# Popular Science Monthly/Volume 8/April 1876/Modern Philosophical Biology II

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* MODERN PHILOSOPHICAL BIOLOGY.
By Dr. E. Cazelles.
TRANSLATED FROM THE FRENCH BY J. FITZGERALD, A.M.
II.
NOT all matter is capable of performing vital acts. Those substances alone possess this property which, owing to their peculiar composition, readily undergo molecular changes; that is to say, whose parts are grouped in very unstable equilibrium, and which are always ready to form other combinations. This state of instability is the result of complex combinations of six simple bodies, which at common temperatures have a very weak affinity for one another, but a strong affinity for elements outside of these combinations. Of the six, four, namely oxygen, hydrogen, carbon, and nitrogen, enter into these combinations in large proportions, while of the other two, namely sulphur and phosphorus, only a few atoms enter; and these latter elements are so readily oxidized that their presence increases the instability of the compound. Besides, the atoms of these simple bodies, though occurring in identical proportions, may be grouped according to different modes of aggregation (isomerism and polymerism), and the organic compounds which they make up stand midway between liquids and solids; their molecules are highly inconstant, whence result two well-known properties: the plasticity of organic matter, and its permeability to other substances. These properties are further causes of instability, inasmuch as they expose the organic substances to a number of disturbing influences. Thus, organic matter is not only subject to decomposition by light and heat, but also by the direct or indirect chemical action of bodies entering it, or acting on it from without. In such cases the effect of the disturbance is to cause the organic substance to pass from a state of relative instability to one of relative stability, or even to the state of compounds the most stable in the organic world.
At the same time that it undergoes the action of these external forces—and among external forces we include those developed in organized beings, but applied to other tissues than those producing them—at the same time that under the action of these external forces organic matter suffers decomposition, it becomes the scene of notable reactions. Even very inconsiderable changes in the external forces, which serve as its conditions, produce in it new molecular arrangements which offer a contrast, in their extent and importance, to the comparative insignificance of their cause. These new arrangements, being succeeded by more stable combinations, in turn bring about a disengagement of a great amount of force, in passing from ​a less stable to a more stable equilibrium. The atoms of the organic substance lose part of their latent motion, which is manifested externally under the form of heat, electricity, light, nervous force, or mechanical motion, according to circumstances. Be the cause which produces these changes necessary or not, they are, of necessity, accompanied by a disengagement of force; and we can affirm of any force whatever expended by an organ of a living being, that it is the equivalent of a force acting from without upon that being. This is a consequence of the law of the persistence of force, and it may be presented under two forms: First, in order that a certain amount of force may be expended by a living being, there must have taken place, within that being, a transformation, by decomposition, of a quantity of organic substance capable of holding that force in the latent stale; and, secondly, there can be no transformation of organic matter holding force in the latent state, without an expenditure of force which shall manifest itself in some shape externally.
In general terms, what we have to consider in living things is, first, a substance of special composition, and then expenditures of force by that substance; and this, too, is what we have in general terms to consider, in non-living things. The former are distinguished from the latter by the fact that the changes which constitute their history are heterogeneous; that they form many series which are simultaneous, correlative, held together by a tie of mutual dependence, the result being a high degree of complexity, a phenomenon belonging to one series having antecedents and consequents in other simultaneous series; and above all, that these changes form clearly-defined combinations. This ensemble of characteristics not only enables us to distinguish living from non-living things, but also to distinguish between living things themselves and to class them according to their degree of life. Thus a thing stands all the higher in the vital scale in proportion as, from the beginning to the end of its vital manifestations, it exhibits a larger number of successive and simultaneous changes, and as these changes are more heterogeneous and more closely linked together, and in more definite relations to one another. Between the lowest animals, rhizopods, planaria, etc., and the highest, the birds of prey, manmalia, carnivora, man, there is an enormous dissimilarity; still the definition applies to them all, and serves to define the difference which separates them, as also the difference of the numerous species lying between these extremes of the animal series.
