# The present evolution of man, Reid 1896/Part 1.1

PART I
ORGANIC EVOLUTION
PHYSICAL EVOLUTION
CHAPTER I
"During the quarter of a century which has elapsed since Biology began to occupy itself again with general problems, at least one main fact has been made clear by the united labours of numerous men of science, viz. the fact that the theory of descent, the idea of development in the organic world, is the only conception as to the origin of the latter which is scientifically tenable. It is not only that, in the light of this theory, numerous facts receive for the first time a meaning and a significance; it is not that, under its influence, all the ascertained facts can be harmoniously grouped together; but in some departments it has already yielded the highest results which can be expected from any theory, it has rendered possible the prediction of facts, not indeed with the absolute certainty of calculation, but with a high degree of probability. It has been predicted that man, who in the adult state only possesses twelve pairs of ribs, would be found to have thirteen or fourteen in his embryonic state; it has been predicted that, in this early period in his existence, he would possess the insignificant remnant of a very small bone in his wrist, the so-called os centrale, which must have existed in the adult condition of his extremely remote ancestors. Both predictions have been fulfilled, just as the planet Neptune was discovered after its existence had been predicted from the disturbance induced in the orbit of Uranus.
"That existing species have not arisen independently, but have been derived from other and mostly extinct species, and that on the whole this development has taken place in the direction of greater complexity, may be maintained with the same degree of ​certainty as that with which astronomy asserts that the earth moves round the sun; for a conclusion may be arrived at as safely by other methods as by mathematical calculation.
"If I make this assertion so unhesitatingly I do not make it in the belief that I am bringing forward anything new, nor because I think that any opposition will be encountered, but simply because I wish to begin by pointing out the firm ground on which we stand, before considering the numerous problems which still remain unsolved.
"Such problems appear as soon as we pass from the facts of the case to their explanation; as soon as we pass from the statement, The organic world has arisen by development, to the question, But how has this been effected, by the action of what forces, by what means, and under what circumstances?
"In attempting to answer these questions we are very far from dealing with certainties; and opinions are still conflicting. But the answer lies in the domain of future investigation, that unknown country which we have to explore.
"It is true that this country is not entirely unknown, and if I am not mistaken, Charles Darwin, who in our time has been the first to revive the long dormant theory of descent, has already given a sketch, which may well serve as a basis for the complete map of the domain; although perhaps many details will be added, and many others taken away. In the principle of natural selection, Darwin has indicated the route by which we must enter this unknown land."—Weismann.
"The continuity of the germ plasm, like Darwin's selection, is a fact not a theory."—Haycraft.
"'Struggle for existence,' as perhaps it was in Mr. Darwin's world of advancing beasts and developing vegetables. But now the plan is so turned about by the arrival of man on the scene, and by his civilization, that you cannot watch even Darwin and Huxley themselves without seeing that the struggle that they and other good men wage is no struggle for existence but a struggle against mere existence. The struggle for existence is brutal life; a struggle to do something more than exist is human life—the mission of the human soul. What is the use of alcohol in such a struggle? The question is a wide one. It might lead us to inquire what that is which men want to obtain beyond mere existence. Watching some eminent teachers, you might suppose it to be a very detailed knowledge of the common frog. ​But men are human because they look upwards and to the future, not downwards to the past. And Darwin and Huxley, and even Haeckel, will in time learn that over-scrutinizing insufficient evidence does not make it more complete."—Moxon.
Above I give three extracts—one from the writings of a very eminent naturalist, the second from those of an eminent physiologist, and the third from those of a physician, also eminent. They disclose a difference of opinion which is remarkable. Professor Weismann scarcely deigns to discuss the theory of evolution, since, in his opinion, it must be considered as proved with as much certainty as is the fact that the world moves round the sun, or as if it had been demonstrated mathematically; and he adds, that we have now only to discuss the details, merely to fill in the minutiæ of the map, the outlines of which Darwin has already sketched. Dr. Haycraft also thinks that the theory of evolution has passed from the category of mere hypothesis into the category of that which must be accepted as proven fact. But Dr. Moxon appears to think that, if a struggle for existence leading to evolution ever did occur, it has now ceased so far at least as man is concerned, and the struggle has become one against mere existence, whatever that may mean, and he adds—"Darwin, Huxley, and even Haeckel will in time learn that over-scrutinizing insufficient evidence does not make it more complete."
His attitude is very characteristic of that of the vast majority of the general public, and characteristic also of that of the majority of medical men, who, while observing the effects of disease on man the individual, have signally failed to observe its effects on man the species. Had Dr. Moxon, when he wrote his essay on alcoholism, which in its effects is a disease, occupied a wider outlook, had he considered the species as well ​as the individual, he might perhaps have been led to conclusions which are as true as they are surprising.
Taking into account this attitude of the general public, and in particular that of the mass of medical men, to whom, if to any, this work should prove of interest, and considering also that a wide audience must be appealed to if it is to have that amount of practical usefulness which the author hopes for it, it seems needful, before proceeding with the main body of the work, to set forth as briefly and clearly as possible certain biological data on which the argument is founded; especially as, so far as the author is aware, they have never yet been explained in sufficiently simple terms to be comprehensible by the general reader; and more especially since, in his conception of the process of evolution, the author differs somewhat from accepted views, or rather since in his opinion acknowledged authorities have not recognized or have not laid sufficient stress on certain processes of evolution which appear to him of the greatest importance. This book is therefore divisible into two parts: in the first the problem of evolution in general is very briefly dealt with, but an attempt is made to penetrate somewhat deeper in certain directions than has hitherto been done; in the second part, the conclusions arrived at in the first are applied to the problem of man's present evolution, and an endeavour is made to show that this evolution is proceeding in a direction hitherto altogether unsuspected.
To many, and, surprising as it may seem, even to some medical men, in spite of what ought to be a scientific training, the theory of evolution means nothing more than the theory of the descent of man from the monkey. In reality it means much more; it teaches that plant and animal types have not persisted unchanged from the time life first became possible on the ​cooling surface of the globe, but that all types known to us have arisen by a process of gradual evolution from pre-existing types, and that this process of evolution has generally been in an upward direction from the low to the high, from the small to the great, from the simple to the complex. Pushed to its logical conclusion the theory teaches yet more; it teaches, as the only hypothesis scientifically tenable, that life originally arose by a process of evolution from that which was non-living; that under conditions of which we are ignorant, of moisture, or of heat, light, electricity, or other of the protean forms of energy, non-living chemical compounds did in the beginning of life overpass the border space which divides the non-living from the living, and become living beings. The lowest living beings, or rather those highest chemical compounds which first display signs, however slight, of what is called life, if any such now exist on earth, probably exist in masses so minute and so little differentiated from mere chemical compounds, as to be beyond all devisable means of observation. Regarding them therefore there is no evidence to offer, but as regards the evolution of higher life from lower life, the evidence is so vast and so decisive that it is impossible to avoid coming to the conclusion that Darwin, Huxley, and even Haeckel did not over-scrutinize insufficient evidence, but that Dr. Moxon under-scrutinized overwhelming evidence, that, in fact, he adopted the device familiar to theologians and dear to them, of abusing evidence instead of examining it.
