was added, the multiplication of the Bacteria went on with rapidity, whether the mouth of the containing flask was open or hermetically closed. Hermetically-sealed flasks, containing portions of the normal fluid infected with Bacteria, were submerged in water heated to various temperatures, the flask being carefully shaken, without being raised out of the water, during its submergence.
The result was, that in those flasks which were thus subjected, for an hour, to a heat of 60°-62° C. (140°-143° Fahr.), the Bacteria underwent no development, and the fluid remained perfectly clear. On the other hand, in similar experiments in which the flasks were heated only to 40° or 50° C. (104°-122° Fahr.), the fluid became turbid, in consequence of the multiplication of the Bacteria, in the course of from two to three days.
Both in Kühne's and in Cohn's experiments, which last have lately been confirmed and extended by Dr Roberts of Manchester, it was noted that long exposure to a lower temperature than that which brings about immediate destruction of life, produces the same effect as short exposure to the latter temperature. Thus, though all the Bacteria were killed, with certainty, in the normal fluid, by short exposure to temperatures at or above 60° C. (140° Fahr.), Cohn observed that, when a flask containing infected normal fluid was heated to 50°-52° C. (122°-125° Fahr.) for only an hour, the consequent multiplication of the Bacteria was manifested much earlier, than in one which had been exposed for two hours to the same temperature.
It appears to be very generally held that the simpler vegetable organisms are deprived of life at temperatures as high as 60° C. (140° Fahr.); but Algæ have been found living in hot springs at much higher temperatures, namely, from 168° to 208° Fahr., for which latter surprising fact we have the high authority of Descloiseaux. It is no ex​planation of these phenomena, but only another mode of stating them, to say that these organisms have become "accustomed" to such temperatures. If this degree of heat were absolutely incompatible with the activity of living matter, the plants could no more resist it than they could become "accustomed" to being made red hot. Habit may modify subsidiary, but cannot affect fundamental, conditions.
Recent investigations point to the conclusion that the immediate cause of the arrest of vitality, in the first place, and of its destruction, in the second, is the coagulation of certain substances in the protoplasm, and that the latter contains various coagulable matters, which solidify at different temperatures. And it remains to be seen, how far the death of any form of living matter, at a given temperature, depends on the destruction of its fundamental substance at that heat, and how far death is brought about by the coagulation of merely accessory compounds.
It may be safely said of all those living things which are large enough to enable us to trust the evidence of microscopes, that they are heterogeneous optically, and that their different parts, and especially the surface layer, as contrasted with the interior, differ physically and chemically; while, in most living things, mere heterogeneity is exchanged for a definite structure, whereby the body is distinguished into visibly different parts, which possess different powers or functions. Living things which present this visible structure are said to be organized; and so widely does organization obtain among living beings, that organized and living are not unfrequently used as if they were terms of co-extensive applicability. This, however, is not exactly accurate, if it be thereby implied that all living things have a visible organization, as there are numerous forms of living matter of which it cannot properly be said that they possess either a definite structure or permanently specialized organs: though, doubtless, the simplest particle of living matter must possess a highly complex molecular structure, which is far beyond the reach of vision.
The broad distinctions which, as a matter of fact, exist between every known form of living substance and every other component of the material world, justify the separation of the biological sciences from all others. But it must not be supposed that the differences between living and not-living matter are such as to justify the assumption that the forces at work in the one are different from those which are to be met with in the other. Considered apart from the phenomena of consciousness, the phenomena of life are all dependent upon the working of the same physical and chemical forces as those which are active in the rest of the world. It may be convenient to use the terms "vitality" and "vital force" to denote the causes of certain great groups of natural operations, as we employ the names of "electricity" and "electrical force" to denote others; but it ceases to be proper to do so, if such a name implies the absurd assumption that "electricity" and "vitality" are entities playing the part of efficient causes of electrical or vital phenomena. A mass of living protoplasm is simply a molecular machine of great complexity, the total results of the working of which, or its vital phenomena, depend, on the one hand, upon its construction, and, on the other, upon the energy supplied to it; and to speak of "vitality" as anything but the name of a series of operations is as if one should talk of the "horologity" of a clock.
Living matter, or protoplasm and the products of its metamorphosis, may be regarded under four aspects:—
(1.) It has a certain external and internal form, the latter being more usually called structure;
(2.) It occupies a certain position in space and in time;
(3.) It is the subject of the operation of certain forces in virtue of which it undergoes internal changes, modifies, external objects, and is modified by them; and
(4.) Its form, place, and powers are the effects of certain causes.
In correspondence with these four aspects of its subject, biology is divisible into four chief subdivisions—I. Morphology; II. Distribution; III. Physiology; IV. Ætiology.}}
I. Morphology.
So far as living beings have a form and structure, they fall within the province of Anatomy and Histology, the latter being merely a name for that ultimate optical analysis of living structure which can be carried out only by the aid of the microscope.
And, in so far as the form and structure of any living being are not constant during the whole of its existence, but undergo a series of changes from the commencement of that existence to its end, living beings have a Development. The history of development is an account of the anatomy of a living being at the successive periods of its existence, and of the manner in which one anatomical stage passes into the next.
Finally, the systematic statement and generalization of the facts of Morphology, in such a manner as to arrange living beings in groups according to their degrees of like ness, is Taxonomy.
The study of Anatomy and Development has brought to light certain generalizations of wide applicability and great importance.
Most plants and animals are aggregates of cells
1. It has been said that the great majority of living beings present a very definite structure. Unassisted vision and ordinary dissection suffice to separate the body of any of the higher animals, or plants, into fabrics of different of sorts, which always present the same general arrangement in the same organism, but are combined in different ways in different organisms. The discrimination of these comparatively few fabrics, or tissues, of which organisms are composed, was the first step towards that ultimate analysis of visible structure which has become possible only by the recent perfection of microscopes and of methods of preparation.
Histology, which embodies the results of this analysis, shows that every tissue of a plant is composed of more or less modified structural elements, each of which is termed a cell; which cell, in its simplest condition, is merely a spheroidal mass of protoplasm, surrounded by a coat or sac the cell-wall which contains cellulose. In the various tissues, these cells may undergo innumerable modifications of form the protoplasm may become differentiated into a nucleus with its nucleolus, a primordial utricle, and a cavity filled with a watery fluid, and the cell-wall may be variously altered in composition or in structure, or may coalesce with others. But, however extensive these changes may be, the fact that the tissues are made up of morphologically distinct units the cells remains patent. And, if any doubt could exist on the subject, it would be removed by the study of development, which proves that every plant commences its existence as a simple cell, identical in its fundamental characters with the less modified of those cells of which the whole body is composed.
But it is not necessary to the morphological unit of the plant that it should be always provided with a cell-wall. Certain plants, such as Protococcus, spend longer or shorter periods of their existence in the condition of a mere spheroid of protoplasm, devoid of any cellulose wall, while, at other times, the protoplasmic body becomes enclosed within a cell-wall, fabricated by its superficial layer.
Therefore, just as the nucleus, the primordial utricle, and the central fluid are no essential constituents of the ​morphological unit of the plant, but represent results of its metamorphosis, so the cell-wall is equally unessential; and either the term "cell" must acquire a merely technical significance as the equivalent of morphological unit, or some new term must be invented to describe the latter. On the whole, it is probably least inconvenient to modify
The liistological analysis of animal tissues has led to results and to difficulties of terminology of precisely the same character. In the higher animals, however, the modifications which the cells undergo are so extensive, that the fact that the tissues are, as in plants, resolvable into an aggregation of morphological units, could never have been established without the aid of the study of development, which proves that the animal, no less than the plant, com mences its existence as a simple cell, fundamentally iden tical with the less modified cells which are found in the tissues of the adult.
Though the nucleus is very constant among animal cells, it is not universally present ; and among the lowest forms of animal life, the protoplasmic mass which represents the morphological unit may b3, as in the lowest plants, devoid of a nucleus. In the animal, the cell-wall never has the character of a shut sac containing cellulose ; and it is not a little difficult, in many cases, to say how much of the so- called " cell-wall" of the animal cell answers to the " pri mordial utricle " and how much to the proper " cellulose cell-wall " of the vegetable cell. But it is certain that in the animal, as in the plant, neither cell-wall nor nucleus are essential constituents of the cell, inasmuch as bodies which are unquestionably the equivalents of cells true morphological units are mere masses of protoplasm, de void alike of cell-wall and nucleus.
For the whole living world, then, it results : that the morphological unit the primary and fundamental form of life is merely an individual mass of protoplasm, in which no further structure is discernible ; that independent living forms may present but little advance on this structure ; and that all the higher forms of life are aggregates of such morphological units or cells, variously modified.
Moreover, all that is at present known tends to the con clusion, that, in the complex aggregates of such units of which all the higher animals and plants consist, no cell has arisen otherwise than by becoming separated from the pro toplasm of a pre-existing cell; whence the aphorism "Omnis cellula e cellula."
It may further be added, as a general truth applicable to nucleated cells, that the nucleus rarely undergoes any considerable modification, the structures characteristic of the tissues being formed at the expense of the more super ficial protoplasm of the cells ; and that, when nucleated cells divide, the division of the nucleus, as a rule, precedes that of the whole cell.
2. In the course of its development every cell proceeds f rom a condition in which it closely resembles every other cell, through a series of stages of gradually increasing divergence, until it reaches that condition in which it presents the characteristic features of the elements of a special tissue. The development of the cell is therefore a gradual progress from the general to the special state.
The like holds good of the development of the body as a whole. However complicated one of the higher animals or plants may be, it begins its separate existence under the form of a nucleated cell. This, by division, becomes con verted into an aggregate of nucleated cells : the parts of this aggregate, following different laws of growth and multiplication, give rise to the rudiments of the organs ; and the parts of these rudiments again take on those modes of growth and multiplication and metamorphosis which are needful to convert the rudiment into the perfect structure.
The development of the organism as a whole, therefore, repeats in principle the development of the cell. It is a progress from a general to a special form, resulting from the gradual differentiation of the primitively similar mor phological units of which the body is composed.
Moreover, when the stages of development of two animals are compared, the number of these stages which are similar to one another is, as a general rule, proportional to the closeness of the resemblance of the adult forms ; whence it follows that the more closely any two animals are allied in adult structure, the later are their embryonic conditions distinguishable. And this general rule holds for plants no less than for animals.
The broad principle, that the form in which the more complex living things commence their development is always the same, was first expressed by Harvey in his famous aphorism, "Omne vivum exovo," which was intended simply as a morphological generalization, and in no wise implied the rejection of spontaneous generation, as it is commonly supposed to do. Moreover, Harvey s study of the development of the chick led him to promulgate that theory of " epigenesis," in which the doctrine that develop ment is a progress from the general to the special is im plicitly contained.
Caspar F. Wolff furnished further, and indeed conclu sive, proof of the truth of the theory of epigenesis ; but, unfortunately, the authority of Haller and the speculations of Bonnet led science astray, and it was reserved for Von Baer to put the nature of the process of development in its true light, and to formulate it in his famous law.
3. Development, then, is a process of differentiation by which the primitively similar parts of the living body become more and more unlike one another.
This process of differentiation may be effected in several ways.