Though this definition is a good one, inasmuch as it applies to all living things, and to them alone, nevertheless it is defective in that it omits the most distinctive peculiarity, namely, the element known as activity, in other words, those operations whereby living beings adapt themselves to their conditions of life. The definition should include the general relations of the living thing to its environment. The environment, too, has its successive and correlative changes ​which, though very diverse, present no definite combinations. Its composition, no doubt, is definite, and equally so its properties; but they are variable, and its variations alter the relations of the environment to the living being. To all changes of the environment there are corresponding changes in the living being, otherwise it would perish. These changes, which follow the laws of vital changes, inasmuch as they are in a definite combination, constitute the activity of the animal; the more numerous and frequent they are, the more active is the life and the higher the rank of the living being in the scale of life. The degree of correspondence between the living thing and its environment is also its degree of life, inasmuch as in effect it connotes an increase in the number and in the mutual dependence of the vital changes which constitute life. A perfect correspondence would imply a perfect life. If to all changes of the environment there were opposed, as a counterbalance, changes in the living thing, natural death would be no more, nor death by disease or by accident, all of which are signs of a lack of correspondence.
A definition of life which possesses these characters, and which expresses in a general formula the law of the changes of structure, and of the changes of function accompanying them; that is to say, which expresses the heterogeneity, the coördination, and the ever-increasing mutual dependence of these changes; and which at the same time expresses the ever-increasing correspondence which attaches them to the changes of the environment by an operation of equilibration—such a definition makes life to be an evolution, a succession of states of unstable equilibrium tending to perfect equilibrium; not ​only an evolution of the individual from the moment when it became more heterogeneous by the differentiation of parts and functions, but also an evolution of the ensemble of living beings, from the first appearance of life in its least differentiated form up to the highest degree of complexity in structure and function.
If life is an evolution, of what is it an evolution? If the question refers to an individual of any given species, the answer is easily given, for we can study the history of its life from the germinal cell to the period of its full development, and to the end of its life. But if the question refers to the ensemble of living Nature, only the middle portion of which is known to us, and the beginning of which we have no idea of save in imagination, then the reply must be only an hypothesis. We find groups differing from one another by their respective degrees of vital evolution, and we regard them as being, not as it were links of one chain, but rather the result of an evolution which has taken different directions owing to different circumstances. Hence we can admit only one starting-point, though the goals are many. The divergent lines which we find in the development of the forms of living things, in the history of life, warrant our supposing the starting-point to be one, and at this point the evolution hypothesis must place the formation of primordial organic matter, whose reactions with its environment present the first crude examples of vital function.
The hypothesis which accounts for the production of life by the spontaneous generation of a complete organism from simple protoplasm is irreconcilable with evolution; this would suppose something more than an evolution, in fact a beginning in the absolute sense, an enormous hiatus between the causes and their supposed effects. But on the theory of evolution we can conceive of another mode of formation. It is possible that even now, under existing cosmical conditions, organic matter is produced; but it is more probable that it was formed in an epoch when the cosmical forces now known to us, especially heat and light, had on earth a greater intensity than at present. The first types must have been more simple, less definite, less fixed in form and structure, than the lowest rhizopods of our day. Indeed, they must have been more nearly allied to protoplasm than even Haeckel's Protogenes; and, before evolution could derive from these types our present infusoria, ages and ages must have elapsed. Strictly speaking, we cannot call the first living thing an organism at all, in the true sense of that term; it is stretching the meaning of words to speak of types in connection with beings whose form must have been perfectly unstable, and whose organization had no structure.
Of this quasi-organism we have merely a symbolic conception, formed by combining two positive, empiric elements, viz., transformations of substances strictly evolutive, such as we see in the laboratory ​of the chemist, where organic matter goes through a series of gradual modifications by which it is adapted to new artificial conditions; and, on the other hand, facts observed in the lowest orders of animals by the biologist. We conceive that, in the primordial world, as now in the laboratory, higher types of organic substance were formed at the expense of lower types, and that gradually, after repeated reactions and under favorable conditions, they resulted in organizable protoplasm, a substance which is very susceptible of modification. Protein, as we know, may exist in upward of one thousand isomeric forms, and, by combination with itself and with other substances, it yields products still more complex, and in countless numbers. Hence we can easily conceive how, under the conditions of heat and light then existing on earth, and with the aqueous, mineral, and atmospheric environment of that epoch, protein may have undergone metamorphoses without end. Under conditions which we can conceive as possible, though we may not be able to define them exactly, products may have been evolved fitted to exhibit the rudimentary vital reactions. In this way we fill up the chasm which divides the positive chemical facts of the higher organic combinations from the biological phenomena of the lower forms of life.
