Evidences Of Organic Descent; Morphology And Embryology
( Originally Published 1909 )
THE facts of biology which admit of adequate explanation only in connection with the theory of descent are grouped by Romanes and other writers on organic evolution under the heads of morphology, embryology, classification, paleontology, distribution and domestication. In all these lines the facts are drawn together by a strong thread of unity. There are numberless similarities and correlations and surprising uniformities. The great variety of life as exhibited in the countless species of plants and animals has been referred to, and yet, great as this variety is, there are, after all, only a few types of structure among all animals and plants, some three or four or eight or ten general modes of development, and all the rest are modifications from these few types.
It is, moreover, true that all living forms are but series of modifications and extensions of one single plan of structure. All have the same ultimate substance the mysterious semi-fluid network of protoplasm, which is, so far as is known, the physical basis of all life ; and the equally mysterious nuclear substance or chromatin which in some fashion presides over all the movements of the protoplasm and is the physical basis of the phenomena of heredity. The same laws of heredity, variability and of response to outside stimulus hold in all parts of the organic world. All organisms have the same need of reproduction. All are forced to make concession after concession to their surroundings, and in these concessions all progress in life consists. And at last each organism or each alliance of organisms must come to the greatest concession of all, which is called death. The unity in life, then, is not less a fact than is life's great diversity. Whatever emphasis is laid upon the diversity of life, the essential unity of all organisms must not be forgotten.
An examination of the facts in each of the lines of evidence makes it clear that the only reasonable explanation for the existence of a fundamental unity in organic life is the theory of descent i.e., that similarities are due to blood relationship and that differences come from adaptive modifications. The facts adduced from morphology, being the result of researches into the structure of adult animals and plants, lead to a preview of certain principles of adaptation, necessary for their interpretation.
First, it must be noted that some structures are not non-adaptive, that is, do not change to fit changed habits or conditions of life. Such structures or organs are most often found internally. For illustration : a change in the locomotive habit of a bird from that of flying to that of an ostrich is associated with an adaptive modification of loco-motor structures, legs and wings, but not in any striking way is there change in the internal organs. Internal organs may persist. unchanged and hence they offer good guides to classification. On the other hand, external structures are likely to undergo adaptation when habits or conditions of life change. Hence, as Jordan has said, "the inside of an animal tells the real history of its ancestry; the outside tells us only where its ancestors have. been."
In the second place, it must be noted that adaptations to similar conditions may result in superficial resemblances. For example, there is a superficial resemblance between the wing of an insect and the wing of a bird both adaptations to an aerial environment; between the heart of an insect and the heart of a vertebrate animal both adaptations for pumping blood ; between the fin of a fish and the paddle of a whale both adaptive swimming organs, yet the resemblance in these cases does not go deeper than the surface it is one of function only. All such cases of resemblance in function but not in detailed plan of structure are called 'analogies,' and mean nothing more than similarity of environment. Turning to more fundamental resemblances, such as the wing of a bat and the wing of a bird, careful study shows detailed internal as well as external similarities of structure. Such cases are 'homologies.'
On the one hand, then, are found structures which are perfectly analogous and yet in no way homologous : totally different structures are modified to perform the same functions. On the other hand are found structures which are perfectly homologous and yet in no way analogous : the structural elements remain, but are profoundly modified to perform totally different functions. Homology thus means identity of structure which is the result of identity of parentage. It is the stamp of heredity. It means blood relationship. These principles of homology are essential to a correct interpretation of the facts of morphology.
The most striking fact of similar structure among plants and among animals is the existence of a common general plan in any group. Since backboned animals are best known to most readers, they may be taken as an illustration. "All vertebrate animals, and none other," says Le Conte, "have an internal jointed skeleton worked by muscles on the outside. The relation of skeleton and muscle in arthropods is exactly the reverse.
"In all vertebrates, and in none other, the axis of this skeleton is a jointed backbone (vertebral column) inclosing and protecting the nervous centers (cerebrospinal axis). These, therefore, may well be called backboned animals.