(1.) The protoplasm of the germ may not undergo divi- tlon sion and conversion into a cell aggregate ; but various parts of its outer and inner substance may be metamorphosed directly into those physically and chemically different materials which constitute the body of the adult. This occurs in such animals as the Infusoria, and in such plants as the unicellular Alyce.
(2.) The germ may undergo division, and be converted into an aggregate of cells, which cells give rise to the tissues by undergoing a metamorphosis of the same kind as that to which the whole body is subjected in the preceding case.
The body, formed in either of these ways, may, as a whole, undergo metamorphosis by differentiation of its parts, and the differentiation may take place without reference to any axis of symmetry, or it may have reference to such an axis. In the latter case, the parts of the body which become dis tinguishable may correspond on the two sides of the axis (bilateral symmetry), or may correspond along several lines parallel with the axis (radial symmetry).
The bilateral or radial symmetry of the body may be further complicated by its segmentation, or separation by divisions transverse to the axis, into parts, each of which corresponds with its predecessor or successor in the series.
In the segmented body, the segments may or may not give rise to symmetrically or asymmetrically disposed pro cesses, which are appendages, using that word in its most general sense.
both animals and plants, is attained by the body when it becomes divided into segments provided with appendages; when the segments not only become very different from one another, but some coalesce and lose their primitive distinct ness ; and when the appendages and the segments into ​which they are subdivided similarly become differentiated
It is in virtue of such processes that the flowers of plants, and the heads and limbs of the Arthropoda and of the Vertebrata, among animals, attain their extraordinary diversity and complication of structure. A flower-bud is a segmented body or axis, with a certain number of whorls of appendages ; and the perfect flower is the result of the gradual differentiation and confluence of these primitively similar segments and their appendages. The head of an insect or of a crustacean is, in like manner, composed of a number of segments, each with its pair of appendages, which by differentiation and confluence are converted into the feelers and variously modified oral appendages of the adult.
In some complex organisms, the process of differentiation, by which they pass from the condition of aggregated embryo cells to the adult, can be traced back to the laws of growth of the two or more cells into which the embryo cell is divided, each of these cells giving rise to a particular por tion of the adult organism. Thus the fertilized embryo cell in the archegonium of a fern divides into four cells, one of which gives rise to the rhizome of the young fern, another to its first rootlet, while the other two are con verted into a placenta-like mass which remains embedded in the prothallus.
The structure of the stem of Chara depends upon the different properties of the cells, which are successively derived by transverse division from the apical cell. An inter-nodal cell, which elongates greatly, and does not divide, is succeeded by a nodal cell, which elongates but little, and becomes greatly subdivided ; this by another inter-nodal cell, and so on in regular alternation. In the game way the structure of the stem, in all the higher plants, depends upon the laws which govern the manner of division and of metamorphosis of the apical cells, and of their continuation in the cambium layer.
In all animals which consist of cell-aggregates, the cells of which the embryo is at first composed arrange them selves by the splitting, or by a process of invagination, of the blastoderm into two layers, the epiblast and the hypoblast, between which a third intermediate layer, the mesoblast, appears, and each layer gives rise to a definite group of organs in the adult. Thus, in the Vertebrata, the epiblast gives rise to the cerebro-spinal axis, and to the epidermis and its derivatives ; the hypoblast, to the epithelium of the alimentary canal and its derivatives ; and the mesoblast, to all the intermediate structures. The tendency of recent inquiry is to prove that the several layers of the germ evolve analogous organs in invertebrate animals, and to indi cate the possibility of tracing the several germ layers back to the blastomeres of the yelk, from the subdivision of which they proceed.
It is conceivable that all the forms of life should have presented about the same differentiation of structure, and should have differed from one another by superficial characters, each form passing by insensible gradations into those most like it. In this case Taxonomy, or the classification of morphological facts, would have had to confine itself to the formation of a serial arrangement representing the serial gradation of these forms in nature.
It is conceivable, again, that living beings should have differed as widely in structure as they actually do, but that the interval between any two extreme forms should have been filled up by an unbroken series of gradations ; in which case, again, classification could only effect the formation of series the strict definition of groups would be as impossible as in the former case.
As a matter of fact, living beings differ enormously, not only in differentiation of structure, but in the modes in which that differentiation is brought about ; and the intervals between extreme forms are not filled up in the existing world by complete series of gradations. Hence it arises that living beings are, to a great extent, susceptible of classification into groups, the members of each group resembling one another, and differing from all the rest, by certain definite peculiarities.
No two living beings are exactly alike, but it is a matter of observation that, among the endless diversities of living things, some constantly resemble one another so closely that it is impossible to draw any line of demarcation between them, while they differ only in such characters as are associated with sex. Such as thus closely resemble one another constitute a morphological species ; while different morphological species are defined by constant characters which are not merely sexual.
The comparison of these lowest groups, or morphological species, with one another, shows that more or fewer of them possess some character or characters in common some feature in which they resemble one another and differ from all other species and the group or higher order thus formed is a genus. The generic groups thus constituted are susceptible of being arranged in a similar manner into groups of successively higher order, which are known as families, orders, classes, and the like.
The method pursued in the classification of living forms is, in fact, exactly the same as that followed by the maker of an index in working out the heads indexed. In an alphabetical arrangement, the classification may be truly termed a morphological one, the object being to put into close relation all those leading words which resemble one another in the arrangement of their letters, that is, in their form, and to keep apart those which differ in structure. Headings which begin with the same word, but differ otherwise, might be compared to genera with their species ; the groups of words with the same first two syllables to families ; those with identical first syllables to orders ; and those with the same initial letter to classes. But there is this difference between the index and the Taxonomic arrangement of living forms, that in the former there is none but an arbitrary relation between the various classes, while in the latter the classes are similarly capable of co-ordination into larger and larger groups, until all are comprehended under the common definition of living beings.
The differences between "artificial " and "natural " classi fications are differences in degree, and not in kind. In each case the classification depends upon likeness ; but in an artificial classification some prominent and easily observed feature is taken as the mark of resemblance or dissemblance ; while, in a natural classification, the things classified arc arranged according to the totality of their morphological resemblances, and the features which are taken as the marks of groups are those which have been ascertained by ob servation to be the indications of many likenesses or un- likenesses. And thus a natural classification is a great deal more than a mere index. It is a statement of the marks of similarity of organization ; of the kinds of struc ture which, as a matter of experience, are found universally associated together; and, as such, it furnishes the whole foundation for those indications by which conclusions as to the nature of the whole of an animal are drawn from o knowledge of some part of it.
bone or of a shell to the nature of the animal to which that bone or shell belonged, he is guided by the empirical morphological laws established by wide observation, that such a kind of bone or sJiell is associated with such and such structural features in the rest of the body, and no ​others. And it is these empirical laws which are embodied
II. Distribution.
Living beings occupy certain portions of the surface of the earth, inhabiting either the dry land or the fresh or salt waters, or being competent to maintain their existence in either. In any given locality, it is found that those different media are inhabited by different kinds of living beings ; and that the same medium, at different heights in the air and at different depths in the water, has different living inhabitants.
Moreover, the living populations of localities which differ considerably in latitude, and hence in climate, always present considerable differences. But the converse proposition is not true ; that is to say, localities which differ in longitude, even if they resemble one another in climate, often have very dissimilar faunæ and floræ.
It has been discovered by careful comparison of local faunae and floras that certain areas of the earth s surface are inhabited by groups of animals and plants which are not found elsewhere, and which thus characterize each of these areas. Such areas are termed Provinces of Distribution. There is no parity between these provinces in extent, nor in the physical configuration of their boundaries ; and, in reference to existing conditions, nothing can appear to be more arbitrary and capricious than the distribution of living beings.
The study of distribution is not confined to the present order of nature ; but, by the help of geology, the naturalist is enabled to obtain clear, though too fragmentary, evidence of the characters of the faunas and florae of antecedent epochs. The remains of organisms which are contained in the stratified rocks prove that, in any given part of the earth s surface, the living population of earlier epochs was different from that which now exists in the locality ; and that, on the whole, the difference becomes greater the farther we go back in time. The organic remains which are found in the later Cainozoic deposits of any district are always closely allied to those now found in the province of distribution in which that locality is included ; while in the older Cainozoic, the Mesozoic, and the Paleozoic strata, the fossils may be similar to creatures at present living in some other province, or may be altogether unlike any which now exist.
In any given locality, the succession of living forms may appear to be interrupted by numerous breaks the asso ciated species in each fossiliferous bed being quite distinct from those above and those below them. But the ten dency of all palasontological investigation is to show that these breaks are only apparent, and arise from the incom pleteness of the series of remains which happens to have been preserved in any given locality. As the area over which accurate geological investigations have been carried on extends, and as the fossiliferous rocks found in one locality fill up the gaps left in another, so do the abrupt demarcations between the faunae and floras of successive epochs disappear a certain proportion of the genera and species of every period, great or small, being found to be continued for a longer or shorter time into the next suc ceeding period. It is evident, in fact, that the changes in the living population of the globe which have takenplace during its history, have been effected, not by the sudden replacement of one set of living beings by another, but by a process of slow and gradual introduction of new species, accompanied by the extinction of the older forms.
It is a remarkable circumstance, that in all parts of the globs in which fossiliferous rocks have yet been examined, tho successive terms of the series of living forms which have thus succeeded one another are analogous. The life of the Mesozoic epoch is everywhere characterized by the abundance of some groups of species of which no trace is to be found in either earlier or later formations ; and the like is true of the Palaeozoic epoch. Hence it follows, not only that there has been a succession of species, but that the general nature of that succession has been the same all over the globe ; and it is on this ground that fossils are so important to the geologist as marks of the relative ago of rocks.
The determination of the morphological relations of the species which have thus succeeded one another is a problem of profound importance and difficulty, the solution of which, however, is already clearly indicated. For, in several cases, it is possible to show that, in the same geographical area, a form A, which existed during a certain geological epoch, has been replaced by another form B, at a later period ; and that this form B has been replaced, still later, by a third form C. When these forms, A, B, and C, are com pared together they are found to be organized upon the same plan, and to be very similar even in most of the de tails of their structure ; but B differs from A by a slight modification of some of its parts, which modification is carried to a still greater extent in C.
In other words, A, B, and C differ from one another in the same fashion as the earlier and later stages of the embryo of the same animals differ ; and in successive epochs we have the group presenting that progressive specialization which characterises the development of the individual. Clear evidence that this progressive specialization of structure has actually occurred has as yet been obtained in only a few cases (e.g., Equidce, Crocodilia), and these are confined to the highest and most complicated forms of life ; while it is demonstrable that, even as reckoned by geolo gical time, the process must have been exceedingly slow. Among the lower and less complicated forms the evi dence of progressive modifications, furnished by compari son of the oldest with the latest forms, is slight, or absent ; and some of these have certainly persisted, with very little change, from extremely ancient times to the present day. It is as important to recognize the fact that certain forms of life have thus persisted, as it is to admit that others have undergone progressive modification.
It has been said that the successive terms in the series of living forms are analogous in all parts of the globe. But the species which constitute the corresponding or homotaxic terms in the series, in different localities, are not identical. And, though the imperfection of our knowledge at present precludes positive assertion, there is every reason to believe that geographical provinces have existed through out the period during which organic remains furnish us with evidence of the existence of life. The wide distribu tion of certain Palasozoic forms does not militate against this view ; for the recent investigations into the nature of the deep-sea fauna have shown that numerous Crustacea, Echinodermata, and other invertebrate animals, have as wide a distribution now. as their analogues possessed in the Silurian epoch.