But another hypothesis is still necessary. "When we come down to the substances out of which living bodies are formed, we find groups and sub-groups of manifold and divergent compounds, the units of which are large, heterogeneous, and unstable in a high degree. Why should we suppose that these combinations must stand still at the complex colloids which enter into the composition of organic matter? Is it not more probable that, in addition to these colloids, there are developed by a higher combination atoms still more heterogeneous and compounds still more numerous? If colloids are unstable, extremely modifiable by very slight incident forces, and incapable of assuming the equilibrated form of crystallization, then a fortiori these new organic atoms are unstable, very modifiable, and of many different species." They would surpass protein in instability and plasticity as much as protein surpasses organic matter. Furthermore, these atoms would possess one fundamental property, without which no explanation is possible in biology, viz., the property of arranging themselves in certain forms peculiar to the various groups to which they belong—a property but little understood, though its existence is unquestionable. We call it polarity, for want of a better term, to indicate the power of manifesting actions in a certain fixed direction. These atoms we denominate physiological units. They are developed in every living thing, differentiating themselves from one another in different organisms by the same causes which differentiate the organisms themselves, and in this way acquiring a diversity which corresponds to that of the creature they constitute by their aggregation. They follow, step by step, in their modifications the ​modifications of the aggregate to which they belong. They undergo the influence of the environment, though indirectly, through this aggregate. Their modifications are new directions, amplitudes of new vibrations, which place them in equilibrium with the forces which the ensemble of the aggregate, as modified by the environment, brings to bear upon them. These modifications endure as long as equilibrium endures, and are ever transmitted to the new units which spring from the former ones, until, on the equilibrium being disturbed, a new breaking-up of the existing relations necessitates others.
The hypothesis of physiological units is a necessity, not only in order to fill up the gap which separates the highest products of organic chemistry from those irreducible elements revealed by the microscope which we call morphological elements, but also in order to furnish a substratum for the positive property which serves to account for the great facts of biology, and to refer them, by formulæ expressed in terms of mechanics, to first principles.
Let us now consider the great facts of biology.
The growth of an organism is an operation essentially like the growth of a crystal. "Around a plant there exist certain elements that are like the elements which form its substance; and its increase of size is effected by continually integrating those surrounding like elements with itself. Nor does the animal fundamentally differ in this respect from the plant or the crystal. Its food is a portion of the environing matter that contains some compound atoms like some of the compound atoms constituting its tissues; and, either through simple inhibition or through digestion, the animal eventually integrates with itself units like those of which it is built up, and leaves behind unlike units."
Organic growth differs from inorganic in this, that it has limits. All conditions remaining the same (a proviso that must always be made in biology), and the quantity of integrated substance not varying, we find that, by the principle of the persistence of force, the growth of the living being must depend on the expenditure. The only portion of the integrated substance that can serve for growth is the unexpended residue, the excess of nutrition over expenditure—a quantity which is essentially variable, and which transfers its variations to the growth, limiting it and diminishing it more or less rapidly from the moment when the body of the living thing has attained its full development. Experience shows that the limit of growth is fixed for those organisms which have large expenditure, and that for those which have hardly any expenditure this limit gradually recedes; of this the crocodile is an instance. But there is another element which must be taken into account, namely, that the definitive volume of an organism, being the sum of its initial volume and of its successive increments, must depend upon the initial volume. The definitive ​volume depends also on the organization, which enables the living thing to assimilate substances in large quantity and to dispose of an amount of nutrition in excess of the expenditure, just as a large capital, while it gives the means of undertaking great enterprises, at the same time yields increased profits.