"All vertebrates, and none other, have a number of their anterior vertebral joints enlarged and consolidated into a box to form the skull, in order to inclose and protect a similar enlargement of the nervous center, viz., the brain; and also usually, but not always, a number of posterior joints, enlarged and consolidated to form the pelvis, to serve as a firm support to the hind-limbs.
"All vertebrates, and none other, have two cavities, in-closed and protected by the skeleton, viz., the neural cavity above, and the visceral or body cavity below, the vertebral column.
"All vertebrates, with few exceptions, and no other animals, have two and only two pair of limbs. The exceptions are of two kinds, viz.: (a) some lowest fishes, amphioxus and lampreys, which probably represent the vertebrate condition before limbs were acquired ; and (b) degenerate forms like snakes and some lizards, which have lost their limbs by disuse.
"So much concerns the general plan of skeletal structures and is strongly suggestive of in fact it is inexplicable without common origin. But much more re-mains which is not only suggestive, but demonstrative of such origin. By extensive comparison in the taxonomic and ontogenic series, the whole vertebrate structure in all its details in different animals may be shown to be modifications one of another. Sometimes a piece is enlarged, sometimes diminished, or even becomes obsolete; some-times several pieces are consolidated into one ; but, in spite of all these obscurations, corresponding parts usually may be made out."
These remarkable similarities in the common general plan alone are convincing evidences of descent, but attention may be called to a like similarity extending to the details of structure. For example : the wings of a bat (a mammal), a bird and a fossil flying reptile all show the same bones adaptively modified; a series of either fore or hind limbs of a mammal with one toe (horse), two toes (sheep), four toes (hog) and five toes (dog) exhibit a remarkable series of homologies pointing to a five toed sncestor, and any other series of organs of vertebrates would give the same evidence of fundamental resemblances (homologies). For such a series of facts the reader must be referred to special books like Wiedersheim's 'Comparative Anatomy of the Vertebrates,' Romanes's 'Darwin and After Darwin,' and Le Conte s 'Evolution.'
The existence of great similarities in vertebrate structure is not always fully recognised. To the superficial observer the bodies of animals of different classes seem to differ fundamentally in plan, to be entirely different machines, made each for its own purposes, at once, out of hand. Extensive comparison, on the contrary, shows them to be the same, altho the essential identity is obscured by adaptive modifications. The simplest, in fact the only scientific, explanation of the phenomena of vertebrate structure is the idea of a primal vertebrate, modified more and more through successive generations by the necessities of different modes of life.
See, then, the difference between man's mode of working and Nature's. A man having made a steam engine, and desiring to use it for a different purpose from that for which it was first designed and used, will nearly always be compelled to add new parts not contemplated in the original machine. Nature rarely makes new parts never, if she can avoid it but, on the contrary, adapts an old part to the new function. It is as if Nature were not free to use any and every device to accomplish her end, but were conditioned by her own plans of structure; as, indeed, she must be according to the derivation theory. Thus, in the fin of a fish, the fore-paw of a reptile or a mammal, the wing of a bird, and the arm and hand of a man, is found the same part, variously modified for many purposes.
Another striking class of the facts of morphology which admit of scientific explanation only along the line of homology are the thousands of cases of rudimentary or vestigial structures to be found. Throughout both the animal and vegetable kingdoms dwarfed and useless representatives of organs are constantly met with, which in other and allied kinds of animals and plants are of large size and functional utility. Thus, for instance, the unborn whale has rudimentary teeth, which are never destined to cut the gums; and throughout its life this animal retains, in a similarly rudimentary condition, a number of organs which never could have been of use to any kind of creature save a terrestrial quadruped.
Other well-known examples among vertebrates are : Vestiges of hind limbs in certain snakes, reduced wings in the Apteryx and ostriches, rudiments of eyes in cave fishes, hind limbs beneath the skin of whales, the vermiform appendix in man, as well as useless muscles to move the ears and the skin, and also a very much reduced hairy covering over the surface of the body. Wiedersheim has recorded more than one hundred and eighty such structural reminiscences in man.