III. Physiology.
Thus far living beings have been regarded merely as definite forms of matter, and Biology has presented no considerations of a different order from those which meet the student of Mineralogy. But living things are not only natural bodies, having a definite form and mode of structure, growth, and development. They are machines in action ; and, under this aspect, the phenomena which they present have no parallel in the mineral world.
and these functions, varied as they are, may be reduced to ​three categories. They are either (1), functions which affect the material composition of the body, and determine its mass, which is the balance of the processes of waste on the one hand and those of assimilation on the other. Or (2), they are functions which subserve the process of re production, which is essentially the detachment of a part endowed with the power of developing into an independent whole. Or (3), they are functions in virtue of which one part of the body is able to exert a direct influence on another, and the body, by its parts or as a whole, becomes a source of molar motion. The first may be termed susten- tative, the second generative, and the third correlative
Of these three classes of functions the first two only can be said to be invariably present in living beings, all of which are nourished, grow, and multiply. But there are some forms of life, such as many Fungi, which are not known to possess any powers of changing their form; in which the protoplasm exhibits no movements, and reacts upon no stimulus ; and in which any influence which the different parts of the body exert upon one another must be transmitted indirectly from molecule to molecule of the common mass. In most of the lowest plants, however, and in all animals yet known, the body either constantly or temporarily changes its form, either with or without the application of a special stimulus, and thereby modifies the relations of its parts to one another, and of the whole to surrounding bodies ; while, in all the higher animals, the different parts of the body are able to affect, and be affected by, one another, by means of a special tissue, termed nerve. Molar motion is effected on a large scale by means of another special tissue, muscle; and the organism is brought into relation with surrounding bodies by means of a third kind of special tissue that of the sensory organs by means of which the forces exerted by surrounding bodies are transmuted into affections of nerve.
In the lowest forms of life, the functions which have been enumerated are seen in their simplest forms, and they are exerted indifferently, or nearly so, by all parts of the protoplasmic body; and the like is true of the functions of the body of even the highest organisms, so long as they are in the condition of the nucleated cell, which constitutes the starting point of their development. But the first pro cess in that development is the division of the germ into a number of morphological units or blastomeres, which, eventually, give rise to cells; and as each of these pos sesses the same physiological functions as the germ itself, it follows that each morphological unit is also a physio logical unit, and the multicellular mass is strictly a com pound organism, made up of a multitude of physiologically independent cells. The physiological activities manifested by the complex whole represent the sum, or rather the resultant, of the separate and independent physiological activities resident in each of the simpler constituents of that whole.
The morphological changes which the cells undergo in the course of the further development of the organism do not affect their individuality ; and, notwithstanding the modification and confluence of its constituent cells, the adult organism, however complex, is still an aggregate of morphological units. Nor is it less an aggregate of physio logical units, each of which retains its fundamental inde pendence, though that independence becomes restricted in various ways.
Each cell, or that element of a tissue which proceeds from the modification of a cell, must needs retain its sustentative functions so long as it grows or maintains a condition of equilibrium ; but the most completely meta morphosed cells show no trace of the generative function, and many exhibit no correlative functions. On the other hand, those cells of the adult organism which are the un- metamorphosed derivatives of the germ, exhibit all the primary functions, not only nourishing themselves and growing, but multiplying, and frequently showing more or less marked movements.
Organs are parts of the body which perform particular functions. In strictness, perhaps, it is not quite right to speak of organs of sustentation or generation, each of these functions being necessarily performed by the morphological unit which is nourished or reproduced. What are called the organs of these functions are the apparatuses by which certain operations, subsidiary to sustentation and genera tion, are carried on.
Thus, in the case of the sustentative functions, all those organs may be said to contribute to this function which are concerned in bringing nutriment within reach of the ultimate cells, or in removing waste matter from them ; while in the case of the generative function, all those organs contribute to the function which produce the cells from which germs are given off ; or help in the evacuation, or fertilization, or development of these germs.
On the other hand, the correlative functions, so long as they are exerted by a simple undifferentiated morphological unit or cell, are of the simplest character, consisting of those modifications of position which can be effected by mere changes in the form or arrangement of the parts of the protoplasm, or of those prolongations of the proto plasm which are called pseudopodia or cilia. But, in the higher animals and plants, the movements of the organism and of its parts are brought about by the change of the form of certain tissues, the property of which is to shorten in one direction when exposed to certain stimuli. Such tissues are termed contractile; and, in their most fully developed condition, muscular. The stimulus by which this contraction is naturally brought about is a molecular change, either in the substance of the contractile tissue itself, or in some other part of the body ; in which latter case, the motion which is set up in that part of the body must be propagated to the contractile tissue through the intermediate substance of the body. In plants there seems to be no question that parts which retain a hardly modified cellular structure may serve as channels for the transmis sion of this molecular motion ; whether the same is true of animals is not certain. But, in all the more complex animals, a peculiar fibrous tissue nerve serves as the agent by which contractile tissue is affected by changes occurring elsewhere, and by which contractions thus ini tiated are co-ordinated and brought into harmonious com bination. While the sustentative functions in the higher forms of life are still, as in the lower, fundamentally de pendent upon the powers inherent in all the physiological units which make up the body, the correlative functions are, in the former, deputed to two sets of specially modified units, whick constitute the muscular and the nervous tissues.
as physiological machines, they are found to differ as machines of human construction do. In the lower forms, the mechanism, though perfectly well adapted to do the work for which it is required, is rough, simple, and weak ; while, in the higher, it is finished, complicated, and powerful. Considered as machines, there is the same sort of difference between a polype and a horse as there is between a distaff and a spinning-jenny. In the progress from the lower to the higher organism, there is a gradual differentiation of organs and of functions. Each function is separated into many parts, which are severally entrusted to distinct organs. To use the striking phrase of Milne-Edwards, in passing from low to high organisms, there is a division of physiological labour. And exactly the same process is observable in the development of any of the higher organ- ​isms ; so that, physiologically, as well as morphologically,
Thus far, the physiological activities of living matter have been considered in themselves, and without reference to anything that may affect them in the world outside the living body. But living matter acts on, and is powerfully affected by, the bodies which surround it; and the study of the influence of the "conditions of existence" thus deter mined constitutes a most important part of Physiology.
The sustentative functions, for example, can only be exerted under certain conditions of temperature, pressure, and light, in certain media, and with supplies of particular kinds of nutritive matter; the sufficiency of which supplies, again, is greatly influenced by the competition of other organisms, which, striving to satisfy the same needs, give rise to the passive "struggle for existence." The exercise of the correlative functions is influenced by similar condi tions, and by the direct conflict with other organisms, which constitutes the active struggle for existence. And, finally, the generative functions are subject to extensive modifi cations, dependent partly upon what are commonly called external conditions, and partly upon wholly unknown agencies.
In the lowest forms of life, the only mode of generation at present known is the division of the body into two or and more parts, each, of which then grows to the size and assumes the form of its parent, and repeats the process of multiplication. This method of multiplication by fission is properly called generation, because the parts which are separated are severally competent to give rise to individual organisms of the same nature as that from which they arose.
In many of the lowest organisms the process is modified so far that, instead of the parent dividing into two equal parts, only a small portion of its substance is detached, as a bud which develops into the likeness of its parent. This is generation by gemmation. Generation by fission and by gemmation are not confined t&lt;- the simplest forms of life, however. On the contrary, both modes of multipli cation are common not only among plants, but among animals of considerable complexity.
The multiplication of flowering plants by bulbs, that of annelids by fission, and that of polypes by budding, are well-known examples of these modes of reproduction. In all these cases, the bud or the segment consists of a multi tude of more or less metamorphosed cells. But, in other instances, a single cell detached from a mass of such un- differentiated cells contained in the parental organism is the foundation of the new organism, and it is hard to say whether such a detached cell may be more fitly called a bud or a segment whether the process is more akin to fission or to gemmation.
In all these cases the development of the new being from the detached germ takes place without the influence of other living matter. Common as the process is in plants and in the lower animals, it becomes rare among the higher animals. In these, the reproduction of the whole organism from a part, in the way indicated above, ceases. At most, we find that the cells at the end of an amputated portion of the organism are capable of reproducing the lost part ; and, in the very highest animals, even this power vanishes in the adult ; and, in most parts of the body, though the undifferentiated cells are capable of multi plication, their progeny grow, not into whole organisms like that of which they form a part, but into elements of the tissues.
Throughout almost the whole series of living beings, however, we find concurrently with the process of agamogenesis, or asexual generation, another method of genera tion, in which the develoDment of the germ into an organism resembling the parent depends on an influence exerted by living matter different from the germ. This is gamogenesis, or sexual generation. Looking at the facts broadly, and without reference to many exceptions in detail, it may be said that there is an inverse relation between agamogenetic and garnogenetic reproduction. In the lowest organisms gamogenesis has not yet been observed, while in the highest agamogencsis is absent. In many of the lower forms of life agamogenesis is the common and predominant modo of reproduction, while gamogenesis is exceptional ; on the contrary, in many of the higher, while gamogenesis is the rule, agamogenesis takes place exceptionally. In its simplest condition, which is termed " conjugation" sexual generation consists in the coalescence of two similar masses of protoplasmic matter, derived from different parts of the same organism, or from two organisms of the same species, and the single mass which results from the fusion develops into a new organism.
In the majority of cases, however, there is a marked morphological difference between the two factors in the process, and then one is called the male, and the other the female element. The female element is relatively large, and undergoes but little change of form. In all the higher plants and animals it is a nucleated cell, to which a greater or less amount of nutritive material, constituting the food-yolk, may be added.
The male element, on the other hand, is relatively small. It may be conveyed to the female element by an out growth of the wall of its cell, which is short in many Algce and Fungi, but becomes an immensely elongated tubular filament, in the case of the pollen cell of flowering plants. But, more commonly, the protoplasm of the male cells becomes converted into rods or filaments, which usu ally are in active vibratile movement, and sometimes are pro pelled by numerous cilia. Occasionally, however, as in many Nematoidea and Arthropoda, they are devoid of mobility.
The manner in which the contents of the pollen tube affect the embryo cell in flowering plants is unknown, as no perforations through which the contents of the pollen tube may pass, so as actually to mix with the substance of the embryo cell, have been discovered ; and there is the same difficulty with respect to the conjugative processes of. some of the Gryptogamia. But in the great majority of plants, and in all animals, there can be no doubt that the substance of the male element actually mixes with that of the female, so that in all these cases the sexual pro cess remains one of conjugation ; and impregnation is the physical admixture of protoplasmic matter derived from two sources, which may be either different parts of the same organism, or different organisms.
The effect of impregnation appears in all cases to be that the impregnated protoplasm tends to divide into portions (blastomeres), which may remain united as a single cell-aggregate, or some or all of which may become separate organisms. A larger or shorter period of rest, in many cases, intervenes between the act of impregnation and the commencement of the process of division.