The integration by an organism of substances homologous with its own has for its effect a segregation which increases the difference between the organism and the environment, and at the same time makes this difference stable. While the organism is being integrated, at the expense of the environment, by deriving from it special materials, each organ is being integrated at the expense of the organism, from which it derives, as from an environment, its special materials. Like the organism, each organ diverges more and more, by a gradual segregation, from the organs around about it. The organic units which constitute it attract other units with the same polarity, diffused throughout the fluids. This is not always the case, and homologous units do not always exist ready made in the nutrient fluid. More generally the organic units find in the fluids only the elements necessary for the production of homologous units, and segregation is perfected by a phenomenon of the nature of a genesis. Still in this case, as in the preceding, the result is a more perfect differentiation of the parts of the organism, an increase of heterogeneity, and augmentation of the distinction between the different parts, and ultimately the formation of a structure and of an actual organism. This result is called development.
Expressed in general terms, development is the transition from a state of incoherent homogeneity to a state of coherent and definite heterogeneity; from a state wherein the parts are all alike, or rather, where there are no distinct parts, to a state wherein there are parts clearly defined, with fixed forms and attributes. The bud of a plant consists of a hemispherical or subconical projection which, at its apex especially, is made up of a transparent mass of cells not yet organized into tissues. This mass grows owing to the rapid multiplication of the cells, lengthens, sends forth other similar projections having a like homogeneous structure; from this come leaves. As the branch develops, the cells, which at first were identical, assume different characters, till at last they lose all resemblance to one another. The same thing takes place in man. His arm is at first simply a little budding prominence on one side of the embryo, consisting of simple cells without any signs of arrangement. Soon there appear vessels, and later the cartilaginous parts from which are produced the bones, the gelatin-like bands which afterward are transformed into muscles, etc. In the individual we see the first phase of existence characterized by a state of homogeneity wherein nothing is distinct, and we follow step by step the gradations of its transition to a greater complexity, and to states characterized by increasing distinction of parts, as their ​dissimilarity becomes greater. And what is true of the individual animal or plant is equally true of the whole organic world. Baer's law would lead us to suppose that the organic world has developed like the individual; that, starting from homogeneity, it has resulted in heterogeneity. In the early stages of their existence, all organisms are alike in most of their characters; somewhat later their structure resembles that found at the corresponding period in a smaller group; at each subsequent stage the organism acquires traits which distinguish the developing embryo from one after another of the groups which before it resembled; till finally the class of organisms which it resembles includes only the species to which the embryo belongs. Thus, in the process of differentiation, the embryo first acquires those characters which determine the sub-kingdom to which it belongs, then the class, then the genus, finally the species. In the series of organisms we should thus find a succession of states like those which constitute the history of the individual, with this difference, that in the individual we can make out the link which connects the primitive homogeneity with the final heterogeneity, while in the series of organisms all we can do is to connect, with a considerable degree of probability, the hypothetical starting-point with the positive goal.
Side by side with heterogeneity and distinction of parts in the structure, we have a correlative result of this same segregative operation, viz., differentiation, which tends to produce heterogeneity and distinction of functions. The expenditure of the force that is stored up in the shape of materials takes place through the parts of the organism, however little heterogeneous these may be supposed to be, and this force is in fact for the parts an incident force which, by the law of the multiplication of effects, must break up in the process of differentiation, when applied to heterogeneous parts. The functions are simply the variously-modified forms assumed by the forces disengaged by the organism as they traverse specialized parts; and, the more diversified the organs, the more diversified are the functions they manifest. Of these some may be denominated static, inasmuch as they serve only to withstand external forces by equilibrating them; such, for example, are the functions of the woody axis in plants and of the skeleton in the vertebrata; others may be called dynamic, as producing motion and giving it direction; such, for example, are the functions of the circulatory apparatus and its belongings in both kingdoms of the organic world, and of the muscular apparatus in animals.
Like structure, function obeys the law of evolution; it proceeds from the homogeneous, the undefined, the incoherent, to the heterogeneous, the definite, the coherent. Like structure, function proceeds from the simple to the composite, from the general to the special. An important corollary results from this law—one that settles the dispute which has so long divided physiologists upon the question as to which precedes the other, function or structure. If the starting-point ​be homogeneity, and if the transition from a structureless to a structural state is a phenomenon of vital action, then vital action precedes structure. Life is a system of internal actions adapted to equilibrate external actions; actions are the substance of life, its form comes from structure. Hence action must of necessity precede the fixation of the structure, which produces the adaptation and gives definite form to the function. From first to last, function is the determining cause of structure. But in justice to those who maintain the precedence of structure, it must be added that function, which, as we hold, is anterior to structure, nevertheless, regarded as an activity modified and different from what it was, assumes its differential, distinguishing characters only in proportion as the adaptation becomes perfect, and as equilibrium is established between that portion of internal reaction which it represents and the external action which it withstands.
