The Principles of Biology Vol. I/Chapter III.6
CHAPTER VI.
THE ARGUMENTS FROM MORPHOLOGY.
§ 133. Leaving out of consideration those parallelisms among their modes of development which characterize organisms belonging to each group, that community of plan which exists among them when mature is extremely remarkable and extremely suggestive. As before shown ( § 103), neither the supposition that these combinations of attributes which unite classes are fortuitous, nor the supposition that no other combinations were practicable, nor the supposition of adherence to pre-determined typical plans, suffices to explain the facts. An instance will best prepare the reader for seeing the true meaning of these fundamental likenesses.
Under the immensely-varied forms of insects, greatly elongated like the dragon-fly or contracted in shape like the lady-bird, winged like the butterfly or wingless like the flea, we find this character in common—there are primarily seventeen segments.[1] These segments may be distinctly marked or they may be so fused as to make it difficult to find the divisions between them, but they always exist. What now can be the meaning of this community of structure throughout the hundred thousand kinds of insects filling the air, burrowing in the earth, swimming in the water? Why under the down-covered body of a moth and under the hard wing-cases of a beetle, should there be discovered the same number of divisions? Why should there be no more somites in the Stick-insect, or other Phasmid a foot long, than there are in a small creature like the louse? Why should the inert Aphis and the swift-flying Emperor-butterfly be constructed on the same fundamental plan? It cannot be by chance that there exist equal numbers of segments in all these multitudes of species. There is no reason to think it was necessary, in the sense that no other number would have made a possible organism. And to say that it is the result of design—to say that the Creator followed this pattern throughout, merely for the purpose of maintaining the pattern—is to assign an absurd motive. No rational interpretation of these and countless like morphological facts, can be given except by the hypothesis of evolution; and from the hypothesis of evolution they are corollaries. If organic forms have arisen from common stocks by perpetual divergences and re-divergences—if they have continued to inherit, more or less clearly, the characters of ancestral races; then there will naturally result these communities of fundamental structure among creatures which have severally become modified in multitudinous ways and degrees, in adaptation to their respective modes of life. To this let it be added that while the belief in an intentional adhesion to a pre-determined pattern throughout a whole group, is negatived by the occurrence of occasional deviations from the pattern; such deviations are reconcilable with the belief in evolution. As pointed out in the last chapter, ancestral traits will be obscured more or less according as the superposed modifications of structure, have or have not been furthered by the conditions of life and development to which the type has been subjected.
§ 134. Besides these wide-embracing and often deeply-hidden homologies, which hold together different animals, there are the scarcely-less significant homologies between different organs of the same animal. These, like the others, are obstacles to the supernatural interpretations and supports of the natural interpretation.
One of the most familiar and instructive examples is furnished by the vertebral column. Snakes, which move sinuously through and over plants and stones, obviously need a segmentation of the bony axis from end to end; and inasmuch as flexibility is required throughout the whole length of the body, there is advantage in the comparative uniformity of this segmentation. The movements would be impeded if, instead of a chain of vertebræ varying but little in their lengths, there existed in the middle of the series some long bony mass that would not bend. But in the higher Vertebrata, the mechanical actions and reactions demand that while some parts of the vertebral column shall be flexible, other parts shall be inflexible. Inflexibility is specially requisite in that part of it called the sacrum; which, in mammals and birds, forms a fulcrum exposed to the greatest strains the skeleton has to bear. Now in both mammals and birds, this rigid portion of the vertebral column is not made of one long segment or vertebra, but of several segments fused together. In man there are five of these confluent sacral vertebræ; and in the ostrich tribe they number from seventeen to twenty. Why is this? Why, if the skeleton of each species was separately contrived, was this bony mass made by soldering together a number of vertebræ like those forming the rest of the column, instead of being made out of one single piece? And why, if typical uniformity was to be maintained, does the number of sacral vertebræ vary within the same order of birds? Why, too, should the development of the sacrum be by the round-about process of first forming its separate constituent vertebræ, and then destroying their separateness? In the embryo of a mammal or