But quite apart from all evidence that evolution has occurred, the conditions of life are such that we may ​infer from them with mathematical certainty that it must have occurred. It is a matter of common knowledge and experience (1) that in both the animal and vegetable kingdoms every individual differs somewhat from every other individual, but that (2) while the offspring is never an exact copy of the parent, there is yet a tendency for the peculiarities of the parent to reappear in the offspring in a greater or lesser degree. Whence it is clear, if some individuals of a species possess a peculiarity, such as a superior keenness of sight, or of hearing, or of scent, or a power of muscle, or a capacity of resisting heat, or cold, or hunger, or thirst, or disease, &c., in a greater degree than that possessed by the other members of the species, under conditions which render this peculiarity of importance in the struggle for existence, that the individuals which possess the peculiarity in the greater degree will be at an advantage, and will, on the whole, survive in greater numbers, and, consequently, other things equal, have a more numerous offspring than those which possess the peculiarity in the lesser degree. And further, since the offspring, while inheriting their parents' peculiarities, tend, to vary somewhat from them, there will, in the next generation, probably be some who have the peculiarity in a greater degree than their parents, and others who have it in a lesser degree, when, if the conditions remain the same, there will be such a new survival of the fit, and such a fresh elimination of the unfit, as will leave the survivors in the third generation with the peculiarity developed, in however slight a degree, more than it was developed in their grandparents; and it follows that this process, repeating itself through innumerable generations, and during long epochs of time, will at length develop the peculiarity to a point beyond which any further increase is no longer useful, or to a point beyond which it is so little ​useful, that those individuals who possess the increase are no longer at such an advantage in the struggle for existence as to survive to an appreciable extent beyond those who have it not. Therefore since individuals vary each one from all others, since the offspring, while varying somewhat from their parents, tend to transmit the peculiarities of their parents, and since individuals that vary favourably tend of course to survive and have offspring, whereas individuals that vary unfavourably tend to be eliminated and have no offspring, it is deductively so certain that whenever there is a struggle for existence,—and throughout nature there is always such a struggle,—the survival of the fittest must lead to evolution, that it is scarcely necessary to appeal to facts for inductive confirmation. If, however, we do seek such confirmation, we find it in three great bodies of facts collected mainly in different fields of research, each of which is separately decisive, and which collectively furnish confirmation so absolute that, practically speaking, no student of biological science now believes otherwise than that the whole organic world arose by a process of evolution.
The first great mass of evidence is furnished by the science of Comparative Anatomy, especially by that branch of it which deals with Comparative Embryology. In the interests of the general reader, we may with advantage defer the consideration of it.
The second great mass of evidence is furnished by the science of Paleontology, which teaches, on the evidence of fossil remains, that the earth has not always been inhabited by the same forms of life, but that during the whole vast period which intervened between the deposit of the earliest fossiliferous rocks and the present age, there occurred a constant but gradual change of form, as a result of which type shaded into type, generally in an upward direction, and which can ​have been due to evolution only, or, as the single other alternative, to an immense series of special creations, made not only when life began, but also during its entire continuance, in exact imitation of a process of evolution—a preposterous theory, for which there is no more warrant than for the hypothesis that our various breeds of dogs have not descended from a common ancestry, but were separately created.
The third great mass of evidence is that furnished by experiment. It is therefore the most conclusive of all, since any theory which can be proved experimentally may be said to be proved decisively. For instance, if we have a theory that the boiling-point of water varies with the atmospheric pressure, and find on experiment that it does so vary, we may conclude that we have proved our theory beyond cavil. As a disputed point is involved, which it will be advantageous to discuss thus early, we will consider this part of the evidence more at length than we did the rest. The theory of evolution may be put to the test of experiment. We may take any species of plant and animal, and by copying the process of natural selection, by destroying those individuals of the species which we regard as unfit,—i.e. those which vary unfavourably as regards any peculiarity we wish to develop,—and by allowing only those individuals who vary favourably as regards that peculiarity to continue the race, we may develop the chosen peculiarity in the chosen species to almost any extent; the one condition being, that our operations shall extend over a sufficient number of generations. The efforts of man to improve his cultivated plants and domesticated animals may be described as a lengthened series of such experiments, conducted on a gigantic scale, with the result that he has been able to vary plant and animal structures almost at will, and so greatly that, in the absence of scientific and historic testimony, no one ​could guess that the peach and nectarine, for instance, were cultivated varieties of the wild almond, or that the various breeds of dogs have a common and, geologically speaking, very recent ancestry.
It may be objected however, and it often is objected, that natural selection is not the same thing as artificial selection; but the objection cannot be sustained, for in effect it is the same thing, artificial selection being merely natural selection with man as the governing agent. Man, for instance, has caused the evolution of speed in the greyhound and in the race-horse by breeding, on the whole, from the swiftest animals of each species; in like manner the carnivora have caused the evolution of speed in the hare and the antelope by permitting, other things equal, only the swiftest to continue the race. Artificially caused evolution differs from naturally caused evolution only in that it is usually more rapid; for man, in his endeavours to produce a wished-for evolution, fixes his attention on but a few traits, and breeds with the intention of developing those traits only. Thus in the case of the greyhound he has bred, generally speaking, with the intention of producing a very swift animal with sight keen enough to see the prey, and jaws and teeth adapted to seize and kill it. In the case of the racehorse he has bred with the intention of producing another swift animal, strong enough and tractable enough to carry him. But natural selection in the hare and the antelope has developed not only speed, but also hearing, watchfulness, and many other traits which are as essential.
Other things—i.e. other essential traits—equal, preeminence in one or more essential traits is favourable to survival. But because in a state of nature many traits are essential, evolution by natural selection must proceed along many lines, and, in consequence, be corre​spondingly slow. To take an example—a buck and a doe antelope, whose sight and hearing respectively are keener than the average, but whose other qualities are equal to the mean of the species, are at an advantage as regards the sense of sight and hearing respectively. If they mate, the offspring, while attaining to the specific mean in the other qualities, will in general surpass it as regards sight and hearing, but will fall below the male parent as regards the sight, and the female parent as regards the hearing; for the comparative dullness of the male's hearing will militate in the offspring against the keenness of the female's hearing, and vice versa as regards the sight. So also with the offspring of a buck pre-eminent for endurance and a doe pre-eminent for speed. Then, if the offspring of the two pairs mate, their offspring will surpass, but in a lessening degree, the specific mean in keenness of sight and hearing, in speed and in endurance, but will fall below each of the grandparents as regards the one quality in which that grandparent excelled, though they will surpass each of them as regards all the other qualities. Therefore if the essential qualities are numerous,—and they always are so in the case of wild animals,—the descendants of numerous ancestors, each one of whom was pre-eminent in one of these qualities, will tend more and more, generation after generation, to approach the general racial mean as regards all these qualities;—which appears to lead to the absurd conclusion, that in the end there will be no evolution at all. But in coming to that conclusion we shall have forgotten that the offspring never present an exact mixture of the qualities of the parents, but that spontaneous variations, caused we know not how, continually arise, in consequence of which one or more of the offspring of the keen-sighted father and the quick-hearing mother (for instance) may surpass both parents ​as regards both qualities; and though there is reason, as we shall presently see, to conclude that on the whole the offspring are usually inferior to the parents, nevertheless, since they are generally numerous, and the process of selection almost always severe, the survivors in each generation, as a rule, surpass their parents in the sum of their qualities; and as a result, there is usually evolution as regards the whole race even when the essential qualities are many. Moreover, we have assumed for the sake of simplicity that each ancestor was pre-eminent in one quality only, but of course, as constantly happens in nature, a single individual may approach pre-eminence in several or many qualities, or conversely, may fall below the specific mean in several or many qualities: in the latter case he tends of course to be eliminated, and to leave no offspring, and therefore to have no influence on the future of the race; but in the former he tends, even more than if he were eminent in one quality only, to survive and to leave offspring who, inheriting his characteristics, tend as a result to survive in greater numbers than the offspring of the less happily endowed, and therefore to render concurrent evolution in several directions more rapid.
Lastly, the statement that artificial selection differs from natural selection in that but a few qualities are developed by it, is true in a very limited sense only; for instance, by causing the evolution of the greyhound man has necessarily caused the evolution of a great number of subsidiary qualities, each subserved by a number, in some cases by an immense number, of finely co-ordinated structures. Therefore though speed alone has been aimed at, yet to obtain it evolution in many directions has been necessary (e.g. in "wind," "limb," circulatory and nervous systems, &c.). On this score then there is no essential difference between artificial ​and natural selection, and the evolution which results from the former therefore furnishes experimental (i.e. conclusive) proof of the evolution which results from the latter.