As a general rule, the female cell which directly receives the influence of the male is that which undergoes division and eventual development into independent germs ; but there are some plants, such as the Florideoe, in which this is not the case. In these the protoplasmic body of the trichogyne, which unites with the molecular spermatozoids, does not undergo division itself, but transmits some influ ence to adjacent cells, in virtue of which they become subdivided into independent germs or spores.
processes of the Infusoria ; but, in the Vorticellidce, it would appear that conjugation merely determines a condition of ​the whole organism, which gives rise to the division of the endoplast or so-called nucleus, by which germs are thrown off; and if this be the case, the process would have some
On the other hand, the process of conjugation by which two distinct Diporpce combine into that extraordinary double organism, the Diplozoon paradoxum, does not directly give rise to germs, but determines the development of the sexual organs in each of the conjugated individuals ; and the same process takes place in a large number of the Infusoria, if what are supposed to be male sexual elements in them are really such.
The process of impregnation in the Floridece is remark ably interesting, from its bearing upon the changes which fecundation is known to produce upon parts of the parental organism other than the ovum, even in the highest animals and plants.
The nature of the influence exerted by the male element upon the female is wholly unknown. N"o morphological distinction can be drawn between those cells which are capable of reproducing the whole organism without im pregnation, and those which need it, as is obvious from what happens in insects, where eggs which ordinarily re quire impregnation, exceptionally, as in many moths, or regularly, as in the case of the drones among bees, develop without impregnation. Even in the higher animals, such as the fowl, the earlier stages of division of the germ may take place without impregnation.
In fact, generation may be regarded as a particular case of cell multiplication, and impregnation simply as one of the many conditions which may determine or affect that process. In the lowest organisms, the simple protoplasmic mass divides, and each part retains all the physiological pro perties of the whole, and consequently constitutes a germ whence the whole body can be reproduced. In more ad vanced organisms, each of the multitude of cells into which the embryo cell is converted at first, probably retains all, or nearly all, the physiological capabilities of the whole, and is capable of serving as a reproductive gerrn ; but as division goes on, and many of the cells which result from division acquire special morphological and physiological properties, it seems not improbable that they, in proportion, lose their more general characters. In proportion, for example, as the tendency of a given cell to become a muscle cell or a car tilage cell is more marked and definite, it is readily con ceivable that its primitive capacity to reproduce the whole organism should be reduced, though it might not be alto gether abolished. If this view is well based, the power of reproducing the whole organism would be limited to those cells which had acquired no special tendencies, and conse quently had retained all the powers of the primitive cell in which the organism commenced its existence. The more extensively diffused such cells were, the more generally might multiplication by budding or fission take place ; the more localized, the more limited would be the parts of the organism in which such a process would take place. And even where such cells occurred, their development or non- development might be connected with conditions of nutri tion. It depends on the nutriment supplied to the female larva of a bee whether it shall become a neuter or a sexually perfect female ; and the sexual perfection of a large pro portion of the internal parasites is similarly dependent upon their food, and perhaps on other conditions, such as the temperature of the medium in which they live. Thus the gradual disappearance of agamogenesis in the higher animals would be related with that increasing specialization of function which is their essential characteristic ; and when it ceases to occur altogether, it may be supposed that no cells are left which retain unmodified the powers of the primi tive embryo cell. The organism is like a society in which every one is so engrossed by his special business that he has neither time nor inclination to marry.
Even the female elements in the highest organisms, little as they differ to all appearance from undifferentiatcd cells, and though they are directly derived from epithelial cells which have undergone very little modification from the condition of blastomeres, are incapable of full develop ment unless they are subjected to the influence of the male element, which may, as Caspar Wolff suggested, be compared to a kind of nutriment. But it is a living nutri ment, in some respects comparable to that which would be supplied to an animal kept alive by transfusion, and its molecules transfer to the impregnated embryo cell all the special characters of the organism to which it belonged.
The tendency of the germ to reproduce the characters of its immediate parents, combined, in the case of sexual generation, with the tendency to reproduce the characters of the male, is the source of the singular phenomena of hereditary transmission. No structural modification is so. slight, and no functional peculiarity is so insignificant in either parent that it may not make its appearance in the offspring. But the transmission of parental peculiarities depends greatly upon the manner in which they have been acquired. Such as have arisen naturally, and have been hereditary through many antecedent generations, tend to appear in the progeny with great force ; while artificial modifications, such, for example, as result from mutilation, are rarely, if ever, transmitted. Circumcision through in numerable ancestral generations does not appear to have reduced that rite to a mere formality, as it should have done, if the abbreviated prepuce had become hereditary in the descendants of Abraham ; while modern lambs are born with long tails, notwithstanding the long-continued prac tice of cutting those of every generation short. And it remains to be seen whether the supposed hereditary trans mission of the habit of retrieving among dogs is really what it seems at first sight to be ; on the other side, Brown- Sequard s case of the transmission of artificially induced epilepsy in guinea-pigs is undoubtedly very weighty.
Although the germ always tends to reproduce, directly or indirectly, the organism from which it is derived, the result of its development differs somewhat from the parent. Usually the amount of variation is insignificant ; but it may be considerable, as in the so-called " sports ; " and such variations, whether useful or useless, may be trans mitted with great tenacity to the offspring of the subjects of them.
In many plants and animals which multiply both asexually and sexually, there is no definite relation between the agamogenetic and the gamogenetic phenomena. The organism may multiply asexually before, or after, or concurrently with, the occurrence of sexual generation.
But ill a great many of the lower organisms, both animal and vegetable, the organism. (A) which results from the impregnated germ produces offspring only agamo- genetically. It thus gives rise to a series of independent organisms (B, B, B, . . .), which are more or less different from A, and which sooner or later acquire generative organs. From their impregnated germs A is reproduced. The pro cess thus described is "what has been termed the " alterna tion of generations " under its simplest form, for example, as it is exhibited by the Salpce. In more complicated cases, the independent organisms which correspond with B may give rise agamogenetically to others (BJ, and these to others (B 2 ), and so on (e.g., Aphis). But, however long the series, a final term appears which develops sexual organs, and reproduces A. The "alternation of genera tions " is, therefore, in strictness, an alternation of asexual with sexual generation, in which the products of the one process differ from those of the other.
The Hydrozoa offer a complete series of gradations between those cases in which th term B is represented by a free, self-nourishing organism (e.g., Cyan&a], through those in which it is free but unable to feed itself (Caly cophoridce), to those in which the sexual elements are developed in bodies which resemble free zooids, but are never detached, and are mere generative organs of the body on which they are developed (Cordylophora).
In the last case, the " individual " is the total product of the development ot the impregnated embryo, all the parts of which remain in material continuity with one another The multiplication of mouths and stomachs in a Cordylophora no more makes it an aggregation of different individuals than the multiplication of segments and legs in a centipede converts that Arthropod into a compound animal. The Cordylophora is a differentiation of a whole into many parts, and the use of any terminology which implies that it results from the coalescence of many parts into a whole is to be deprecated.
In CordylopMra the generative organs are incapable of maintaining a separate existence; but in nearly allied Hijdrozoa the unquestionable homologues of these organs become free zooids, in many cases capable of feeding and growing, and developing the sexual elements only after they have undergone considerable changes of form. Mor phologically, the swarm of Medusa; thus set free from a Hydrozoon are as much organs of the latter, as the multi tudinous pinnules of a Comatula, with their genital glands, are organs of the Echinoderm. Morphologically, there fore, the equivalent of the individual Comatula is the Hydrozoic stock + all the Medusae which proceed from it.
No doubt it sounds paradoxical to speak of a million of Aphides, for example, as parts of one morphological indivi dual ; but beyond the momentary shock of the paradox no harm is done. On the other hand, if the asexual Aphides are held to be individuals, it follows, as a logical conse quence, not only that all the polypes on a Cordylophora tree are " feeding individuals," and all the genital sacs "generative individuals," while the stem must be a " stump individual," but that the eyes and legs of a lobster are "ocular" and "locomotive individuals." And this conception is not only somewhat more paradoxical than the other, but suggests a conception of the origin of the com plexity of animal structure which is wholly inconsistent with fact.
IV. Ætiology.
Morphology, Distribution, and Physiology investigate and determine the facts of Biology. ^Etiology has for its object the ascertainment of the causes of these facts, and the explanation of biological phenomena, by showing that they constitute particular cases of general physical laws. It is hardly needful to say that aetiology, as thus conceived, is in its infancy, and that the seething controversies, to whioh the attempt to found this branch of science made in the Origin of Species has given rise, cannot be dealt with in the limits of this article. At most, the general nature of the problems to be evolved, and the course of inquiry needful for their solution, may be indicated.
In any investigation into the causes of the phenomena of life, the first question which arises is, whether we have any knowledge, and if so, what knowledge, of the origin of living matter?
In the case of all conspicuous and easily-studied organisms, it has been obvious, since the study of nature began, that living beings arise by generation from living beings of a like kind ; but before the latter part of the 17th century, learned and unlearned alike shared the conviction that this rule was not of universal application, and that multitudes of the smaller and more obscure organisms were produced by the fermentation of not-living, and especially of putrefying dead matter, by what Avas then termed generatio cequivoca or spontanea, and is now called aliogtnesis. Redi showed that the general belief was erroneous in a multitude of instances ; Spallanzani added largely to tho list ; while the investigations of the scientific helmintholo- gists of the present century have eliminated a further category of cases in which it was possible to doubt the applicability of the rule " omnc vivum e vivo " to the more complex organisms which constitute the present fauna and flora of the earth. Even the most extravagant supporters of abiogeuesis at the present day do not pretend that organisms of higher rank than the lowest Fungi and Protozoa are produced otherwise than by generation from pre-existing organisms. But it is pretended that Bacteria, Torulce, certain Fungi, and " Monads " are developed under conditions which render it impossible that these organisms should have proceeded directly from living matter.
The experimental evidence adduced in favour of this proposition is always of one kind, and the reasoning on which the conclusion that a biogenesis occurs is based may be stated in the following form:—
All living matter is killed by being heated to n degrees.
The contents of the closed vessel A have been heated to n degrees.
Therefore, all living matter which may have existed therein has been killed.
But living Bacteria, &c., have appeared in these contents subsequently to their being heated.
Therefore, they have been formed abiogenetically.
No objection can be taken to the logical form of this reasoning, but it is obvious that its applicability to any particular case depends entirely upon the validity, in that case, of the first and second propositions.
Suppose a fluid to be full of Bacteria in active motion, what evidence have we that they are killed when that fluid is heated to n degrees? There is but one kind of conclusive evidence, namely, that from that time forth no living Bacteria make their appearance in the liquid, supposing it to be properly protected from the intrusion of fresh Bacteria. The only other evidence, that, for example, which may be furnished by the cessation of the motion of the Bacteria, and such slight changes as our microscopes permit us to observe in their optical characters, is simply presumptive evidence of death, and no more conclusive than the stillness and paleness of a man in a swoon are proof that he is dead. And the caution is the more neces sary iu the case of Bacteria, since many of them naturally pass a considerable part of their existence in a condition in which they show no marks of life whatever save growth and multiplication.