Now, rudimentary organs of this kind are of such frequent occurrence, that almost every species of organism presents one or more of them usually, indeed, a consider-able number. How, then, are they to be accounted for? Of course the theory of descent with adaptive modification has a simple answer to supply-namely, that when, from changed conditions of life, an organ which was previously useful becomes useless, it will be suffered to dwindle away in successive generations, under the influence of certain natural causes.
On the other hand, the theory of special creation can only maintain that these rudiments are formed for the sake of adhering to an ideal type.
"Now, here again the former theory appears to be triumphant over the latter," says Romanes, "for, without waiting to dispute the wisdom of making dwarfed and use-less structures merely for the whimsical motive assigned, surely if such a method were adopted in so many cases, we should expect that in consistency it would be adopted in all cases. This reasonable expectation, however, is far from being realized. In numberless cases, such as that of the fore-limbs of serpents, no vestige of a rudiment is present. But the vacillating policy in the matter of rudiments does not end here; for it is shown in a still more aggravated form where within the limits of the same natural group of organisms a rudiment is sometimes present and sometimes absent. For instance, altho in nearly all the numerous species of snakes there are no vestiges of limbs, in the Python we find very tiny rudiments of the hind-limbs. Now, is it a worthy conception of Deity that, while neglecting to maintain his unity of ideal in the case of nearly all the numerous species of snakes, he should have added a tiny rudiment in the case of the Python and even in that case should have maintained his ideal very inefficiently, inasmuch as only two limbs, instead of four, are represented ?"
Convincing as are the evidences of descent recorded in the structure of plants and animals, these evidences have been in the past thirty years somewhat over-shadowed by the far more surprising evidences of descent discovered in the development of plant and animal embryos. A dozen volumes would be necessary to present the mass of embryological evidence, but a few salient facts will illustrate the kind of evidence to be deduced from embryology.
Most remarkable of all the principles which have been discovered by embryologists is the 'Recapitulation Doctrine,' which, briefly stated, is that individual development (ontogeny) recapitulates ancestral history (phylogeny). Illustrations quoted from the works of Romanes and Le Conte will make this principle clear. "It is an observable fact," says Romanes, "that there is often a close correspondence between developmental changes as revealed by any chronological series of fossils which may happen to have been preserved, and developmental changes which may be observed during the life history of now existing individuals belonging to the same group of animals. For instance, the successive development of prongs in the horns of deer-like animals, which is so clearly shown in the geological history of this tribe, is closely reproduced in the life-history of existing deer. Or, in other words, the antlers of an existing deer furnish in their development a kind of 'resume,' or recapitulation, of the successive phases whereby the primitive horn was gradually superseded by horns presenting a greater and greater number of prongs in successive species of extinct deer.
"Now, it must be obvious that such a recapitulation in the life-history of an existing animal of developmental changes successively distinctive of sundry allied, tho now extinct species, speaks strongly in favor of evolution. For as it is of the essence of this theory that new forms arise from older forms by way of hereditary descent, we should antecedently expect, if the theory is true, that the phases of development presented by the individual organism would follow, in their main outlines, those phases of development through which their long line of ancestors had passed. The only alternative view is that as species of deer, for instance, were separately created, additional prongs were successively added to their antlers; and yet that, in order to be so added to successive species, every individual deer belonging to later species was required to repeat in his own lifetime the process of successive additions which had previously taken place in a remote series of extinct species. Now I do not deny that this view is a possible view; but I do deny that it is a probable one. According to the evolutionary interpretation of such facts, we can see a very good reason why the life-history of the individual is thus a condensed 'resume' of the life history of its ancestral species. But according to the opposite view no reason can be assigned why such should be the case."