The reduction towards the specific mean which interbreeding tends to bring about in the qualities of individuals as exhibited in their descendants, has caused some biologists to insist that isolation is a necessary antecedent to evolution. But if we bear in mind the fact (to be more fully discussed in a future page) that evolution proceeds not on lines of traits, however favourable, which occur infrequently or abnormally, but on lines of traits common to all the individuals of the whole species, that is, in which every individual rises above or falls below the specific average; and bear in mind also, that generally those individuals who on the whole vary fortunately as regards these traits, survive and have offspring, whereas generally those who on the whole vary unfortunately are eliminated and have no offspring, we shall have no difficulty in understanding that evolution is quite possible in the absence of isolation. What is not possible in the absence of isolation is evolution on diverging lines. For instance, if the whole of a species of antelope inhabited an open plain under conditions common to all there might be evolution, but it would be in a direction common to the whole species. But if, owing to any circumstance, such as a deficiency of food supply, a portion of the species separated from the rest, and migrated permanently to forest land, then the two divisions of the species, being unable to interbreed, ​and differently conditioned as to the environment, would develop on divergent lines of evolution.
The conclusion, deductively arrived at, that the conditions under which life has existed and still exists, are such that evolution must have occurred and still occurs, is therefore decisively confirmed by the conclusion, inductively arrived at, that evolution certainly has occurred, and therefore, though in the subsequent pages of this work many proofs will incidentally be afforded of the actuality of Organic Evolution, in future it will be assumed that the truth of it is admitted, and we shall endeavour only to fill in the details of the map as to the fidelity of the outlines of which no well-informed man any longer entertains a doubt.
The upward march of life from the earliest beginnings may be compared to that of a horde of men, leaving their old habitations and entering new lands; travelling ever forwards, but ever sending out branch swarms that part from the parent horde, never to reunite with it, and ever leaving some of their members behind on the way, some of whom may journey backwards; such hordes as those which in ancient times came from the East, settled the countries they passed over, sent offshoots to the North and South, and rolled on the tide of conquest till they destroyed the old Roman Empire. The lowest, or in other words the least differentiated and specialized forms of life, may be compared to those members of the horde that stayed behind in the original habitat, the intermediate forms to those that halted and settled by the way, and the highest forms to those that journeyed till they reached the farthest limits of the wanderings. The comparison is made yet closer if we imagine, as generally true of life, that which is generally true of emigrant swarms of men, namely, that those that stayed in the original habitat, that those that halted, diverged, or ​retrogressed by the way, and that those that reached the farthest limits, did not remain altogether unchanged, but changed somewhat as ages passed and generations lived and died; those that remained in the original habitat having changed least, those that wandered farther having changed more, and those that wandered farthest having changed most, and this for the reason that though the environment nowhere remained absolutely unaltered, yet for those that travelled farthest it altered most; so those Aryans who dwell in or near the original habitat of the race have changed least, those who halted by the way have changed more, and those that journeyed farthest to the West, or diverged to the North or South, have changed most. So also in the march of life, the lowest organisms may be considered as having stayed behind at or near the original starting-point; the sponges as having diverged from the main road at an early period; the Cœlenterates, the Mollusks, the Annulosa to have halted by the way at a later stage; whereas the Vertebrates, with mammals at their head, and man at the head of the mammals, may be considered as having reached the farthest limits. But just as it becomes increasingly difficult the farther we penetrate into historical antiquity to trace the descent of the nations of any race of human beings,—the Teutonic for example,—not only for the reason that the peoples who have travelled farthest have changed so much, have undergone evolution, and because the peoples who halted by the way or stayed behind have changed also, but because some of the links of the chain are broken, because some of the nations which halted by the way are lost and cannot be recognized, or because they have perished utterly in the struggle for existence; so also the farther we penetrate into geological antiquity, the harder it becomes to trace the descent of species, the more numerous become the ​broken links, the wider the gaps; nevertheless, however numerous the broken links, however wide the gaps, just as it is in general possible to trace with more or less accuracy the descent of the nations, so it is in general possible to trace the descent of species.
CHAPTER II
It has already been pointed out, that the conditions of life are usually such as to cause the number of traits necessary for survival to be numerous, and that this again causes the process of evolution to be slow, though it is no bar to it as some biologists have supposed. But however slow the multiplicity of essential traits may render evolution, it is difficult at first sight to understand why evolution should be so extremely slow as it generally is. Every plant or animal pair usually gives origin to numerous offspring, sometimes the offspring are numbered by millions (e.g. the offspring of cod-fish); yet the number of individuals in each species does not increase as a rule, whence it is clear that nature, by destroying the majority of the offspring, exercises her power of selection with extreme stringency. Under the circumstances we might à priori expect evolution to be rapid. But what are the facts? Some types, such as the lamp-shells, have persisted almost unchanged during enormous epochs of time, and in almost all cases nature requires hundreds, nay thousands or tens of thousands of years to bring about comparatively trifling changes of structure. Why is this?
The answer is probably found if we consider certain groups of apparently unconnected facts. Children who do not especially resemble their parents often resemble their grandparents, or even more remote ancestors; at any rate they more often resemble an ancestor than ​they do any other given individual. For instance, if before a child is born we indicate any individual of our acquaintance, the chances are immensely greater that the child when grown will resemble a particular ancestor than that he will resemble the person indicated. That is to say, there is a greater tendency for the child to vary from its parents in the direction of its ancestry than in any other given direction. Now if the parent has varied from the grandparent in any other direction than towards the ancestry, this, the child's variation towards the ancestry, is evidently an omission of the last step made in the evolution of the race. It is what is known as atavism and is sometimes carried so far that the variations (i.e. the evolution) of hundreds of ancestors nearest it are omitted, and the child resembles some extremely remote ancestor, being ape-like in features, especially in times of famine, when want of nourishment checks the development, and sometimes being covered with long hair. So also the progeny of a pair of thoroughbred horses is sometimes an "arrant weed," or it may revert to an even more remote ancestor, by showing zebra-like stripes. So also "a blue pigeon, like the ancient Columba Livia, may be hatched in the dovecot." On the other hand, the progeny of a pair of ordinary horses never has the peculiar characteristics of the racer; nor has the progeny of a blue pigeon ever those of the pouter or fantail.
It is well known that race-horses have been developed by an extremely stringent process of artificial selection. I believe I am right in saying that when this process ​began, the rate of speed evolution (which depends on structural evolution) was comparatively rapid, but that this rate has been gradually slowing, so that now the race-horses of one decade do not greatly surpass the race-horses of the previous decade. To some extent this, of course, may be due to the fact that to obtain any increment of speed a more than corresponding increment of force must be put forth, as is exemplified by the fact that an oarsman, to increase his speed from two to four miles an hour, must use more than twice the amount of force, and to increase his speed to six miles an hour, must use a still more disproportionate amount of force. But the race-horse surpasses the ordinary horse not so much because it is more powerful, but because it is better shaped for speed, just as the racing boat surpasses in speed the ordinary rowing boat for the same reason. We can hardly conclude that the evolution of the race-horse is now slow because he has nearly reached the perfection of shape for speed, for another of the equidæ, the wild ass, a smaller animal, is said to be swifter. The decreasing rate of evolution, therefore, must be set down to another cause.
If we mate two ordinary horses there is a fair prospect that some at least of the offspring will be as fine, or even finer, animals than either the sire or the dam. But if we mate a Derby winner with a good mare—a better mare than his dam—we shall find that the majority of his progeny are inferior to himself, and that only very exceptionally does he procreate a son or a daughter that can match him for speed. Thus, I venture to say that very few, if any, of the sons or daughters of Ormonde will be his equals. Rapid evolution, therefore, rapidly becomes more slow. Why? Evidently because there is a tendency for the offspring to omit more or less of the latest evolution of the race and revert to the ancestry, sometimes the remote ​ancestry, and therefore in cases in which evolution has been rapid, as with race-horses, this reversion produces wide divergences of structure from the parent, but in cases in which the evolution has been slow, as with lamp-shells, this reversion produces very slight divergences of structure. For instance, to take an extreme example, that amount of reversion which in the offspring of a man would result in a fish, would in the offspring of a lamp-shell result in another lamp-shell, only a little modified. For this reason it is that characters long present in a race are, as is well known, little capable of undergoing retrogression. Since racehorses have been rapidly evolved under a process of extremely stringent selection, reversion of the offspring to a comparatively recent ancestor causes a considerable amount of divergence from the parent. Let us try to imagine what would happen among race-horses if the stringency of selection were relaxed, or rather if selection in this case were altogether abolished, so that the inferior animals were allowed to propagate the species as well as the superior. I think we may prophesy with tolerable confidence. The selective action being withdrawn, the tendency towards reversion, towards atavism, would operate uncontrolled, and the race would rapidly revert to its ancestor, the ordinary horse.