If indeed it could be proved that, in cases which are not open to doubt, living matter is always and invariably killed at precisely the same temperature, there might be some ground for the assumption, that, in those which are obscure, death must take place under the same circum stances. But what are the facts ? It has been pointed out at the commencement of this article, that the range of high temperatures between the lowest, at which some living things are certainly killed, and the highest, at which others certainly live, is rather more than 100 Fahr., that is to say, between 104 Fahr. and 208 Fahr. It makes no sort of difference to the argument how living beings have come to be able to bear such a temperature as the last mentioned; the fact that they do so is sufficient to prove that, under certain conditions, such a temperature is not sufficient to destroy life.
Thus it appears that there is no ground for the assump tion that all living matter is killed at some given tempera ture between 104 and 208 Fahr,
But, further, there is very strong reason for believing that the influence of temperature on life is greatly modified, first, by the nature of the medium in which organisms are placed, and, secondly, by the length of time during which any given temperature is kept up.
On this point recent experiments made by Dr Roberts of Manchester are of great importance. He found, for example, as every other careful experimenter has done, that ordinary infusion of hay boiled for a few minutes was sterilized, that is to say, no development of Bacteria took place in it, however long it might be kept ; while if the infusion was rendered alkaline with ammonia or liquor potassae, it was not sterilized except after an exposure to the heat of boiling water for more than an hour. Some times it became productive after two hours, and once after three hours of such exposure. Is it to be imagined that, in the case of the alkalized hay infusion, the heat applied really killed the Bacteria which existed in the infusion, and that Bacteria of identically the same kind were generated afresh out of the dead matter 1 ? or is it more probable that the powers of resistance of the Bacteria to heat were simply increased by the alkalinity of the infusion] The statement of the questions surely render it unnecessary to answer them.
Dr Roberts further proves that there are two factors in the induction of sterilization, the degree of heat on the one hand, and the duration of its application on the other. A longer exposure to a lower temperature was equivalent to a shorter exposure to a higher temperature. "For example, speaking roughly, an exposure of an hour and a half to a heat of 212 Fahr. appeared to be equivalent to an ex posure for fifteen minutes to a heat of 228 Fahr.
It is hard to conceive what explanation can be offered of this fact, except that, under the conditions of the experiment, the organisms were either all affected by the first incidence of the heat in such a way as only to arrest some of their vital functions, and to leave a potentiality of life in them, such as exists in some kinds of dried living matter ; or that they individually differed very much in their powers of resistance, and that some were able to withstand heat much longer than others.
Under these circumstances it will be evident, that no experimental evidence that a liquid may be heated to n degrees, and yet subsequently give rise to living organisms, is of the smallest value as proof that abiogenesis has taken place, and for two reasons : Firstly, there is no proof that organisms of the kind in question are dead, except their permanent incapacity to grow and reproduce their kind ; and secondly, since we know that conditions may largely modify the power of resistance of such organisms to heat, it is far more probable that such conditions existed in the experiment in question, than that the organisms were generated afresh out of dead matter.
Not only is the kind of evidence adduced in favour of abiogenesis logically insufficient to furnish proof of its occurrence, but it may be stated as a well-based induction, that the more careful the investigator, and the more com plete his mastery over the endless practical difficulties which surround experimentation on this subject, the more certain are his experiments to give a negative result ; while positive results are no less sure to crown the efforts of the clumsy and the careless.
It is argued that a belief in abiogenesis is a necessary and corollary from the doctrine of Evolution. This may be true of the occurrence of abiogenesis at some time ; but if the present day, or any recorded epoch of geological time, be in question, the exact contrary holds good. If all living beings have been evolved from pre-existing forms of life, it is enough that a single particle of living protoplasm should once have appeared on the globe, as the result of no matter what agency. In the eyes of a consistent evolutionist any further independent formation of protoplasm would be sheer waste.
The production of living matter since the time of its first appearance, only by way of biogenesis, implies that the specific forms of the lower kinds of life have undergone but little change in the course of geological time, and this is said to be inconsistent with the doctrine of evolu tion. But, in the first place, the fact is not inconsis tent with the doctrine of evolution properly understood, that doctrine being perfectly consistent with either the progression, the retrogression, or the stationary condition of any particular species for indefinite periods of time; and secondly, if it were, it would be so much the worse for the doctrine of evolution, inasmuch as it is unquestionably true, that certain, even highly organized, forms of life have persisted without any sensible change for very long periods. The Terebratula psittacea of the present day, for example, is not distinguishable from that of the Cretaceous epoch, while the highly organised Teleostean fish, Beryx, of the Chalk differed only in minute specific characters from that which now lives. Is it seri ously suggested that the existing Tcrebratulce and Beryccs are not the lineal descendants of their Cretaceous ancestors, but that their modern representatives have been inde pendently developed from primordial germs in the interval ? But if this is too fantastic a suggestion for grave con sideration, why are we to believe that the Gloligerince of the present day are not lineally descended from the Cretaceous forms ? And if their unchanged generations have succeeded one another for all the enormous time represented by the deposition of the Chalk and that of the Tertiary and Quaternary deposits, what difficulty is there in supposing that they may not have persisted unchanged for a greatly longer period?
The fact is, that at the present moment there is not a shadow of trustworthy direct evidence that abiogenesis does take place, or has taken place, within the period during which the existence of life on the globe is recorded. But it need hardly be pointed out, that the fact does not in the slightest degree interfere with any conclusion that may be arrived at deductively from other considerations that, at some time or other, abiogenesis must have taken place. If the hypothesis of evolution is true, living matter must have arisen from not-living matter ; for by the hypothesis, the condition of the globe was at one time such that living matter could not have existed in it, life being entirely incompatible with the gaseous state. But living matter once originated, there is no necessity for another origina tion, since the hypothesis postulates the unlimited, though perhaps not indefinite, modifiability of such matter.
Of the causes which have led to the origination of living matter, then, it may be said that we know absolutely of nothing. But postulating the existence of living matter endowed with that power of hereditary transmission, and with that tendency to vary which is found in all such matter, Mr Darwin has shown good reasons for believing that the interaction between living matter and surrounding conditions, which results in the survival of the fittest, is sufficient to account for the gradual evolution or plants and animals from their simplest to their most complicated forms, and for the known phenomena of Morphology, Physiology, and Distribution.
Mr Darwin has further endeavoured to give a physical explanation of hereditary transmission by his hypothesis of Pangenesis ; while he seeks for the principal, if not the only, cause of variation in the influence of changing conditions.
It is on this point that the chief divergence exists among those who accept the doctrine of Evolution in its general outlines. Three views may be taken of the causes of variation:—
a. In virtue of its molecular structure, the organism may tend to vary. This variability may either be indefinite, or may be limited to certain directions by intrinsic condi tions. In the former case, the result of the struggle for existence would be the survival of the fittest among an indefinite number of varieties ; in the latter case, it would be the survival of the fittest among a certain set of varieties, the nature and number of which would be predetermined by the molecular structure of the organism.
b. The organism may have no intrinsic tendency to vary, but variation may be brought about by the influence of conditions external to it. And in this case also, the varia bility induced may be either indefinite or defined by in trinsic limitation.
c. The two former cases may be combined, and variation may to some extent depend upon intrinsic, and to some extent upon extrinsic, conditions.
At present it can hardly be said that such evidence as would justify the positive adoption of any one of these views exists.
If all living beings have come into existence by the gradual modification, through a long series of generations, of a primordial living matter, the phenomena of embryonic development ought to be explicable as particular cases of the general law of hereditary transmission. On this view, a tadpole is first a fish, and then a tailed amphibian, pro vided with both gills and lungs, before it becomes a frog, because the frog was the last term in a series of modifica tions whereby -some ancient fish became an urodele amphi bian; and the urodele amphibian became an anurous amphi bian. In fact, the development of the embryo is a recapitu lation of the ancestral history of the species.
If this be so, it follows that the development of any organism should furnish the key to its ancestral history ; and the attempt to decipher the full pedigree of organisms from so much of the family history as is recorded in their development has given rise to a special branch of biological speculation, termed phylogeny.
In practice, however, the reconstruction of the pedigree of a group from the developmental history of its existing members is fraught with difficulties. It is highly probable that the series of developmental stages of the individual organism never presents more than an abbreviated and condensed summary of ancestral conditions; while this summary is often strangely modified by variation and adaptation to conditions ; and it must be confessed that, in most cases, we can do little better than guess what is genuine recapitulation of ancestral forms, and what is the effect of comparatively late adaptation.
The only perfectly safe foundation for the doctrine of Evolution lies in the historical, or rather archaeological, evidence that particular organisms have arisen by the gradual modification of their predecessors, which is fur nished by fossil remains. That evidence is daily increasing in amount and in weight ; and it is to be hoped that the comparison of the actual pedigree of these organisms with the phenomena of their development may furnish some criterion by which the validity of phylogenetic conclusions, deduced from the facts of embryology alone, may be satis factorily tested.
Bibliography.—Haeckel, Generelle Marphologie ; H. Spencer, Principles of Biology.
(t . h. h.)
LIMITS AND CLASSIFICATION OF THE VEGETABLE KINGDOM.
The fundamental difference which separates the vegetable kingdom from the animal kingdom is to be found in the modes of nutrition which obtain in each. If we compare a plant and animal reduced to their simplest terms, and consisting, therefore, in each case of a single cell, i.e., of a minute mass of protoplasm invested with a cell-wall, while the unicellular plant draws its nutri ment by simple imbibition through the cell-wall from the surrounding medium a process which implies that all its nutriment passes into it in a liquid form the unicellular animal is able to take in solid nutriment by means of interruptions in the continuity of the cell-wall, and is also able afterwards to reduce this solid food, if of a suitable composition, to the liquid state. And not merely is there a difference of this kind in the mode, there is also one no less important, although less general, in the materials of nutrition. While under present terrestrial conditions those substances, or chemical combinations, which are required for the nutrition of animal organisms, are, as far as we know, nowhere spontaneously produced that is to say, nowhere apart from the influence of living organisms materials derived wholly from the in organic world are sufficient to sustain directly nearly the whole of vegetable life, and therefore, indirectly, of all other life as well. Roughly speaking, while plants are able to use for the purposes of nutrition binary compounds, such as carbon dioxide (CO.,), water (H 2 0), and ammonia (Nil), animals are essentially dependent on the same elements as enter into these compounds, but mostly in a higher state of chemical aggregation than the binary. Plants, therefore, are the " hewers of wood and drawers of water " for other living things. And this property which they so largely possess of constructing, from materials not directly available for animal nutrition, substances which are so, is found to be uniformly attended with the presence of a peculiar green colouring matter, known as chlorophyll, with which a portion of the protoplasm of their cells is tinged. Many plants, however, such as the whole group of Fungi, as well as some flowering plants, draw their nutriment from compounds derived from other organisms, and therefore in a higher state of chemical aggregation than those the green plants make use of. So far they approach animals in the mode of their nutrition.