bird, the central element of the vertebral column is, at the outset, continuous. The segments that are to become vertebræ, arise gradually in the adjacent mesoderm, and enwrap this originally-homogeneous axis or notochord. Equally in those parts of the spine which are to remain flexible, and in those parts which are to grow rigid, these segments are formed; and that part of the spine which is to compose the sacrum, having acquired this segmental structure, loses it again by coalescence of the segments. To what end is this construction and re-construction? If, originally, the spine in vertebrate animals consisted from head to tail of separate moveable segments, as it does still in fishes and some reptiles—if, in the evolution of the higher Vertebrata, certain of these moveable segments were rendered less moveable with respect to one another, by the mechanical conditions they were exposed to, and at length became relatively immovable; it is comprehensible why the sacrum formed out of them, should continue ever after to show its originally-segmented structure. But on any other hypothesis this segmented structure is inexplicable. "We see the same law in comparing the wonderfully complex jaws and legs in crustaceans," says Mr. Darwin: referring to the fact that those numerous lateral appendages which, in the lower crustaceans, most of them serve as legs, and have like shapes, are, in the higher crustaceans, some of them represented by enormously-developed claws, and others by variously-modified foot-jaws. "It is familiar to almost every one," he continues, "that in a flower the relative position of the sepals, petals, stamens, and pistils, as well as their intimate structure, are intelligible on the view that they consist of metamorphosed leaves arranged in a spire. In monstrous plants we often get direct evidence of the possibility of one organ being transformed into another; and we can actually see in embryonic crustaceans and in many other animals, and in flowers, that organs, which when mature become extremely different, are at an early stage of growth exactly alike." ... "Why should one crustacean, which has an extremely complex mouth formed of many parts consequently always have fewer legs; or conversely, those with many legs have simpler mouths? Why should the sepals, petals, stamens, and pistils in any individual flower, though fitted for such widely-different purposes, be all constructed on the same pattern?"
To these and countless similar questions, the theory of evolution furnishes the only rational answer. In the course of that change from homogeneity to heterogeneity of structure displayed in evolution under every form, it will necessarily happen that from organisms made up of numerous like parts, there will arise organisms made up of parts more and more unlike: which unlike parts will nevertheless continue to bear traces of their primitive likeness.
§ 135. One more striking morphological fact, near akin to some of the facts dwelt on in the last chapter, must be here set down—the frequent occurrence, in adult animals and plants, of rudimentary and useless organs, which are homologous with organs that are developed and useful in allied animals and plants. In the last chapter we saw that during the development of embryos, there often arise organs which disappear on being replaced by other organs discharging the same functions in better ways; and that in some cases, organs develop to certain points and are then re-absorbed without performing any functions. Very generally, however, the partially-developed organs are retained throughout life.
The osteology of the higher Vertebrata supplies abundant examples. Vertebral processes which, in one tribe, are fully formed and ossified from independent centres, are, in other tribes, mere tubercles not having independent centres of ossification. While in the tail of this animal the vertebræ are severally composed of centrum and appendages, in the tail of that animal they are simple osseous masses without any appendages; and in another animal they have lost their individualities by coalescence with neighbouring vertebræ into a rudimentary tail. From the structures of the limbs analogous facts are cited by comparative anatomists. The undeveloped state of certain metacarpal bones, characterizes whole groups of mammals. In one case we find the normal number of digits; and, in another case, a smaller number with an atrophied digit to make out the complement. Here is a digit with its full number of phalanges; and there a digit of which one phalange has been arrested in its growth. Still more remarkable are the instances of entire limbs being rudimentary; as in certain snakes, which have hind legs hidden beneath the integument. So, too, is it with dermal appendages. Some of the smooth-skinned amphibia have scales buried in the skin. The seal, which is a mammal considerably modified in adaptation to an aquatic life, and which uses its feet mainly as paddles, has toes that still bear external nails; but the manatee, which is a much more transformed mammal, has nailless paddles which, when the skin is removed, are said, by Humboldt, to display rudimentary nails at the ends of the imbedded digits. Nearly all birds are covered with