We are now in a position to understand why evolution caused by natural selection is so extremely slow. In every species natural selection as a cause of evolution, and atavism as a cause of retrogression, are constantly at war. If the one force predominate we have evolution; if the other, retrogression; if the two forces balance one another, the species in which this occurs undergoes for the time neither evolution nor retrogression. Rapid evolution is accompanied by an increasing tendency to retrogression, because in such a case any ​ancestor reverted to differs more in structure from the last member of the race than a corresponding ancestor in a race that has been more slowly evolved. I must, however, guard myself against possible misconception. When I speak of atavism as a cause of retrogression, I do not necessarily mean that in each case of atavism the whole organism in all its parts retrogresses. This must rarely happen, especially when evolution has been slow. What I do mean is that, generally speaking, atavism, by causing various small retrogressive changes in various parts of the organism, is by its cumulative effects a check to evolution, and even sometimes a cause of retrogression. An organism may undergo retrogressive changes in some of its parts owing to the severity of natural selection becoming relaxed as regards them, at the same time that, owing to increased severity of natural selection, it is undergoing evolution as regards other parts. Thus in the case of a species of bird, the habits of which are becoming less and less aerial, natural selection will cause the legs to undergo evolution, but atavism the wings to undergo continued retrogression backwards, successively from recent ancestors to remoter ancestors, till, if the power of flight no longer influences the survival rate, there will be an approximation in structure, as far as the wings are concerned, to the structure of a remote wingless or rather limbless ancestor; but an approximation only, for, as regards such complex organs as wings, we must not expect to find that a degenerate example in any stage of the retrogression resembles very closely an ancestral wing, in a corresponding stage of the evolution; for instance, we must not suppose that the wing of the apteryx is a close reproduction of a remote ancestral form, for the reason that though the wing in all its parts undergoes retrogression, yet, like the parts of a complex organism, all the parts of the wing do not ​undergo equal retrogression. Various causes prevent the retrogression from being everywhere equal, since the conditions under which retrogression takes place cannot be an exact reversal of those under which evolution occurred; for instance, the retrogressing wing of the apteryx is attached to a body which has not undergone a pari passu retrogression. Moreover, even while the wing was undergoing evolution, parts of it must have undergone retrogression; this and that structure or part of a structure in it, useless in a wing, but useful in the organ which preceded it, must have tended to disappear at the same time that other structures or parts of structures, more useful in a wing than in the organ that preceded it, tended to increase; the two warring forces of natural selection and atavism thus operating to bring the wing and the organism to which it belongs into completer harmony with the environment. Thus all the parts of the degenerated wing of an apteryx cannot have undergone equal retrogression, and therefore the resemblance between the degenerated wing and the ancestral form in any of its stages cannot be very close.
It follows, if the above theory be correct, that while it is not possible by means of selection, natural or artificial, to bring about rapid and extensive evolution, since such evolution must soon be checked by the increasing tendency towards far-reaching reversion, it is possible by means of selection to bring about rapid and extensive, indeed unlimited, retrogression. This hypothesis, however, is at variance with accepted doctrines. Mr. Herbert Spencer says—
The domesticated dog is presumably descended from one or more of the different wild varieties, or from their relatives the wolves. Now, considering the length of time dogs have been domesticated, and the severity of the selection to which they have been subjected, our largest dogs, the St. Bernards, Newfoundlands, mastiffs, boarhounds, do not very greatly exceed wild dogs or wolves in size, nor do our most intelligent dogs greatly surpass them in intelligence; but our smallest dogs, some of them little bigger than rats, are very much smaller, and some of our tame breeds are exceedingly stupid. Clearly as regards dogs, we have been able to produce little evolution, but great retrogression. In some breeds there has undoubtedly been ​evolution in this or that respect—in the St. Bernard of size, in the mastiff of strength, in the greyhound of speed, in the bulldog of courage; but on the whole the tendency has been towards retrogression—a retrogression which has operated unequally on the animal parts, physical and mental, and has not infrequently been concomitant with partial evolution. To this unequal retrogression, combined in some cases with limited evolution, is probably to be attributed the diversity of shapes and capacities which characterize our many domesticated breeds. Thus the lapdog and the pug are probably examples mainly of retrogression, not evolution; probably their peculiarities of shape and character are due mainly to the fact that man has permitted or caused retrogression in them in certain directions by selecting the most degenerate individuals in these respects, at the same time that he has checked retrogression, and even caused some evolution, in other directions. Thus also the bulldog has undergone retrogression as regards size, but evolution as regards courage; the greyhound retrogressed as regards strength and power of scent, but evolved as regards speed; the St. Bernard has gained in size, but lost in speed; and so also with all other breeds.
Dogs among civilized peoples especially have often been the mere playthings of their owners, who, taking unconscious advantage of the tendency towards retrogression, have gratified their caprices by rearing many grotesque varieties. As regards the equidæ and bovidæ the case has been entirely different. Reared almost solely for use, their powers and capabilities preserved to the utmost, the domesticated varieties show little retrogression and some evolution. But suppose we selected a number of the swiftest race-horses, or the largest dray-horses, or the best milk-giving cows, and bred from them, selecting afterwards in succeeding ​generations always the slowest, the smallest, or the least milk-giving individuals respectively wherewith to continue the race, is there any limit to the retrogression which would result in "assigned" time? We certainly should not bring about the reappearance, for instance, of the remotely ancestral horse, the five-toed Eohippus, for the form of the modern horse, like that of other animals, has resulted not only from evolution, but also from retrogression, the disappearance of the four lateral toes being examples mainly of the latter, while the great size of the middle toe is an example of the former; but it cannot be doubted that we should cause, in a time comparatively short as compared to that which elapsed during the evolution, such extreme retrogression as would result in an animal quite incapable of existence.
The improvements in our cultivated plants are the result of centuries of stringent selection; and here again it cannot be doubted, that if the process by which they were evolved were reversed, were we to propagate only from the most inferior plants, that the rate of retrogression would be much more rapid than was the rate of evolution.
It is to be noted, however, that some cultivated plants exhibit comparatively extreme evolution, and in fruit or flower or other particular greatly surpass the wild individuals of the species, e.g. peach, apple, pear, rose, and therefore furnish apparent exceptions to the law that rapid evolution is soon checked by an increasing tendency towards retrogression. In reality they afford the strongest proof of it. I think I am right in saying, that in every instance such plants have been propagated principally by cuttings and not by seed; i.e. they are not descendants in any true sense of their immediate predecessors, but detached portions of them. Their evolution appears to have been effected as ​follows. A plant that varied favourably has been chosen, and a vast number of plants have been reared from slips cut from it, the process being repeated through any number of pseudo-generations. In this way a single plant with a favourable variation is multiplied into many similar plants, and the variation is preserved for an indefinite time. Seeds reared from such plants generally develop into inferior plants, but exceptionally a superior plant develops, and is then propagated as before by means of slips. The parent variety is then ignored for breeding purposes and is used only for stock on which to engraft the newer variety. The more improved variety thus takes the place of the variety which is less improved. This process, the repetition of which has resulted in such extreme examples of rapid evolution as the peach and garden rose, is therefore a tremendously stringent process of selection. Practically speaking the most favourable individual of a species has been chosen and multiplied by means of slips, the rest of the species being eliminated; and in each new seminal generation the same process has been followed. It is as though, in an endeavour to increase the height of men, we chose the tallest man in the world, eliminated the rest of his species, multiplied him—not by seminal generation, but by some process analogous to the multiplication of plants by slips—into thousands of men and women as tall as himself, chose the tallest individual among the children of these, and repeated the process, and continued to do so through several seminal generations. The process of selection under which the cultivated rose and the peach have been evolved has therefore been, as I say, tremendously severe—a thousand times more severe than it is possible to make it among annual plants and among the higher animals; and their evolution has therefore been extremely rapid—even ​more rapid than would at first sight appear, for it is probable that but a few seminal generations intervene between the most divergent cultivated plants and their wild progenitors. But now suppose we chose any one of these highly divergent varieties, and without using any selection, bred from seed alone, what would happen? There is ample evidence leading us to believe that in the vast majority of instances the variety would swiftly (i.e. in a very few generations) revert to something very like the wild stock from which they originally descended;—but not to the wild stock precisely, for no doubt while the cultivated species was undergoing evolution in one direction, it was under the changed conditions undergoing retrogression in other particulars, and in these the reverted variety would differ from the original stock.