At first there are only two functions, corresponding to the structural distinctions of endoderm and ectoderm, viz., the functions of accumulation and of expenditure of force. In proportion as each of the apparatus and each of the corresponding functions become differentiated and subdivided into specialized parts, a third function appears and takes root; at first this is a very simple affair, and it employs an ill-developed apparatus, but gradually it becomes more complex, and ultimately, in the higher animals, is divided into very definitely specialized parts. This is the circulatory apparatus, which performs those operations whereby materials containing latent force are distributed throughout the organism.
But differentiation is not the only change produced in the organism. The functions, as they multiply and are better defined, combine, become dependent on each other, are integrated. Labor is divided, as they say in political economy, but it is also centralized, and coordinated. Alongside of division of labor we have coöperation: an organ does not work for itself alone; it has a special function, but this function serves to facilitate, or even to render possible, the special function of some other organ.
As the formation of an organ depends on the function, so the growth of an organ depends on the growth of the function, and when once produced it is maintained only when the increase of function persists. And not only its growth, but also its development (including the differentiation of structure which accompanies it), depends on the development of the function, or, in other words, on the differentiation of the reactions of the organism to the forces of the environment.
We shall all the better understand the mechanism of the adaptation and of the modifications produced in one another by function and structure, if we consider what must of necessity occur when an augmentation of function in an organ answers to an augmentation of the demand for work made by the external conditions. In virtue of the law of universal rhythm, the result of excess of function is excess of ​wear, and consequent relative impotence of the organ. Thus excess of function in the organ A cannot go on forever unless the losses are constantly made good, the wear compensated, its power renovated; and this cannot be without an augmentation of function in one or more organs, B, C, D, etc, on the activity of which its own activity depends. The increase of function in these organs once established by a definite structure, the organ A not only can preserve its increase of structure and function, but it has now a firmer basis for growing still more, for producing another excess of function, and for going farther in the same direction than otherwise it could have gone. But adaptive modifications have a limit, and it is always near at hand, though it slowly retreats from generation to generation. This we learn from the mechanism of adaptation. As the function of an organ cannot be permanently increased save on condition that the functions of those organs on the action of which it depends have gained a permanent increment, and as they in turn are conditioned on a permanent increment in the functions of other organs, it is plain that there is needed nothing short of a reconstruction of the whole organism upon a plan which shall insure normal provision for the organ that is subject to an excess of function, and in which this excess of function shall be in fact a normal process. If equilibrium be disturbed at one point, it is reestablished only by propagating its own disturbance to all the internal equilibria; and, in order that it may itself endure, it must not be disturbed by a perturbation of reaction from within; the internal equilibria must be restored at the expense of the forces developed by the nutrition, and must be fixed by modifications of structure.
So long as this rearrangement of the internal equilibria remains unconsolidated by a reconstruction of the general structure, so long will the equilibrium produced by the adaptive modification, at the point affected by the initial disturbance, remain instable. And if, now, the disturbing conditions from without cease to exist, then the new structure, no longer sustained, so to speak, by an excess of temporary function, and receiving from the auxiliary organs, which are not yet adapted to this service, no permanent excess of function, can only furnish the same amount of action which it furnished originally. Little by little the imperfectly modified parts return to their original functions, and the whole scheme of adaptation comes to naught. Thus we see that, in virtue of the general laws set forth in the "First Principles," an adaptive change must quickly find a term beyond which it cannot progress save slowly—a fact which explains the apparent fixity of species, or the inconsiderableness of such deviations from a type as can occur during the periods over which our observations extend. It is plain that a modifying cause, the action of which persists only for a short time, can produce only a transient modification; that the complexity of the internal equilibria and their reciprocal dependence constitute the one great obstacle to the ​permanent change of structures and functions; that a disturbing influence, even though it were to extend to many generations, can only modify a race superficially; and, finally, that, the instant that this cause ceases to be, the race resumes, slowly but surely, its original characters.