At first sight it might seem a probable hypothesis that the part played by green plants is one which has always been filled by them from the earliest appearance of life upon the earth. It must, however, be noticed that the presence of chlorophyll in the organism depends upon a specialization of some only of its constituent cells, and of part only of the protoplasmic contents of those cells. The inference, which appears to be justified by general biologi cal principles, is that such a specialization is not a thing of primary origin, but has been gradually attained. We are thus, therefore, led to the supposition that the very earliest plants probably belonging to the same stock as the very oldest animals were destitute of chlorophyll, and were nourished, as Fungi are now, by the imbibition of sub stances fitted for their nutrition, but which, in the condi tions that accompanied the first appearance of life upon the earth s surface, were produced independently of any organisms. The development of chlorophyll would, there fore, on this view, have to be regarded as a later acquirement.
in mind in order to clearly apprehend the relation to one another of the different phases of nutrition which the vegetable kingdom includes. The plants, for example, which ​we collectively terra Fuwji may, and probably do, include descendants of the original stock which existed before plants possessed chlorophyll at all. No doubt also Fungi com prise plants which are destitute of the chlorophyll possessed by their near allies, in consequence of the degeneration due to a parasitic mode of life. Amongst Flowering Plants we cannot doubt that this has been the case with Cuscuta, Orobanche, Lathrcea, and many others. But besides plants which are actually parasitic, there are other degraded allies of green plants, which are content to work up again the imperfectly broken down products of decay. Such plants are termed Saprophytes; many examples of them exist amongst the Orchidacece, such as Neottia, Epipogium, and Corallorrhiza. They live upon the products of the decom position of vegetable matter, and have more or less com pletely lost the characteristic green tint of chlorophyll, which would be useless to them if they possessed it. But perhaps the most curious case of the occasional disposition of even green plants to seize upon nutritive matter in an available state of chemical aggregation, is that which is met with in the numerous examples now known of insectivorous
In these latter cases we certainly find morphological adaptation of considerable complexity for purposes of nutrition. But in the vegetable kingdom generally this is certainly the exception rather than the rnle. In the animal kingdom it is very different. Amongst plants, however, adaptations of structure which have reference to reproduction assume far greater importance, and these have to a large extent to be relied upon for taxonomic purposes. Even in the highest plants the physiological division of labour is very small compared with the extent to which it exists amongst animals. From plants of the simplest structure up to the most complicated, the plan of nutrition retains the same broad features. There are few physiolo gical facts of real importance to be observed iu the highest terms of the series which may not be equally well studied in the lower.
Amongst such of the lower plants as are aquatic in their mode of life the protoplasm of individual cells is often broken up into fragments, very minute in size, which are set free in the surrounding fluid, and being furnished with cilia or motile filamentous prolongations of their protoplasm, rapidly disperse themselves over a consider able area. Such locomotive organisms are usually called zoospores. After a time each is invested with a cell-wall composed of cellulose; this differs entirely in composi tion from protoplasm, especially in containing no nitrogen. The production of the cell-wall is not therefore to be regarded as a modification of any part of the proto plasm, but as a segregation of particles of cellulose which were intermixed with it ; such a segregation goes on repeatedly, wherever life exists in plant tissues. Starch, which is identical in ultimate composition with cellulose, we know to be fabricated from inorganic materials in the chlorophyll-granules (which are specialized portions of protoplasm) under the influence of light. Cellulose is derived from the starch so manufactured, and is dispersed in a state probably of molecular subdivision throughout the protoplasm.
The cellulose wall is not apparently essential to the con ception of a vegetable cell, but it is, perhaps, not going too far to say that its existence has conditioned almost all the histological and morphological peculiarities of plant con struction. The cell, as already pointed out, although bounded with what is relatively a tough and even rigid cell-wall, is by no means debarred from further nutrition and growth. If destitute of chlorophyll, it may take in nutrient matter, which only requires some moderate elabor ation to suit it for incorporation with the protoplasm. If, on the other hand, chlorophyll be present, it will do a good deal of preliminary work in preparing the substances which then, as before, the protoplasm will further appro priate and work upon. In either case the protoplasm of the cell will grow, and as the processes which have been described are generally accompanied by the imbibition of fluid, the cell-wall is subjected in consequence to a con siderable tension. The cell-wall, under these circumstances, grows also, and the experiments of Traube seem to show that, given the conditions under which it is known to take place, this growth is almost entirely a physical process. Carried beyond a certain point, tension must result in rapture ; but just short of this there appears to be a limit at which the intercalation of new molecules of cellulose is permitted, and so the surface of the cell-wall is enlarged. In (Edo- goniv.m there is a peculiar arrangement in which fracture actually does take place repeatedly. A circular cleft is formed, which is repaired within by the apposition of an annular splice.
To the growth of a cell so conceived there would seem to be no limit, and in the Siphophycece, of which Vaucheria is a well-known type, there is apparently none. The vege tative portion of these organisms, however complicated, is always formed by the extension of a single cell ; the pro toplasm is continuous throughout every part, and except when zoospores are formed is never segmented.
This, however, is a rare arrangement. Generally speak ing, there comes a time when the protoplasm, by a phase of contractility, divides itself into two masses, and between these a partition of cellulose is formed in the same way as the coat of the naked zoospores already alluded to. Each cell so formed possesses all the capacity for nutri tion and growth which the whole possessed. It divides therefore in its turn, and in this way we get the first in dication of an aggregate of cells. In the lower plants the cell is complete in itself ; in the higher, its independence is more or less merged in that of the others with which it is associated.
This aggregation seems to begin in a purely mechanical way. In Pleurococcus, for example, cell division repeated a few times may produce aggregation of, at any rate, four cells. If it were not that the adhesion of these cells seems afterwards to fail, there would be no reason why the mere process of cell division should not produce larger aggregates. But the cell-wall common to two adjacent cells splits through its middle lamina, and the two neighbouring cells part company. In Hydrodictyon we have a remarkable example of the formation of an aggregate synthetically, owing to the action of some cause which is quite imperfectly under stood, but which is probably purely physical. An enor mous number of zoospores are formed from the contents of a parent cell, and these, after tumultuously moving within its cavity, come to rest, and at the same time arrange themselves in the well-known net-like fashion which is characteristic of the full-grown plant. The mechanical persistence of aggregates of cells formed by normal cell division is obviously the step which led to the evolution of such organisms as Volvox and Ulva, since these are merely aggregates of simple types, such as Ghlamydococcus and Pleurococcus.
cells would be little impaired. In a Spirogyra or Oscilla- toria, for example, the number of cells present in a fila ment is probably a matter which does not affect the cells themselves individually, and which conversely they have no power of influencing. The constituent cells might go on dividing, and so form filaments of unlimited length, but which occasionally would be liable to be broken up by arbitrary accident. In Cladopkora. however, the cells of a filament cease after a time to divide transversely, and ​branch by the production of lateral swellings, which are elongated and then divided off by cellulose partitions. It seems reasonable to suppose that branching in this case simply means that one part of the cell- wall is weaker, less fitted to withstand internal tension, than the rest. If the cell- wall were uniform it would stretch uniformly, and the want of uniformity in the cell-wall may probably be traced to its being part of an aggregate. The string of cells is moored by one end, and any motion of the water will cause it to assume an oblique position. It is then unequally influenced by gravity and by light, and these two forces are known to be competent to modify growth in an unsymmetrical manner under such circumstances. Reasoning of this kind will at any rate suggest the way in which the specialization of individual cells would be a result of their aggregation. With this specialization they would lose an increasing amount of their capacity for in dependent existence. Specialization of function will, of course, be reflected in corresponding morphological modifi cations, and thus a mechanical aggregate of independent cell elements gradually passes into a physiological aggregate
One obvious penalty of specialization is the loss of the power of reproduction by individual cells when detached. Amongst plants, however, even the highest types preserve some measure of it. It is sufficient to adduce the common horticultural method of propagating Begonias from fragments of the leaves. Such a mode of reproduction in one of the highest plants is essentially the same as that which exists in Pleurococcus, and although reproduction from fragments of structures so specialized as a leaf is comparatively rare, there are in almost all plants provisions for agamogenesis, which depend upon the retention by fragments of the organism of this capacity for independent existence. And it may even happen that the perpetuation of the race for consider able periods may come to depend upon such a property. One of the most striking instances of this is the distri bution over the British Isles of the American water-weed (Anacharis), of which, nevertheless, only the female plant exists in this country. If we had the male, we should have a good instance (and others might be given) of gamo- genesis and agamogenesis proceeding side by side in in dividuals of the same generation, although probably not without the two processes reacting upon one another. Throughout the vegetable kingdom, however, we find gamo- genesis and agamogenesis occurring in separate generations, which are often extremely different. The two modes of reproduction are then subject to a cyclical arrangement, and the comparison of the forms under which this alter nation of generations occurs in different groups throws a good deal of light upon their taxonomic relations. It will be convenient to use the word "Sporophore " for the agamo- genetic generation, in which special cells (spores) are de tached from the parent to serve as a means of propagation ; while for the gamogenetic generation, in which conjuga tion takes place, or in which special cells (oospheres) are fertilized by antherozoids, and become oospores, "Oopkore" may be employed.
Our present knowledge has rather added to than dimi nished the difficulty of devising even a plausible phylogenetic classification of the vegetable kingdom. Greater success has, however, been attained in establishing the primary and larger secondary groups, so as to allow us to feel some confidence that they are really natural assemblages. The relation of these groups to one another is a problem, the more than approximate solution of which will probably have to be some time postponed. Many of them appear to represent the later developments of simpler types or aggre gates than anything that these groups at present contain. The Equisetacece, for example, are extremely ancient, yet we know of no form, living or extinct, which enables U3 to trace the connection of their very remarkable organiza tion with that of other groups. In the animal kingdom the "recapitulation theory" steps in, and obtains from the study of the development of the organism that kind of information which is wanting amongst plants as to the simpler ancestral forms of the different vegetable types. This information, however, is drawn in the animal king dom to a large extent from the phenomena presented by the differentiation of the sustentative organs of the embryo. For reasons already pointed out, there is nothing analogous to this amongst plants, where the rapid extension of sur face is usually the primary object to be attained. The earliest stages in the development of any plant are subject to conditions so simple that there is little room for special ization, and the economy of nutrition has probably gene rally led to the suppression of recapitulative structural details.
On the other hand, if we derive little help from re capitulation in studying the process of development in plants, from the unicellular stage of spore or oospore, it proves extremely suggestive, if we take in the whole cycle comprised between two processes of gamogenesis, and com pare the relations to one another of the gamogenetic and agamogenetic generations, or using the nomenclature intro duced above, of the Oophore and Sporophore.
Thallophyta.
Endlicher, in 183G, divided the vegetable kingdom into Tliallophyta and Cormophyta; and these divisions still hold good, though it is by no means easy to frame characters which will strictly limit them. With the exception of the absence in the one, and presence in the other, of the " opposition of stem and root," none of the distinctions which Endlicher pointed out are available now. Through out the greater part of the Thallophyta anything like a distinct segmentation of an axis furnished with lateral appendages is altogether wanting. As thus limited by the absence of a clear differentiation of root, stem, and lateral appendages, this sub-kingdom comprises an assemblage of plants, which were divided by Bishop Agardh, in 1821, into the three well-known groups of Algce, Fungi, and Lichens. And to these, for reasons which will be presently pointed out, must be added the hitherto problematical group of Characece.
It has long been seen that, with respect to the three former groups, it was impossible to assign morphological characters which would separate them strictly one from the other. Accordingly, Berkeley and Lindley were compelled to fall back on distinctions of a physiological kind. Algce were defined to be generally aquatic in their mode of life; Fungi and Lichens, on the other hand, were aerial, but the former drew their nutriment from the " substratum," while the latter obtained it from the air.