The truth therefore appears to be, that while there is a limit within "assigned" time to evolution, there is practically none to retrogression.
The above considerations may afford an explanation of another set of facts, viz. that cultivated plants and domesticated animals are much more variable than the wild varieties of the same species. In a state of nature plants and animals exist under conditions which, normally, are uniform during long ages; owing to which, and to the circumstance that in a state of nature many traits are essential to survival, evolution is slow, and therefore the traits of wild animals and plants acquire a certain fixity, because if any recent ancestor be reverted to in any particular, the change is not great; moreover, any retrogression must generally cause elimination; therefore since evolution is slow, since any reversion can seldom be great, and since reversion tends to cause elimination, there can seldom be any great or observable change of form. With cultivated plants and domesticated animals fewer ​traits are essential to survival, and therefore evolution is more rapid as regards them. Many traits essential in a wild state for survival, in a cultivated or domesticated state are not essential, and therefore as regards them there is retrogression. In a few generations "sports" appear; but these sports, if I am right, must generally be examples of retrogression, not of evolution (retrogression in traits which were essential to the wild ancestors, but are no longer essential to the cultivated or domesticated descendants, or far-reaching retrogression in traits rapidly acquired under artificial selection); must generally be due to reversion to the ancestral form, not to an advance beyond it.
Again, atavism is not the only cause of retrogression. Evolution may be a cause of apparent retrogression, as in the case of certain insects, which, living as they do in storm-swept islands, are exposed when flying to the danger of being carried to sea. In them natural selection, reversed selection as it is called in such cases, has co-operated with atavism to deprive them of the power of flight. Therefore, if this theory of retrogression be correct, it may afford us a not unimportant insight into the past life-history of species, and enable us to decide what retrogressive changes are due to atavism and what to natural selection. For instance, man as we know has descended from a hairy ancestor: if his present partially hairless condition is due to atavism, then since he has reverted in this respect to a remote hairless ancestry, his embryo will not be hairy, nor will he in cases of atavism be hairy. We know that this is not the case; therefore his present hairlessness is due mainly to natural selection (reversed selection), possibly to that form of natural selection known as sexual selection.
This theory has not, so far as I am aware, been propounded before, and it is opposed to another theory ​(presently to be noticed) which has the support of many eminent men of science. The mere fact, however, that organisms vary more frequently in the direction of their ancestry than in any other direction, appears to me to raise a strong presumption in its favour. But whether it be true or not, this at least is known to be true; if any characteristic become of little or no importance in the struggle for existence it is certain to undergo retrogression, i.e. cessation of natural selection is invariably followed by retrogression.
CHAPTER III
It has already been explained that the lowest organisms are probably quite beyond our ken. Higher organisms than these appear as minute and apparently formless specks, the protogenes of Haeckel, visible only under the highest powers of the microscope, and composed of that transparent jelly, "the formal basis of all life," which is known as protoplasm. About them also we have as yet been able to learn little beyond the fact that they are living beings. Higher in the scale are such organisms as the amœba; about them we are able to learn much that is important and instructive. They occupy that point in the scale of life at which the plant and animal kingdoms begin to diverge the one from the other, and though excessively minute, are larger than the protogenes, and therefore better observable. Each is a little mass of protoplasm in which may be seen a dot, the nucleus, which is usually situated eccentrically, and which, as modern research seems to have established, is the most important part of the organism. Such a speck of living protoplasm as the amoeba is known to biologists as a cell; and of such cells or variations of them the structures of all plants and animals are built up, a plant or animal composed of a single cell being known as a unicellular organism, whereas a plant or animal composed of a plurality of associated cells is known as a multicellular organism. Since such unicellular organisms as the amœba are fairly observable, ​we may conveniently begin the study of life at that stage of evolution which they have reached. Watched under the microscope the amœba is known by its actions to be a living being. It puts forth or withdraws thinner or thicker processes known as pseudopodia. It moves by flowing forward, or by putting forth pseudo-podia and flowing into them. Streaming motions of granules may be observed in its substance. It engulfs food particles, and having assimilated the digestible portions, flows away, leaving the indigestible remainder behind. It shrinks from harmful contact.
In describing the amœba I have described also the white blood corpuscle, the leucocyte of man and other animals. These occur in great numbers, maintaining a separate existence in the blood, or wandering through the tissues. It might be thought that they are parasites, but this is disproved by the fact that if any tissue is injured, as for instance by a cut, they crowd in countless numbers to the spot, and repair the injury with their bodies, which thereupon undergo changes in shape and structure. What is known as pus or "matter," such as flows from an abscess, is a clear fluid rendered turbid by the multitude of their dead. In cases of zymotic disease they have been seen with the invading microbes enclosed in their substance, when either the leucocyte or the microbe perishes.
Both the amoeba and the leucocyte multiply by fission, by dividing the one into two. First the nucleus divides by a complicated process known as Karyokinesis, the completion of which is followed by the division of the cell body. The daughter-cells grow, and in time divide in the same manner as the parent. This process is repeated through an immense though finite number of generations by the leucocyte, and through an infinite number of generations by the amœba. Whence it is clear that the amœba is ​potentially immortal. It may be killed by external circumstances, by starvation, by heat, by cold, by violence, &c., but if the conditions are favourable no death occurs, for, as Weismann pertinently remarks, "If it is mortal, if it dies—What is it that dies? where is the dead body?" The leucocyte, however, is not potentially immortal, for in time it or its descendants die.
If we watch low unicellular organisms of almost any species we see that occasionally two of them come together and fuse more or less completely, so that the two animals become one, or so that an exchange of substance takes place between their nuclei; subsequently the dual animal divides and re-divides many times before fusion again occurs. But however many the number of cell-divisions subsequent to fusion, recent investigations seem to show that the descendants of the conjugated pair ultimately perish, unless fusion again occurs.
"What was the result? At the date referred to the family was observed to have exhausted itself. The members were being born old and debilitated. The asexual division came to a standstill and the powers of nutrition were lost.
"Meanwhile before the generations had exhausted themselves several of the individuals had been restored to their natural condition, where they conjugated with unrelated forms of the species. One of these was isolated and watched for five months and the usual number of successive generations occurred. On to the ​one hundred and thirtieth generation members were removed at different stages, and were observed to conjugate successfully with unrelated forms.
"When the family began to draw near its end even removal to fresh conditions was without effect. About the one hundred and eightieth generation, the strange sight was seen of individuals of the same family attempting to unite with one another. The results were however nil, and the conjugates did not even recover from the effects of their forlorn hope.