In fact, the environment is ever changing, and in the enormous cycles of changes in the conditions surrounding organic life upon the earth the same conditions have never occurred a second time. Organisms must follow this movement of variation; they must be ever undergoing a process of adaptation, in order to be in equilibrium with the altered conditions around them. In this necessity for adaptation we recognize a consequence of our first principles. The state of homogeneity must give way to a state of heterogeneity: a species must be ever growing more and more varied in its forms; old species must be ever breaking up into new. If at one time a species consisted of individuals alike in all respects, the action of the various forces of the environment would soon put an end to this uniformity; at the same time, however, leaving tokens of relationship. But let us go further, and suppose the conditions to be still more profoundly altered, owing, for instance, to a climatic perturbation of the habitat, or to an emigration of the species into other habitats; in that case there will be different sets of conditions, and the groups of individuals will resemble one another, or be unlike, according to the likeness or unlikeness of the conditions. The connection between changes in the conditions, changes in function, and changes in structure, is a consequence of the persistence of force.
The law of heredity, which is antagonistic to the law of variation, may also be traced back to our first principles. This law represents the element of fixity in the domain of life. All the organisms of a given type are descended from organisms of the same type. If we consider heredity in a succession of organisms, it appears to be inexplicable. Many still deny the existence of heredity, and explain the resemblance of the child to its parentage by a special intervention of the creative power of Nature. But, if we compare the heredity of the individual with certain phenomena occurring in the individual, for example, the repair of tissues, the reproduction of worn-out or lost parts—a process which in some animals goes so far as to reproduce highly-complex organs or groups of organs (for instance, in lizards, the reproduction of feet and tail; the reconstruction of the fresh-water hydra; the restoration of the plant Begonia Phyllomaniaca from a fragment of its leaf)—we shall perceive that there exists a tendency to reproduce like products, and that the two orders of phenomena are related. We must suppose them both to be due to the tendency of the physiological units of an organism to arrange themselves in the form proper to that organism. But we need not recognize in this tendency any such mystic entity as an Archæus or a vital ​principle. Sound philosophy should discredit all such fanciful ideas. The tendency merely signifies that these polarities, being complexes of the physiological units, can find equilibrium only in the form of the adult organism to which they belong. To this equilibrium they tend, not only by an internal impulsion, but also under the combined action of external forces: the latter represent the force which arranges the units in a new order, and the former the direction in which this force is exerted. Now, the cells which go to reproduce an organism are in a state of unstable equilibrium and of minimum heterogeneity: but they are not indifferent substances; they are the vehicles of physiological units derived from the parents, and they follow only the tendency impressed upon them by their polarities. The same is to be said of the elements of the plasma from which a tissue or an organ is reproduced. Thus we see that the resemblance of an organism to the organisms from which it is sprung is the result of the tendencies proper to the physiological units which have come from the parents.
In the fecundated germ there are two groups of physiological units, presenting in their structures slight differences, so that by their fundamental resemblance they conspire to form an organism of the species to which the parents belong, and by their differences they give to this organism traits peculiar to each of the two parents. In this way, simultaneously with transmission of generic and specific characters, we have transmission of those which are peculiar to the individual. Further, we see that characters due to variations called accidental or spontaneous, because we are unable to assign their true cause, must also be transmitted as a tendency of the physiological units, provided this character has gained in the individual such a degree of stability as henceforth to find its place in that individual's state of equilibrium. The action of the surrounding conditions will determine whether the tendency of the physiological units is to be realized or frustrated. The tendency of the physiological units expresses an internal equilibrium, and hence heredity is a consequence of our first principles.
One character of living things is the faculty of reproducing themselves, i. e., of emitting parts of themselves which develop into perfect individuals. This property, in all respects analogous to that which reproduces tissues, differs from the latter only as regards the production of new individuals, or only parts of the same individual. There is an analogy between the operation of generation and that of repair, but there is also a difference. In repair the new products are aggregated around the same axis as the old, whereas in generation the new product soon becomes itself the axis around which the increments of nutrition group themselves. In reality, the contrasts are in excess of the analogies; generation is at bottom an operation of disintegration. This is very well seen in those low organisms which produce ​new generations by fission, and abdicate their individuality in favor of a greater or less number of new individualities. It is also to be seen in those organisms on whose surface a new organism is formed by the process of budding. Here the disintegration is perfect, or nearly so, but in the higher organisms the disintegration affects only an insignificant portion of the parent.