developed feathers, severally composed of a shaft bearing fibres, each of which, again, bears a fringe of down. But in some birds, as in the ostrich, various stages of arrested development of the feathers may be traced: between the unusually-elaborated feathers of the tail, and those about the beak which are reduced to simple hairs, there are transitions. Nor is this the extreme case. In the Apteryx we see the whole of the feathers reduced to a hair-like form. Again, the hair which commonly covers the body in mammals is, over the greater part of the human body almost rudimentary, and is in some parts reduced to mere down—down which nevertheless proves itself to be homologous with the hair of mammals in general, by occasionally developing into the original form. Numerous cases of aborted organs are given by Mr. Darwin, of which a few may be here added. "Nothing can be plainer," he remarks, "than that wings are formed for flight, yet in how many insects do we see wings so reduced in size as to be utterly incapable of flight, and not rarely lying under wing-cases, firmly soldered together?" ... "In plants with separated sexes, the male flowers often have a rudiment of a pistil; and Kölreuter found that by crossing such male plants with an hermaphrodite species, the rudiment of the pistil in the hybrid offspring was much increased in size; and this shows that the rudiment and the perfect pistil are essentially alike in nature." And then, to complete the proof that these undeveloped parts are marks of descent from races in which they were developed, there are not a few direct experiences of this relation. "We have plenty of cases of rudimentary organs in our domestic productions—as the stump of a tail in tailless breeds—the vestige of an ear in earless breeds—the re-appearance of minute dangling horns in hornless breeds of cattle." (Origin of Species, 1859, pp. 451, 454.)
Here, as before, the teleological doctrine fails utterly; for these rudimentary organs are useless, and occasionally even detrimental; as is the appendix vermiformis, in Man—a part of the cæcum which is of no value for the purpose of absorption but which, by detaining small foreign bodies, often causes severe inflammation and death. The doctrine of typical plans is equally out of court; for while, in some members of a group, rudimentary organs completing the general type are traceable, in other members of the same group such organs are unrepresented. There remains only the doctrine of evolution; and to this, these rudimentary organs offer no difficulties. On the contrary, they are among its most striking evidences.
§ 136. The general truths of morphology thus coincide in their implications. Unity of type, maintained under extreme dissimilarities of form and mode of life, is explicable as resulting from descent with modification; but is otherwise inexplicable. The likenesses disguised by unlikenesses, which the comparative anatomist discovers between various organs in the same organism, are worse than meaningless if it be supposed that organisms were severally framed as we now see them; but they fit in quite harmoniously with the belief that each kind of organism is a product of accumulated modifications upon modifications. And the presence, in all kinds of animals and plants, of functionally-useless parts corresponding to parts that are functionally-useful in allied animals and plants, while it is totally incongruous with the belief in a construction of each organism by miraculous interposition, is just what we are led to expect by the belief that organisms have arisen by progression.
- ↑ Early in our friendship (about 1855) Prof. Huxley expressed to me his conviction that all the higher articulate animals have twenty segments or somites. That he adhered to this view in 1880, when his work on The Crayfish was published, is shown by his analysis there given of the twenty segments existing in this fluviatile crustacean; and adhesion to it had been previously shown in 1877, when his work on The Anatomy of Invertebrated Animals was published. On p. 398 of that work he writes:—"In the abdomen there are, at most, eleven somites, none of which, in the adult, bear ambulatory limbs. Thus, assuming the existence of six somites in the head, the normal number of somites in the body of insects will be twenty, as in the higher Crustacea and Arachnida." To this passage, however, he puts the note:—"It is open to question whether the podical plates represent a somite; and therefore it must be recollected that the total number of somites, the existence of which can be actually demonstrated in insects, is only seventeen, viz., four for the head, three for the thorax, and ten for the abdomen." I have changed the number twenty, which in the original edition occurred in the text, to the number seventeen in deference to suggestions made to me; though I find in Dr. Sharp's careful and elaborate work on the Insecta, that Viallanes and Cholodkovsky agree with Huxley in believing that there are six somites in the insect-head. The existence of a doubt on this point, however, does not essentially affect the argument, since there is agreement among morphologists respecting the constancy of the total number of somites in insects.