Among these low forms conjugation is evidently the same thing as sexual union among the higher animals, to which, and to the vertebrates in particular, popular observation is almost exclusively directed; and as sexual union among the vertebrates invariably precedes the birth of a new individual, it is naturally supposed that it is the cause of the genesis of a new individual; but this is certainly not so, for various invertebrate species are able to multiply asexually. Some organisms comparatively high in the scale are able to reproduce imperfectly without sexual union; for instance, among bees the offspring of asexual reproduction are drones only, not workers and queens, who are essential to the continuance of the race. Others again, such as aphides, are able to multiply asexually perfectly for many generations before sexual reproduction occurs; and lastly, Weismann has never been able to discover sexual union ​among a species of crustaceans which he has observed for a number of years. Sexual union therefore, when it does occur, appears to be an essential condition, not the cause of reproduction, much as a sufficient supply of nutriment or of heat is an essential condition, not a cause. Various hypotheses have been put forth as to the raison d'être of it. By some it is supposed that its object is the maintenance of the specific average—an absurd hypothesis, for the law governing organic beings is that of the "survival of the fittest," not that of the maintenance of the specific average. Another hypothesis is that sexual union causes a rejuvenescence and revitalization, so that by virtue of sexual union Maupas's infusorians when they conjugated grew young and vigorous and were able to continue the race, but when they were unable to conjugate grew old and feeble and perished, and with them the race. But as Weismann remarks, it is difficult to understand how one aged and debilitated individual can be rejuvenated and revitalized by union with another individual equally aged and debilitated. Another raison d'être must, I think, be sought for sexual conjugation. Weismann's hypothesis appears to be the most probable; he thinks that sexual union is very prevalent because by it is produced an increased amount of variability in the offspring.
On consideration it is apparent that of two individuals the one that produces offspring that vary within certain limits more from itself is the better placed as regards the ultimate survival of its descendants, other things equal, than the other individual which produces offspring that vary less, assuming of course, what must on the average happen, namely, that some of the offspring vary favourably as compared to the parent and some unfavourably. Though some of the descendants of the first, because they have varied extremely unfavourably, ​must perish and leave no offspring, yet some of his descendants, because they have varied extremely favourably, must, when there is a struggle for existence, other things equal, survive in greater numbers, and cause the ultimate elimination of the offspring of the second. Now, of two individuals, one of which produces offspring without collaboration with another individual, and the other in collaboration with another and somewhat dissimilar individual, the offspring of the latter must tend to vary more than the offspring of the former, and therefore in the struggle for existence to bring about their ultimate extinction, whereby only the descendants of the individual that reproduced sexually, and which inherited this peculiarity, would be left to continue the race. Weismann's hypothesis therefore appears to be reasonable, and it is moreover supported by other considerations, for if we assume, as we must, that nonliving chemical compounds, under conditions we are ignorant of, did in the beginning of life pass over the border space and become living beings, it is difficult to imagine how they could possibly have multiplied sexually from the very first—how sexual union can have been the rule in the beginning. It seems more reasonable to suppose that these earliest forms multiplied asexually by fission, and that conjugation only occurred later, the purpose of it being to produce a greater amount of variability among the offspring. Possibly conjugation had its origin in attempts at cannibalism, which, among such excessively simple types as the first must have been, perhaps resulted in coalescence instead of assimilation. The extermination of those that multiplied asexually appears therefore to be the cause of the almost universal prevalence of sexual reproduction among low as well as among high organisms. If it be asked, Why, if sexual reproduction produces variability, and variability is of such importance, do low ​organisms only conjugate at rare intervals? the answer may be given in the following extract—
The above explains also why sexual reproduction does not occur in all instances, e.g. when the specific persistence is secured by extreme rapidity of multiplication rather than by close adaptation to the environment.
In speculating on the origin of species we may conceive it possible, or rather certain, that among the innumerable variations which occurred among the vast multitudes of low unicellular organisms, such a variation occasionally occurred as the following: that when one cell divided into two the resulting cells did not separate, as normally happened, but remained adherent; and further, that this variation, whether for purposes of food-getting, locomotion, protection, &c., proved a fortunate one. This variation, which, like other variations, would tend to be transmitted, and which, if fortunate, would tend to cause the ultimate survival of those organisms that possessed it, would be the first step in the evolution of the multicellular from the unicellular organism. The dual animal which resulted would reproduce by each of its cells dividing into two, so that there would be four single cells which would separate, ​so as again to form unicellular organisms. But each unicellular organism would in general inherit the peculiarities, and repeat the life-history of its grandparent-cell, by dividing into two adherent cells. A race of two-celled organisms would thus be established. We may fairly believe that in time a second variation, which also proved fortunate, occurred, whereby the four grand-daughter cells remained adherent until reproduction; and afterwards other variations of the same nature, till an organism was at length evolved which consisted of a multitude of cells adherent together for the common benefit—the morula. The component cells would all be alike, each would perform all the functions of life, assimilation, locomotion, &c.; and when reproduction (i.e. reproduction of the whole organism) took place the whole would break up into single cells, every one of which, by repeating one by one the variations of its ancestors, would build up, step by step, a multicellular organism similar to that from which its parent cell was derived; the steps of the ontogeny or development of the individual thus following those of the phylogeny, or evolution of the race.
The cells of such a simple multicellular organism, though similar in kind, would yet necessarily differ as regards position, some being internal and some external, and therefore it would obviously be an advantage (1) if the organism assumed such a shape as would enable all its constituent cells to perform their common functions to the greatest advantage; or (2) if among the cells a division of labour took place by virtue of which the internal and external cells performed functions different in kind; or (3) if the two variations were combined so that changes in the shape of the organism were accompanied by differentiations in the functions of the cells, some cells taking on one function, e.g. food-getting or locomotion, some another, e.g. digestion or ​reproduction, accordingly as they were best placed to perform it.
Now, since it cannot be doubted that low organisms vary as well as high organisms, we may legitimately suppose that some such variations as the above did occur, indeed we have abundant evidence that they must have occurred, and that they were so seized upon and accumulated by the action of natural selection (i.e. by the survival of the fittest), that such differentiations were thereby brought about in the forms of multicellular organisms (i.e. in masses of cells adherent for the common benefit), and such differentiations in structure and specializations in function in their component cells as resulted, after long ages and innumerable generations, in all the varied and wonderful forms of plant and animal life; and in the equally wonderful and varied differentiations in structures and specializations in functions of the cells composing those plants and animals; in such lordly- forms as the Wellingtonia Gigantia and the elephant, as well as in such lowly forms as the lichen and the hydra; in such highly differentiated cells as nerve muscle or gland cells, as well as in the white blood corpuscle, which may be likened to an amœba, or the bone cell, which secretes round itself a calcareous envelope like a rhizopod.
The single cell of the amoeba performs of necessity all the functions of life; but even in such low organisms as sponges a great amount of cell-specialization is already observable. In them cells which are differently situated as regards the environment differ somewhat in structure and function. All the cells are to some extent capable of performing all the functions of life, but some cells perform some one function better and other functions less well than other cells differently situated, which in turn display a like peculiarity. Thus as regards the function of reproduction some cells subserve ​it more than others; yet many of the cells of all portions of the whole organism are capable of performing it, for almost any fragment of a sponge, if separated from the parent mass and bedded out, is capable of growing into an "individual" and of continuing the race. If we pass in succession to higher species, we find cell-specialization becoming more and more marked; we find cells becoming more and more capable of performing efficiently some one function and less and less capable of performing all other functions. A division of labour thus occurs in the cell-community similar to that which distinguishes communities of civilized men from savage communities. Every savage does everything for himself, receiving small help from the rest of the community and giving little in return; and the more utterly savage the community is, the more complete in general is the independence of the individual. But while the savage performs all the labours necessary for supporting life, he performs each one less efficiently than he would did he devote himself entirely to it. In civilized communities each man devotes himself to some special work which he performs specially well, but which would not enable him to live unless it were supplemented by the labours of other men, who in like manner perform other special duties equally well. Thus a bricklayer performs a special duty better than he could do were he also a carpenter, a farmer, and a miner; but except he receive help from the labours of others his work does not produce all that he requires to maintain life. It is not, for instance, directly productive of food and clothing. For these he depends on the labours of other specialists; and therefore the higher the state of civilization the more complete is the state of interdependence of the members of the community.
So in higher animal organisms we find that all cells have taken up special duties—have all become specialists. ​Every kind of cell performs one function specially well and the others not so well, or not at all. The result of this cell-specialization and differentiation is, like the results of increased specialization and differentiation in the labours of civilized men, increased efficiency of the organism as a whole, and the result of the increased efficiency is, of course, an increased average rate of survival.