Why this special disintegration? Biology can give no answer, unless we suppose that the genesis of individuals belongs as a genus to a class of facts including all the phenomena of general disintegration which attend growth, and which mark the gradual decline of the organism. This supposition finds its warrant in the fact that, as a general rule, reproduction does not take place until growth and structural development approach their term, when the molecular forces of the physiological units find themselves in equilibrium with the forces of the organism as a whole, and with the forces from without. Disintegration would now set in, or, to speak more exactly, disintegration would now begin to show an excess over integration, for, ever since the earliest vital phenomenon, disintegration has constantly accompanied integration. Among the various modes in which the decline of the organism is gradually brought about, there is one which resembles all the others, inasmuch as it constitutes a loss to the individual, but which differs from them in that it gives rise to new organisms. In a large number of cases among individuals of the lower orders of organisms, units combined in a certain group, and carrying away with them, as we have seen, their own proper tendency to find the equilibrium of their forces in arrangements similar to those in which they were originally integrated, become detached, and form the centre of a new integration. But in a very large number of organisms, and in all higher organisms of both the organic kingdoms, reproduction takes place by the mixture of two products, the one germinal, the other spermatic, coming from slightly different physiological units. In virtue of a property found in the simplest organic elements, and still more markedly present in the complex organic elements of living things, the mixture of substances which differ little from one another gives rise to products that are less stable than their constituent elements. Accordingly, the result of this mixture, namely, the fecundated germ, is farther from the state of equilibrium than were the units emitted by each of the parents, in the shape of germinal and spermatic cells. The faint tendency which existed in each of these groups to produce evolutional phenomena is intensified with the instability of the mixture. From this we may infer, if not the impossibility, at least the difficulty of an agamic genesis, and the necessity of a genesis by concurrence of different sexes. This conclusion, derived from the law of equilibrium, which itself flows from the law of persistence of force, seems to be hardly in agreement with facts, since unquestionably there exists such a thing as agamic ​genesis. But agamo-genesis is not habitual in organisms of very simple structure, which exhibit the first steps in evolution, and in which the absence of highly-specialized tissues shows that integration still possesses its full intensity, and is far removed from equilibrium. Besides, those more complex organisms which exhibit the phenomenon of agamo-genesis, from time to time reproduce by way of gamo-genesis. After a series of agamic generations, the units of the organism will find themselves in an attitude approaching that of mutual equilibrium. The groups of units emitted as germs will no longer be able to assume arrangements which shall give them the form proper to their species, and agamo-genesis will be impossible, or very difficult. The series would come to an end did not sexual generation intervene periodically, restoring a state of instability, which gives back to the organism the power of evolution. Another conclusion, which at first sight appears to contradict the facts, is this, that an organism needs, in order to reproduction, the concurrence of another organism differing slightly from it. This is true of the higher organisms; but lower down in the animal scale, and in most phanerogamous plants, hermaphrodism is apparently the rule. But, not to speak of the fact that most frequently fecundation takes place in monœcious organisms by the intervention of another individual, so that such authors as Huxley and Darwin regard this intervention as the law of reproduction, the hypothesis which we maintain affords an explanation of hermaphrodism in those exceptional cases where it appears to exist beyond question. On the same principles which account for the variable results of the union of near kindred, we can understand how, in the case of hermaphrodites, there may exist simultaneously groups of physiological units coming from each parent, keeping their proper tendencies, which find only partial equilibrium, permitting one or other side to be in excess, and there undergoing the operation of segregation, which produces groups so differentiated that fruitful germs result from their mixture.
Considered in the light of this hypothesis, generation appears as a fact of disaggregation, occurring in an organism in process of equilibration: as a fact of disaggregation, which ever renews the evolution of the species, and which retards its equilibrium by multiplying the conditions under which the species may, under the influence of the incident forces of the environment, undergo a more perfect elaboration, the result of which shall be a better adaptation of the organism to its surroundings. Generation is in fact antagonistic to equilibrium, but this antagonism is only temporary, and causes the organic evolution to obey the law of universal rhythm.
[To be continued.]