There are several grounds on which this arrangement appears to need reconsideration.
Contrasting, in the first place, Algoe and Fungi, we now know that the plants belonging to the former group in variably contain chlorophyll, while those belonging to the latter are equally devoid of it. In their morphological aspects, however, the two groups present a remarkable parallelism.
chlorophyll, and the difference in the mode of life which . results, would have greater weight for classificatory pur poses, were we not familiar with instances in other parts of the vegetable kingdom in which it proves to have no value at all. Amongst flowering plants we are acquainted with many cases where plants closely allied in structure to ​others in which chlorophyll is normally developed, are entirely destitute of it. These considerations have re cently led Cohn and Sachs to treat Fungi and Algce as an assemblage of organisms the classification of which is to be attempted on purely morphological grounds. But the assemblage of plants formed by Algce and Fungi which thus appear to require classification anew, has been further increased by the addition of two other groups, both re garded quite recently as entirely distinct. In 1868 Schwen- dener proposed his now well-known theory as to the true nature of Lichens ; and although his views have been vigorously attacked, chiefly by writers who seemed to feel that they had a vested interest in their autonomy, the weight of testimony, in the case of those who have examined the matter in a wider spirit, has been to strongly confirm Schwendener s hypothesis. Lichens must now be regarded as composite structures, partly consisting of an alga, partly of a fungus. Quite lately Sachs has pointed out (in the 4th edition of his Lehrbuch der Botanik) that Characece may be compared with the structure and mode
Sachs has proposed a classification of the Thallophyta, which appears to be the best that our present knowledge admits of. He divides them into four classes Protophyta, Zygosporece, Oosporece, and Carposporece.
1. The Protophyta include the simplest plants, and those in which at present gamogenesis is not known to occur. In the Cyanophyccoe the protoplasm of the cells is destitute of a nucleus, and, besides containing chlorophyll, is tinged with a peculiar bluish colouring matter, known by the name of phycocyan. In Palmellacece this peculiar pigment is absent. Eugleneoe is a group of well-known but little understood organisms, which must also be placed here provisionally. To these must be added Schizomycetes (Bacteria), which are the agents of putrefactive changes in nitrogenous organic matters, and Saccharomyces (yeast), which bring about the phenomena known as " fermentation." The Schizomycetes appear to bo allied in some respects to Chroococcacece and Oscillatoriaceos amongst the Cyanophycecz. The true posi tion of Saccharomyces must for the present be held as pro blematical ; we are still without evidence to conclusively decide in favour either of its autonomy or of its being a peculiar condition of a member of some group of Fungi of more complex development.
2. The Zygosporece are an assemblage of organisms, none of the members of which attain any high degree of morphological complexity, and in all the subordinate divisions of which the simplest form of gamogenesis, known as conjugation, has been observed to take place. Conjugation only differs from the normal process of fer tilization in the two protoplasmic bodies which take part in it being precisely similar in bulk and form. Till Thwaites pointed out the contrary in 1848, it was not supposed to D3 entitled to recognition as a sexual process. But fer tilization, as ordinarily understood, only differs in the two conjugating bodies being unlike that is, in their having undergone differentiation into antherozoid and Oosphere, the male and female bodies respectively.
The Zygosporece may be divided, perhaps artificially, into two groups, according as the conjugating cells are motile or non-motile.
In many of the simpler green Algce it has long been known that two kinds of zoospores, differing in size, are produced. To these the terms macrozoospore and micro- zoospore may be applied. The function of the macrozoo- spores is purely agamogenetic. The microzoospores, on the other hand, meet in pairs, and fuse into a single pro toplasmic body, which Areschoug has termed a zygozoo- spore.
The process was first observed by Pringsheim in Pan- dorina. It has also been observed in Chlamydomonas. On these grounds Sachs has placed the whole group of the Volvocinacece amongst the Zygosporece. This has, however, been reasonably objected to by Cohn, inasmuch as in Eu- dorina as described by Carter, and in Volvox as observed by himself, there is a true process of fertilization by means of antherozoids, and not a simple conjugation. It may be allowed, therefore, provisionally to break up the Volvo cinacece, or rather to restrict the name to the two last-named genera, which may then be removed to the next class, using that of Pandorinece for Yolvocinaceous forms, which only exhibit conjugation. Hydrodictyece should probably be placed here, and also Confervacece and Ulvacece in which the conjugation of zoospores has been observed by Ares choug.
Sachs has proposed to associate with the Zygosporece the Myxomycetes, in which the formation of the plasmodium is a kind of complex conjugation. Closely allied in some respects to the Myxomycetes are Chytridinece and, more doubtfully, Protomyces.
In the second division, in which the conjugating cells are non-motile, must be placed all the more familiar instances of the process the Desmidiece, the Diatomacece, the Zygne- macece, and the Zygomycetes as a fungoid type.
3. The Oosporeæ must include the Volvocinacece, in the limited sense already explained, and also Sph&roplea a form divorced on much the same grounds from Confervacece. Fucus affords the best known type of reproduction belong ing to this group. The antherozoids are ciliate bodies dis charged from antheridia ; the oospheres are naked proto plasmic masses originally contained in oogonia. The antherozoids gather round them in such numbers as to impart to them a movement of rotation. They eventually completely blend with the oosphere, which becomes sur rounded with a coat, and sinks to the bottom of the fluid in which it has hitherto been suspended. It is now an oospore, and speedily undergoes a process of cell division, which gives rise to an individual of the new generation. Near the Fucacece a place must probably be assigned to the Phceosporece. The remaining groups which fall into Oosporece are Cceloblastce and (Edogcmiece. Cctloblastce have their protoplasm unsegmented throughout the vegeta tive portions of the organism. They include forms which are partly algoid, such as Vaucheria and the Siphophycece generally, partly fungoid, as Peronosporece and Saprolegniece. In all other respects, except the presence or absence of chlorophyll, they closely agree with one another, and the consideration of this fact has led, perhaps more than any other fact, to the breaking down of the barrier between Algse and Fungi.
4. The Carposporeæ agree with the Oosporece in so far that the two sexual organs contribute in very different proportions to the formation of the sexual product. While the male only stimulates its development, the female supplies the material for the whole subsequent growth.
more cells. The male organ varies very considerably in the different subordinate groups. Fertilization may, as in the Oosporece, be effected by antherozoids (which may be actively motile or passively locomotive), or by a kind of conjugation, or even by a mere apposition of the male organ and subse quent diffusion of the fertilizing medium. The product of the act of fertilization is sometimes a single cell developing directly into a new individual (Chara). In other cases the fertilized female organ produces zoospores, and still more commonly a multicellular mass in which spores are finally developed. This involves an alternation of the generations of the type of that met with in the sporocarp of Muscinece. ​And we may have every grade of development, from the simplest case, in which the sporocarp appears as a mere appendage of the parent plant of inconsiderable dimensions, to the most extreme condition in the other direction, in which the sporocarp is capable of independent growth, _ and therefore represents a second generation which is entirely distinct (sporophore). The sporocarp also differs essen tially from the oospore in the fact that cells contribute to its formation which have not been directly influenced by fertilization, and that in consequence the part of the fruit which produces the spores is surrounded by what for want of a more convenient term we may call the pericarp, in which no spores are developed, and which serves as a mere protective investment, or is subsequently drawn upon for purposes of nutrition. In Phycomyces, which belongs to Zygosporece, it is noteworthy that there is a kind of anticipation of the development of a protective investment
The principal types of the Carposporece include all the more highly developed forms both of Algce and Fungi.
In Coleochcvtece the carpogonium terminates in an open canal, and fertilization is effected by motile antherozoids.
In Floridece, in the simplest type (JVemalion), the car pogonium resembles that of Coleochcetece, only the terminal tube (trichogyne) is closed and not open. Fertilization is effected by passively motile antherozoids, which adhere to the trichogyne. In Ceramiece the carpogonium, even before fertilization, consists of numerous cells, a lateral row of which the trichophore bears the trichogyne. Neither trichogyne nor trichophore, however, take any part in the de velopment of the sporocarp. The pericarp is produced by a process of budding from cells beneath the carpogonium.
In the genus Dudresnaya the process of fertilization be comes very complicated, and in fact involves a double pro cess, of which the first stage consists in the application of antherozoids to a trichogyne ; and the second in the de velopment from below the trichophore of a " conducting filament" which conveys the fertilizing influence to the terminal cells of a number of small branches, with which it successively conjugates; at each point of conjugation a sporocarp is developed. Amongst the Ceramiece there is something comparable to this double process ; the fer tilizing influence which is conveyed by antherozoids to the trichogyne must afterwards be communicated by a process of diffusion from the trichophore to the cell from which the spores are developed.
Characece are now to be regarded as a reduced type of Carposporece. The carpogonium is supported by two basal cells with oblique septa, which it seems quite reasonable to accept as the rudiments of a trichophore. There is, of course, no trichogyne, and fertilization is effected by motile antherozoids, and the pericarp is developed before instead of after this process. In Cham the main axis is clothed with a peculiar cortical tissue, which grows over it from the nodes, and_is clearly comparable with that which exists in the Ceramiece.
Fungi contribute to the Carposporece the Ascomycetes, JEcidiomycetes, and Basidiomycetes. In the first and last of these groups the existence of a sexual process preceding the development of the sporocarp is now known. Ascobolus has been studied by Janczewski, and the essential features of the sexual process are closely comparable to those of Ceramiece. The carpogonium consists of a row of cells ; the terminal one is fertilized by the ramified " pollinodium," and from the central cell of the carpogonium a number of filaments branch out which bear the asci, while the peri carp is formed by the branching of the mycelium below the carpogonium.
Lichenes, in so far as the hyphse and fructification are concerned, are essentially ascomycetous fungi. Their gonidia are referable to different groups of algse. The two sets of organisms live together in a kind of modified com- mensality. The algae are able to sustain their companion fungus without succumbing to the demands of its nutrition.
Cormophyta.
Alternation of generations exists amongst the Thallo phyta. But, as in GEdogonium, it assumes the form of an occasional occurrence of gamogeuesis after more or less prolonged periods of agamogenesis ; a series of sporophores is interrupted by the intercalation of an oophore. Amongst the Cormophyta, however, we have generally a regular alter nation of oophore and sporophore. Both may, however, propagate themselves by the detachment of more or less specialized gemmae. But apart from this, the agamogenetic production of spores and the gamogenetic production of oospores regularly follow one another.
Cohn, in a classification of Cryptogams published in 1872, established the groups Bryophyta and Pteridophyta, which, added to Phanerogamce, constitute the groups into which Cormophyta appear best susceptible of division.