CHAPTER IV
One function which, in high animal organisms, is performed exclusively by one class of cells alone, the germ cells, is that of reproduction, that is, reproduction of the whole organism. All the other cells are capable of reproducing their like by that process of fission which, as we have seen, is very prevalent among unicellular organisms. Thus muscle, nerve, or skin cells can severally produce their like, but no muscle, nerve, or skin cell can produce a cell of an unlike kind; still less can it produce a whole organism, in which there are many different kinds of cells arranged in definite relations to one another. Unlike the case in lower animals, such as sponges, then, this power among higher animals, such as vertebrates, is limited to the germ cells alone, each of which presents in addition the remarkable peculiarities unknown among any other of the cells, (1) of conjugating with another cell, and (2) of being incapable of reproducing other cells, like or unlike, except it first conjugates with another germ cell, and that germ cell not a near relation, not a descendant of the pair of germ cells from which the organism of which it forms a part ​is derived, i.e. not a germ cell from the same body, but only with a germ from another body.
Thus these remote cell-descendants of the original pair of cells which conjugated, and from which the body to which they belong is derived, exhibit, after many generations, the same peculiarity which Maupas found distinguished the remote cell-descendants of conjugated unicellular organisms; viz. that after a certain number of cell-generations, unless conjugation occurs anew, they perish, and with them the race. The other cells which do not conjugate cease in time, like Maupas' infusorians, to multiply, and, like those infusorians, perish, and with them the individual of whom they generally form the chief part.
The above seems to bear out the theory that conjugation causes a "rejuvenescence and revitalization" without which the race cannot persist; but on closer examination the facts are found to point the other way. The number of cell-generations following conjugation differs in different individuals of the same species, and may be made to differ in each individual by variations in nutrition, exercise, &c. It differs enormously in different species of plants and animals; thus the number of cell-generations following conjugation is enormously fewer in minute plants and insects than in such great plants and animals as pines and whales. Moreover the number of non-conjugating cell-generations is almost infinitely prolonged along certain lines in such animals as the aphides, which reproduce asexually during the whole summer, and reproduce sexually only on the advent of cold weather, and which, were the warm weather to continue, might reproduce asexually for ever. It is infinitely prolonged in such plants as multiply by means of suckers, or are propagated by cuttings, without their cells ever conjugating. The theory therefore that conjugation causes a "rejuvenes​cence and revitalization," without which death must occur, is utterly disproved, as is also the theory that the purpose of conjugation is to maintain the specific average. For here we have unlimited cell-multiplication without any rejuvenescence by means of conjugation, while the specific average is perfectly maintained by the similarity of the conditions to which the different individuals of the species are exposed. On the other hand, Weismann's contention, that conjugation is not necessarily essential to persistence, but is merely a condition which is usually, but not always, advantageous—so advantageous that in nearly all plants and animals it periodically occurs—is fully borne out.
Death, that is death from internal causes, from failure of the vital powers, not death from external causes, such as cold, hunger, accident, &c., occurs for quite another reason, which may be set forth as follows. In those plants which multiply by means of suckers or are multiplied by means of cuttings, the cells whence the individual is derived are to be regarded as little differentiated and specialized, like the cells of the sponge, cell-differentiation and specialization not having proceeded nearly so far among plants as among animals; for the highest plants are inferior in this respect to animals low in the scale. As a result, while plants high in the scale are able to reproduce otherwise than by means of germ cells, animals comparatively very low in the scale are not so able. They reproduce solely by means of germ cells, for such, owing to specialization in function, is the interdependence of their other cells, that these latter cannot exist apart from one another, any more than can a bricklayer, who does nothing else to support life, exist apart from other men. They must remain part of the organism or perish, while they cannot continue to multiply indefinitely, for then the organism would grow too large for its available supply of nutriment. ​It (the organism) is during life subjected to all manner of external conditions, which tend to destroy it, or so to maim and enfeeble it that it is easily destroyed; for, unlike plants and low animal organisms, the interdependence of the parts as well as of the cells of a higher animal organism is so great, that no one part can be injured without all the other parts being injuriously affected. The ultimate death of all organisms with highly specialized cells is therefore inevitable. To put it another way: all cells or aggregate of cells, which are so specialized as to be incapable of independent existence, cannot continue the race, and must inevitably perish. In high animal organisms the germ cells alone are capable of existing independently, and therefore they alone survive in their descendants. It is true that in the highest animals the germ cells after conjugating are still retained within the body of, and supplied with the nutriment by, one of the parents till development is considerably advanced, but in the sense in which I write they as truly lead a separate existence as the child whom the mother suckles.
But though conjugation is not universally necessary to unending reproduction it is yet generally true; and the point I wish to emphasize is this—that in the highest animals, as well as in the lowly infusorians, unless conjugation occurs after a certain number of cell-generations, cell-multiplication ceases, and the race perishes.
The germ cell, like the unicellular organism, on conjugating, divides and redivides many times without conjugation ever occurring again among its descendant cells, the successive generations of which may be compared to successive non-conjugating generations of infusorians, so that the body of a multicellular organism in the successive stages of its ontogeny is comparable to later and later generations of infusorians; but, unlike ​the cell-descendants of conjugated infusorians, the cell-descendants of germ cells, (1) instead of separating remain adherent, and (2) the mass thus formed, as cell-proliferation proceeds, takes definite shapes, at first resembling those of very low multicellular organisms, then with more or less indistinctness those of higher organisms, and lastly that of the parent organisms, from which the pair of germ cells which conjugated, and from the union of which the other cells resulted, was derived; and (3) each successive generation of cells shows greater and greater degrees of differentiation and specialization, till such highly differentiated and specialized cells as skin, nerve, gland, blood, &c. as are present in the fully developed organism appear. In other words, the development of the individual is a short, rapid, blurred recapitulation of the evolution of the species; which, if there is any truth in the theory of evolution, is exactly what was to be expected, and which therefore affords convincing proof of its truth.
We have seen that evolution depends on three factors: (1) that offspring in general inherit (i.e. recapitulate) the traits of their parents; (2) that offspring vary somewhat from their parents; (3) that there is invariably a struggle for existence, during which natural selection causes evolution by preserving favourable and eliminating unfavourable variations. Now if the son recapitulates the traits of the parent, the parent the traits of the grandparent, the grandparent the traits of the great-grandparent, and so on, it is evident that the son, the last of the race, must recapitulate the traits of each ancestor up to the remotest, or at least up to the unicellular organism, which for convenience of language we may call the first ancestor. In other words, the last descendant recapitulates the traits of the first ancestor, plus the traits, in their order, of all subsequent ancestors, beginning with the traits ​of the first and ending with those of the last, and therefore in his development he presents a fleeting resemblance to each ancestor in turn.
It follows, therefore, that what is known as atavism is nothing other than a failure to recapitulate in the ontogeny the last stages of the phylogeny; i.e. it is an arrest of development, the individual halting at a stage reached by a remote ancestor, and developing no farther. A race may differ from its ancestry in three ways: (1) through evolution as a result of selection; (2) through evolution as a result of reversed selection; and (3) through retrogression occurring in the absence of selection. As regards traits evolved under the influence of selection, atavism is a simple arrest of development; that is, the ancestral form is approximated to, because the last stages of the phylogeny are omitted in the ontogeny. As regards traits evolved under the influence of reversed selection, atavism is also an arrest of development; the ancestral form reappears because there is an omission in the last stages of the ontogeny to retrace steps previously made, as was done in the phylogeny. As regards traits suppressed through retrogression, i.e. through a lapsing by the race of the last steps of the evolution, there can of course be no atavism; for instance, if the lost toes of the horse have disappeared through retrogression, i.e. through a return, in the absence of selection, to a very remote ancestral condition when they did not exist, then their reappearance in the modern horse would be an instance of evolution, not of retrogression, since it would be a return from a more ancient to a much more modern condition; on the other hand, if the toes have disappeared as a result of reversed selection, the embryo should exhibit them, and they should be present in cases of atavism.