"It is well known," likewise comments Le Conte, "that the embryo or larva of a frog or toad, when first hatched, is a legless, tail-swimming, water-breathing, gill-breathing animal. It is essentially a fish, and would be so classed if it remained in this condition. The fish retains permanently this form, but the frog passes on. Next, it forms first one pair and then another pair of legs ; and meanwhile it begins to breathe also by lungs. At this stage it breathes equally by lungs and by gills i.e., both air and water. Now, the lower forms of amphibians, such as siredon, menobranchus, siren, etc., retain permanently this form, and are therefore called 'perennibranchs,' but the frog still passes on. Then the gills gradually dry up, as the lungs develop, and they now breathe wholly by lungs, but still retain the tail. Now this is the permanent, mature condition of many amphibians, such as the triton, the salamander, etc., which are therefore called 'caducibranchs,' but the frog still passes on. Finally, it loses the tail, or rather its tail is absorbed and its material used in further development, and it becomes a perfect frog, the highest order (anoura) of this class.
"Thus, then, in ontogeny the fish goes no further than the fish stages. The perennibranch passes through the fish stage to the perennibranch amphibian. The caducibranch takes first the fish form, then the perennibranch form, and finally the caducibranch form, but goes no further. Last, the anoura takes first the fish form, then that of the perennibranch, then that of the caducibranch, and finally becomes anoura. Now, this is undoubtedly the order of succession of forms in geological times i.e., in the phylogenic series. Fishes first appeared in the Devonian and Upper Silurian in very reptilian or rather amphibian forms. Then in the Carboniferous, fishes still continuing, there appeared the lowest i.e., most fish like forms of amphibians. These were undoubtedly perennibranchs. In the Permian and Triassic higher forms appeared, which were certainly caducibranch. Finally, only in the Tertiary, so far as we yet know, do the highest form (anoura) appear. The general similarity of the three series is complete.
"It is a curious and most significant fact that the successive stages of the development of the individual in the higher forms of any group (ontogenic series) resemble the stages of increasing complexity of differentiated structure in ascending the animal scale in that group (taxonomic series), and especially the forms and structure of animals of that group in successive geological epochs (phylogenic series). In other words, the individual higher animal in embryonic development passes through temporary stages, which are similar in many respects to permanent or mature conditions in some of the lower forms in the same group.
"Surely this fact is wholly inexplicable except by the theory of derivation or evolution. The embryo of a higher animal of any group passes now through stages represented by lower forms, because in its evolution (phylogeny) its ancestors did actually have these forms. From this point of view the ontogenic series (individual history) is a brief recapitulation, as it were, from memory, of the main points of the philogenic series, or family history. We say brief recapitulation of the main points, because many minor points are dropped out. Even some main points of the earliest stages of the family history may be dropped out of this sort of inherited memory.
"This resemblance between the three series must not, however, be exaggerated. Not only are many steps of phylogeny, especially in its early stages, dropped out in the ontogeny, but, of course, many adaptive modifications for the peculiar conditions of embryonic life are added. But it is remarkable how even these for example, the umbilical cord and placenta of the mammalian embryo are often only modifications of egg organs of lower animals, and not wholly new additions. It is the similarity in spite of adaptive modifications that shows the family history."
But even these recapitulations are not so convincing as are those of many internal structures which are not now of any possible use. In the fishes, the lowest class of vertebrates, gills are the organs of breathing. Water taken into the mouth is ejected from the throat through the gill-slits, which are lined with delicate vascular membranes. Blood circulating through these membranes absorbs oxygen from the water and gives off carbon dioxide gas. Thus in the fishes the gills play the part performed by lungs in higher forms, and are essential organs throughout life. Passing up the scale of animal life, amphibian embryos develop gill-slits and they are present and functionally active for a few days in the early life of certain tadpoles. In all amphibians the slits close soon and the fully developed frogs and salamanders do not use them for breathing. Certain amphibians have gills throughout life, but they are not the gills corresponding to those of fishes.