1. Bryophyta.—In such a case as (Edogonium, sporophore and oophore each attain a tolerably equal degree of vegetative development. In others, however, this may be very much curtailed in one or the other generation. Amongst the Thallophyta the sporophore not uncommonly suffers this reduction. Thus in Mucor the zygospore only develops a short filament, which terminates in a spo rangium filled with spores. If we suppose the filament suppressed, the spores belonging to the second generation will . be produced directly from the oospore itself. Thus, amongst the Ccelollastcc, in Cystopus the oospore produces zoospores, and, according to Cienkowski, this is also the case with Volvox. In such cases the oospore itself is the sporophore. Amongst the Bryophyta there is an alternation of generations which is hardly less abridged. The oophore which in Hepaticce is often a thalloid body, while in mosses it is a leafy plant bears female organs (archegonia) and male organs (antheridid). The oospore, or fertilized central cell of the archegonium, gives rise to a complicated struc ture which produces the spores. The first division of the oospore appears to be inclined to the axis of the arche gonium. The sporophore is retained in a kind of attach ment to the oophore, and never attains to any vegetative development of its own. From the fact that the vege tative structure of some Hepaticoe is thalloid and leafless (Ancura), while in others (Marchantia) leaves are present in a very reduced condition, it may be inferred that the former have lost their leaves rather than that they repre sent an original leafless condition. The ancestral type of the Bryophyta was probably more like a moss than like any of the Hepaticce. And it is worth while considering what claims Chora may have to be regarded as a transition from the Thallophyta, of which it is an anomalous type in its possession of distinct lateral appendages to the axis. Cohn placed it amongst the Bryophyta; but its removal to the Thallophyta probably assigned it its true position. The investment of the carpogonium, however, cannot have more than an analogy with an archegonium. The fertilized carpogonium (sporocarp) might perhaps be regarded as a very reduced form of that of the Bryophyta, producing only a single spore. It is certainly an interesting and it may be a significant point, that the spore of Cliaracece produces in germinating a filamentous chain of cells, appa rently comparable to the protonetna of mosses, and from this the fully developed sexual plant is produced as a lateral bud.
the high vegetative development of the sporophore and ​gradually increasing suppression of that of the oophore. This never, however, as in the succeeding group, com
The whole direction of the attempts at classifying the Pteridophyta, which correspond to the so-called vascular Cryptogams, has been completely changed by Fankhauser s discovery of the long-sought reproduction of Lycopodium. It proves to be totally unlike that of Selaginella, which, from the close agreement that exists between the two types in every other respect, it might have been expected to closely resemble. On the contrary, in the formation of a monoeci ous subterranean prothallus, it may be compared to Ophio- glossece. Yet, as Sachs has pointed out, it is impossible to use this striking divergence for the purpose of breaking up a group of plants which in all respects is perfectly con gruous. The anomaly of forms so closely allied, exhibiting two modes of gamogenesis, has now, however, found a parallel amongst the Filicales, in which group RhizocarpeoB seem to find their proper place.
The Pteridophyta are divisible into three probably natural classes, which may be briefly defined by the following characters, in themselves, no doubt, artificial:—
i. Filicales.—Leaves highly developed and bearing numerous sporangia.
ii. Equisetaceæ.—Leaves rudimentary, reduced to sheaths on the barren stems, the fertile ones bearing 5-10 spor angia.
iii. Dichotomæ.—Leaves small, simple; sporangia soli tary.
The Filicales include, according to the arrangement pro posed by Sachs, three orders. In Stipulates the sporangia are not trichomes, but are either wholly endogenous pro ducts as in Ophiocflossu?n, or are formed from cellular pro tuberances, and the tissue beneath the epidermis takes part in the formation of spores (Marattiacece). In Filices pro per the sporangia are, as already mentioned, trichomes, and these are developed upon the normal fronds. In Rhizocarpea} the sporangia are borne by metamorphosed leaves, which are united into a capsular body in a way com parable to that in which carpels bearing ovules are united into a syncarpous ovary. The germination of the spores also presents important differences in the different orders amongst the Stipulates. In Ophioglossacece it gives rise to a tuberous monoecious oophore (prothallium), destitute of chlorophyll. In Osmunda the prothallium exhibits some tendency to become dioecious ; it resembles the normal typo of prothallium in ferns, but has a kind of mid-rib, and produces adventitious shoots from marginal cells. These shoots become detached, and constitute a mode of agarnogenesis. Such a structure cannot fail to recall the thalloid Hepaticce, which also possess a mode of agamo- genesis by means of gemmules (e.g., Marckantia). In Rhizocarpeae the dioecious condition of the oophore has been carried so far that it is indicated even in the spores themselves ; the microspores are male, the macrospores female. In Salvinia there is a rudimentary prothallium bearing the anthericlia ; and a last trace of such a structure appears to exist in Marsilea and Pilularia. The macro- spore also develops a prothallium, in which numerous archegonia are produced, one only of which is fertilized. The development of the prothallium is so far external to the macrospore that it effects its rupture at the apical papilla. In the Rhizocarpece the development of the oophore has been almost entirely compressed within the limits of the spore.
Equisetaceæ call for little remark as regards the oophore. It is dioecious, and irregularly branched, but in other respects there is a close agreement with the oophore of ferns.
The Dichotomæ present in the morphology of the oophore a parallel series with the Filicales. The Lycopodiacece develop from the single form of spores which they possess a monoecious subterranean prothallium like that of Ophio- glossece. The Ligulatce (comprising Selaginella and Isoetes) produce, like Rhizocarpecc, microspores and macrospores. In this group the suppression of the oophore is carried still further ; the microspores possess, according to Millardet, the merest rudiment of a male prothallium. The macrospores produce a female prothallium, which is an endogenous structure in even a higher degree than in the Rhizocarpeoe. The first divisions of the oospore have not as yet been sufficiently studied. But while in Ferns proper, Equise- tacece and Ophioglossacece, the first septum is more or less inclined to the axis of the archegonium, it is parallel with it in Rhizocarpece (a position into which a very oblique inclination would readily pass), while in Selaginella it is at right angles with it.
3. Phanerogamæ. In at any rate two of the three groups of the Pteridophyta we see that the progressive tendency of the oophore (prothallium) is to lose its inde pendence. In Opliioglossece, Filices, and Equisetacece, its growth, independent of the spore, often continues fora con siderable period. In Rhizocarpeoz and Lycopodiacece, where male and female spores are produced, the oophore remains attached to the spore, although, protruding from it. In Isoetes it fills the cavity of the macrospore as a mass of tissue, and the bounding wall is only ruptured to allow the access of the antherozoids to the archegonia. In the Phanerogamce even so small an assertion of independence as this is suppressed. The macrospore (embryo-sac) is never detached from the sporophore previous to fertiliza tion, and the oophore, which is moderately developed in the Gyinnospermw, but in the Angiospermce is reduced to the merest rudiment, always remains entirely enclosed within the macrospore.
In the Gymnospermæ the endosperm is the homologue of the prothallium (oophore). The so-called "corpuscula" are the archegonia, or rather the central cells of reduced arche gonia (secondary embryo-sacs of Henfrey). The neck of the archegonium is represented by the "rosette" first described by Hofmeister. Strasburger has pointed out that a small portion of the contents of the central cells is divided off at the upper end, and this is the rudiment of the canal cell which, running the length of the neck of the archegonium in the Eryopliyta and Pteridophyta, leaves a pervious track by its deliquescence for the access of the antherozoids to the central cell. Strasburger considers all the rest of the contents of the embryo-sac to be the equivalent of the oosphere. The result of fertilization is to cause a trans verse partition of the lower part of the oo^iphere, as in Selaginella. From the lower cell thus constituted, by suc cessive longitudinal and transverse partitions, the suspensors are developed, the ends of each of which bear an embryo. It is noteworthy that the development of a suspensor as an intermediate structure between the fertilized oosphere and the embryo is also met with in Selaginella.
the comparable structures in Angiospermce. Here the pro- thallium (oophore) has completely disappeared, unless we regard, with Sachs, the " antipodal cells " as a last rudi mentary trace; their appearance is, however, inconstant. Within the embryo-sac (macrospore) "embryo-vesicles" arc formed, usually two or three in number. One of these is the oosphere, and may be compared to the central cell of an evanescent archegonium, to which another of the " embryo- vesicles " performs the function of a canal cell; for while it is that which is usually in nearest proximity to the pollen tube, it only apparently transmits the fertilizing influence. In a few cases this dormant embryo-vesicle is replaced by a somewhat more highly developed arrangement the ​filamentary apparatus the homology of which, with a
The foregoing sketch indicates, in a very brief manner, the outlines of the classification of the vegetable kingdom, which has been constructed in the light of the most recent studies of the comparative structure of different types. It is sufficient to say, that a detailed consideration of the new relations in which many of the groups now stand to one another by no means diminishes, but strongly confirms, the soundness of the arrangement.
The following table gives the classification in a synoptic form:—
VEGETABILIA. SUB-KINGDOM A. THALLOPHYTA. With Chlorophyll. Without Chlorophyll (ALG.E.) * ( FUNGI.) Class 1. PROTOPHYTA. i. Cyanophycecc. Chroococcacere. Oscillatorieae, &c. Ji. PallmellacecE. ""? Euglenese.] i. Sckizomycclcs. ii. Saccharomyccs. Class 2. ZYGOSPORE.E. a. Conjugating cells motile. i. Pandorinece. i. Myxomycctcs. [? Hydrodictyese.] ii. Confervacecc. ii. Chytridinece. iii. Ulvacece. ft. Conjugating cells non- motile, iv. Conjugate. [? Protomyccs.] Desmidiese, &c. Class 3. OOSPORE..E. iii. Zygomycetes. Mucorini, &c. i. Volvocinece. ii. Sphceroplece. iii. Cwlollastcc . Siphophycese. iv. (Edogonieae. v. Fucaccce. [Phseosporese." i. CceloUastce. Ancylisteae. Saprolegniea. Fcronosporeae. Class 4. CAEPOSPOEE.E. W *h Chlorophyll. Without Chlorophyll (ALG.E.) (FUNGI.) i. Colecchcctcce. i. Ascomycetes. Lichenes, &c. ii. Ilorideas. ii. ^Ecidiomycctcs. Nemaliese. Ceramieoe, &c. iii. Characccc. iii. Basidiomycctcs. SUB-KINGDOM B. CORMOPHYTA. Series I. BEYOpnYTA. Class 1. Musci. Class 2. Hepaticse. Series II. PTERIDOPHYTA. Class 1. Filicales. i. Stipulate. Ophioglossese. Marattiaceae ft. Filiccs. iii. Rliizocarpcce. Class 2. Equisetaceas. Class 3. Dichotomy. i. Lycopodiacccc. Lycopodieoe. Psiloteae. Phylloglosseae. ii. Ligulatce. Selaginelleae. Isoetece. Series III. PHANEEOGAM-E. Class 1. Gymnosperma3. Class 2. Angiospermaj. i. Monocotyledoncs. ii. Dicotylcdoncs.
See A Text-Book of Botany, Morpliologica.l and Physiological, by Julius Sachs (3d edition), translated by A. "W. Bennett, assisted by W. T. Thiselton Dyer ; Lehrbuch der Botanik, von Dr Julius Sachs, vierte Auflage ; Quarterly Journal of Microscopical Science, new series, vols. i.-xv.
(w. t. t. d.)
* ↑ Proceedings of the Royal Society, No. 152, p. 290.
* ↑ It makes no difference if we adopt Sir W. Thomson s hypothesis, and snppose that the germs of living things have been transported to our globe from some other, seeing that there is as much reason for supposing that all stellar and planetary components of the universe are or have been gaseous, as that the earth has passed through this sfeige.
* ↑ See Sachs s Lehrbuch, 4th ed. pp. 240-243.