It is now easy to understand why all races tend to ​retrogress unless that tendency is checked by selection; for, as regards any trait, an individual may vary from his parent primarily in two ways. He may vary from him either towards the ancestry or away from it, i.e. he may either undergo retrogression or evolution; and, so far as we know, the chances are equal of his doing either the one or the other. But if he vary away from the ancestral type, it does not necessarily follow that the variation will constitute an extension of the previous evolution. It may constitute a reversal of it, or a divergence in an altogether new direction, and, therefore, in the absence of selection, the variation of the offspring from the parent must tend on the whole to bring about retrogression—a tendency which is checked and reversed in an evolving species only by a sufficiently severe process of selection.
Though each multicellular organism has its starting-point in a unicellular organism (the germ cell), yet nevertheless each germ cell, counting from the time of the first evolution of the multicellular organism, when all the cells were more or less germ cells, through succeeding generations, during which ever-increasing differentiation in structure and specialization in function took place, must have become in one sense a more and more complex entity, differing more and more from its ancestor, the unicellular organism, in that it was the starting-point of a more and more complex and heterogeneous multicellular organism. In other words, each successive conjugating germ cell differed more and more from its ancestor, the conjugating unicellular organism, not only in that its non-conjugating descendants, however remote, remained adherent in one mass, but also in two other particulars.
First, in that definite lines of its cell-descendants multiplied at increasingly different rates, whereby were produced differentiations in the shape of the whole ​mass; for instance, the mass of cells constituting a man owes its particular shape, the human shape, to certain definite inequalities which occur in the rates of multiplication in the lines of the cell-descendants of the germ whence the man is derived. Were there no inequalities in the rates of multiplication, did each line of cell-descendants multiply at the same rate, a solid, spherical mass of cells must result, whereas, owing to their unequal but definite rates of multiplication, the shape of the mass is irregular (i.e. not spherical), but nevertheless definite for man and every species of animal. An ox, therefore, differs in shape from a man mainly because the inequalities in the rates of multiplication in the various lines of the cell-descendants of the germ whence it is derived are different from the inequalities in the rates of multiplication in the lines of cells that spring from a human germ; but it resembles other oxen in shape, because the inequalities in the rates of cell-multiplication are much the same. Sometimes, however, though rarely, this or that cell-descendant of the germ reverts to the remote unicellular ancestral type, in so far that it does not multiply at a rate bearing a definite proportion to the rates of multiplication of its other co-descendants of the germ, but at a rate that has no definite proportion to them, and is only proportionate to its supply of nutriment and powers of assimilation. There then results the "morbid" condition which is known as a "tumour," in which the cell-descendants of a cell which has so reverted to the remote ancestral type form a more or less spherical mass which neither bears a definite proportion to the whole mass of the cell-community, nor performs definite functions beneficial to it, and is therefore an encumbrance or worse. But this tendency of cells to revert to the ancestral unicellular type, to multiply at a rate that is only proportionate to the supply of nutriment ​and the powers of assimilation, is checked by natural selection, which, by placing at a disadvantage, brings about the ultimate elimination of multicellular organisms, in which the tendency is displayed.
On the other hand, this or that cell-descendant of the germ may vary from the normal in a direction which is not towards the ancestral type, but in a different direction, and its descendant cells then form a mass or group which also differs more or less from the normal. The bud variations which have been observed on several cultivated plants, by taking advantage of which gardeners have sometimes been able to establish new varieties, probably result from this cause.
Secondly, each successive germ cell must differ from the ancestral conjugating unicellular organism more and more, in that different lines of its cell-descendants differentiate more and more from one another in structure and function, whence arise in high animals differentiated and specialized tissues such as muscle, bone, skin, gland, nerve, &c.
Moreover, just as each germ cell during the phylogeny differed more and more from its prototype, the unicellular organism, in that it was the starting-point of a more and more complex organism (mass of cell-descendants), so each embryo during the whole process of its ontogeny differs from its prototypes in the phylogeny, in that it carries within it the potentiality of developing beyond those prototypes. We cannot, however, discern in the macroscopic or microscopic appearances of germ or embryo any peculiarities of structure which imply this potentiality. They lie beyond the ken in the minute structure of the cells, probably in those portions of them which are known as the nuclei.
It may be objected, (1) that it is impossible that during the short period of the development of the ​individual all the vast numbers of variations which occurred among the millions of ancestors from the unicellular organism downwards can be recapitulated; and (2) as regards the highest animals, that it is obvious that the embryo in its various stages cannot present exact copies of its remote ancestors, for the simple reason, that in its various stages it is incapable of living apart from the parent. For instance, no animal that even approximately resembled the human embryo of two months could be capable of living outside the body of its mother. But these objections are met when we take into consideration what is undoubtedly true, viz. that variations occur, and that natural selection (reversed selection in this case), and particularly cessation of natural selection, act, not only at the end of the ontogeny, but during the entire period of development; natural selection during that period seizing upon as favourable and accumulating all such variations as tend to shorten and simplify the process of development, and cessation of natural selection tending to bring about the disappearance of all characters which during the phylogeny had been useful, but which to the embryo, living under different conditions, protected and sustained as it is within the body of its parent in the highest animals, or advantageously placed as regards the environment by the parent in the case of lower animals, are no longer useful.
One other point remains to be cleared up. It has been observed that the embryo of a high animal, e.g. man, resembles the embryos of lower animals, never the lower animals themselves; for instance, the embryo of a man in one stage of its ontogeny resembles the embryo of a frog, never the adult frog. The reason for this is obvious; man has not descended from the frog, but the frog and man have descended from a common ancestry, and therefore both animals as they recapitulate ​in their ontogenies the evolution of their respective races, develop along parallel lines up to the point at which the races diverged, after which they develop along lines which are no longer parallel. It follows that the nearer the relationship between any two species, the longer does the course of the development from germ to adult proceed along parallel lines. Mr. Herbert Spencer puts the matter very clearly.
Following the above train of thought it may be remarked, that any trait which appears late in the ​development of the individual, as for instance the horns of deer and the beards of men, must have appeared late in the evolution of the species. The young hornless deer and the young beardless man should therefore correspond to very recent stages in the evolution of their respective species. Whence if there is any truth in my theory of retrogression by atavism, it follows that comparatively hornless deer and comparatively beardless men ought to be comparatively frequent, for the ancestor reverted to would in either case be comparatively recent. As regards the beard, we know that comparatively beardless men are common in a well-bearded race such as the Anglo-Saxon. As regards the horns of deer I cannot speak with authority, not having sufficient knowledge, but I doubt not that others who have had better opportunities for observation will confirm my conclusion in this matter. I think it will be found, for example, that comparatively hornless deer are much more common than comparatively hairless animals of the species.
* ↑ This is of course the theory of spontaneous generation, which is popularly supposed to be quite exploded. What is exploded is that such highly organized beings as the infusorians arose spontaneously.
* ↑ "Without isolation, or the prevention of free intercrossing, organic evolution is in no case possible. Isolation has been the universal condition of modification. Heredity and variability being given, the whole theory of organic evolution becomes a theory of the causes and conditions which lead to isolation "—Romanes.
* ↑ 'Atavism, which is the name given to the recurrence of ancestral traits, is proved by many and varied facts. In the picture-galleries of old families, and on the monumental brasses in adjacent churches, are often seen types of feature that are still, from time to time, repeated in members of their families."—Herbert Spencer's Principles of Biology, vol, i. p. 252.
* ↑ p. 231/2 in the 1910 ed. (Wikisource-Ed.)
* ↑ Any interest this chapter may possess will be greatly increased if in connection with it Mr. Darwin's book, Animals and Plants under Domestication, be read. In that great mine of information many facts bearing on the subject under discussion are to be found.
* ↑ "This is very generally the case, but it is not universal. 'Self' fertilization—that is, union of the eggs and sperms of the same organism—has been proved to occur in several trematodes, and to be almost universal in cestodes. This may be one of the conditions of the degeneracy of these parasites, for, frequent as hermaphroditism is among plants and animals, self-fertilization is extremely rare."—Evolution of Sex, p. 71.