Now, in interpreting these facts, it should be noted that the embryos of the higher classes, reptiles, birds and mammals, never have access to water and yet in every species whose development is known gill-slits are present in the embryos. For example, in a chick of three to five days' incubation there are four slits on the side of the neck. Likewise there are several gill-slits in human embryos of three to five weeks' development. In all reptiles, birds and mammals the gill-slits, however, are temporary; they serve no function and close long before hatching or birth. The only interpretation which appeals to the biologist as reasonable is that gill-slits in the higher vertebrates are reminiscences of ancestral history and, originally fish organs, they appear regularly in the fish-like stage of every embryo of higher forms which in their development pass through stages comparing in a general way to the adults of lower forms.
There are dozens and dozens of similar cases known to occur in the embryology of vertebrates. The notochord, which is the dorsal stiffening axis in the lowest vertebrate (amphioxus), appears in the embryos of all higher forms. In them it is purely temporary and disappears as the backbone is developed around it. The lungs of amphibia and higher forms develop in the embryo identically with the air bladder of fishes.
In the embryonic development of man and other mammals three pairs of kidneys are formed, only one remaining at birth. The first kidneys (pronephros) develop in the stage when gills and other structures correspond to a fish-embryo stage, and in some of the lowest fishes this pair becomes the permanent kidneys. The second kidneys (mesonephros) correspond to an amphibian reptilian stage, and this pair persists throughout life in the amphibia, in the embryos of which the first kidneys are temporarily present. The third kidneys (metanephros) succeed the first and second in mammalian development and remain throughout life. In the higher forms the first and second kidneys are absolutely useless. The only reasonable explanation of this fact is that the appearance of the first kidneys in amphibian embryos and of the first and second in mammalian embryos is due to this repetition or recapitulation of ancestral history.
If space permitted it would be easy to present abundance of additional evidence to the same effect from the development of the skeleton, the skull, the brain, the sense-organs, and, in short, of every constituent part of the vertebrate organization. Even without any anatomical dissection, the similarity of all vertebrate embryos at comparable stages of development admits of being strikingly shown, if the embryos are merely placed one beside the other. Here, for instance, are the embryos of a fish, a salamander, a tortoise, a bird, and four different mammals. In each case three comparable stages of development are represented. Now if the series is read horizon-tally, it can be seen that there is very little difference between the eight animals at the earliest of the three stages represented all having fish like tails, gill slits, and so on. In the next stage further differentiation has taken place, but it will be observed that the limbs are still so rudimentary that even in the case of man they are considerably shorter than the tail. But in the third stage the distinctive characters are well marked.
So far examples have been drawn entirely from the vertebrate animals ; it should be pointed out, however, that there is a repetition of just the same kind of evidence in all the other groups of animals, and as well in the plant kingdom. When all his mass of evidence is taken together, it is not to be wondered at that the science of comparative embryology should be considered as the principal witness to the theory of descent.
But why should ontogeny repeat the steps of phylogeny? Professor Le Conte replies that "the general answer is doubtless to be found in the law of heredity that wonderfuI law, so characteristic of living things. We have compared it to a brief recapitulation from memory the minor points, especially if they be also early, dropping out. But . can we not explain it further? It is probable that we find a more special explanation in 'the law of acceleration,' first brought forward by Prof. Cope. By the law of heredity each generation repeats the form and structure of the previous, and in the order in which they successively appeared. But there is a tendency for each successively appearing character to appear a little earlier in each successive generation; and by this means time is left over for the introduction of still higher new characters. Thus, characters which were once adult are pushed back to the young, and then still back to the embryo, and thus place and time are made for each generation to push on still higher. The law of acceleration is a sort of young Americanism in the animal kingdom. If our boys acquire knowledge and character similar to that of adults of a. few generations back, they will have time while still young and plastic to press forward to still higher planes."
These similarities or homologies of embryology appeal to the thinker, as has been said, as explainable only on the theory of descent. Agassiz, however, held an opposing view. His statements on this point are classical in biological literature. He says : "If we now pass to the highest type of the Animal Kingdom, the Vertebrates, there is no lack of evidence to show the identity in their mode of development, as well as the striking resemblance of the young in their earliest stages of growth. The young Fish, the young Reptile, the young Bird, the young Mammal, resemble one another to an astonishing degree, while they have not one feature in their mode of growth which recalls either the Articulate, the Mollusk, or the Radiate. It is, therefore, not true, tho so often stated, that in their development the higher animals pass successively through the condition of all the lower ones; while it is emphatically true that in each of the four great branches of the Animal Kingdom there is a common mode of development. It is equally true that in certain features the higher classes of each branch in their younger condition recall the characteristic features of the lower ones, tho each class has its own structural character, and early diverges from the common starting-point.
"Indeed, modern Embryology leads at once to the consideration of the most occult problem, as to the origin of animals, suggested by these comparisons. What do these resemblances mean, from some of which we shrink as unnatural and even revolting? If we put a material inter pretation upon them, and believe that even Man himself has been gradually developed out of a Fish, they are repugnant to our better nature. But looked at in their intellectual significance, they truly reveal the unity of the organic conception of which Man himself is a part, and mark not only the incipient steps in its manifestation, but also, with equal distinctness, every phase in its gradual realization. They mean that when the first Fish was called into existence, the Vertebrate type existed as a whole in the creative thought, and the first expression of it embraced potentially all the organic elements of that type, up to Man himself. To me the fact that the embryonic form of the highest Vertebrate recalls in its earlier stage the first representatives of its type in geological times and its lowest representatives at the present day, speaks only of an ideal relation, existing not in the things themselves, but in the mind that made them.
"It is true that the naturalist is sometimes startled at these transient resemblances of the young among the higher animals in one type to the adult condition of the lower animals in the same type; but it is also true that he finds each one of the primary divisions of the Animal Kingdom bound to its own norm of development, which is absolutely distinct from that of all the others; it is also true, that, while he perceives correspondences between the early phases of the higher animals and the mature state of the lower ones, he never sees any one of them diverge in the slightest dgree from its own structural character never sees the lower rise by a shade beyond the level which is permanent for the group to which it belongs never sees the higher ones stop short of their final aim, either in the mode or the extent of their trans-formation.
"I cannot repeat too emphatically, that there is not a single fact in Embryology to justify the assumption that the laws of development, now known to be so precise and definite for every animal, have ever been less so, or have ever been allowed to run into each other. The philosopher's stone is no more to be found in the organic than the inorganic world; and we shall seek as vainly to transform the lower animal types into the higher ones by any of our theories, as did the alchemists of old to change the baser metals into gold."
Thus did Agassiz reject the theory of descent in the face of the most overwhelming evidence. His view that similarity of development is simply an expression of type in creative thought appeals to scientists today as absurd. Agassiz was the last prominent naturalist who could believe that such things as gill-slits were put into embryos of higher forms in order to adhere to an ideal plan "in the mind that made them."
The paragraphs quoted above from Agassiz suggest that it may not be known what hidden reasons there may have been for the creation of animals with such remarkable similarities in development or adult structure. But as Romanes has pointed out, a man "may quite legitimately say, Assuming that the theory of special creation is true, it is not for us to anticipate the form or method of the process. But where the question is as to whether or not the theory is true, it becomes a mere begging of this question to take refuge in the argument from ignorance, or to represent in effect that there is no question to be discussed. And if, when the form or method is investigated, it be found everywhere charged with evidence in favor of the theory of descent, the case becomes the same as that of a supposed revelation, which has been discredited by finding that all available evidence points to a natural growth.
"In short, the argument from ignorance is in any case available only as a negative foil against destructive criticism. In no case has it any positive value or value of a constructive kind. Therefore, if a theory on any subject is destitute of positive evidence, while some alternative theory is in possession of such evidence, the argument from ignorance can be of no logical use to the former, even tho it may be of such use to the latter. For it is only the possession of positive evidence which can furnish a logical justification of the argument from ignorance. In the absence of such evidence even the negative value of the argument disappears and it then implies nothing more than the gratuitous assumption of a theory."