CHAPTER VII.

PROOFS FROM EMBRYOLOGY, OR COMPARISON IN THE ONTOGENIC SERIES.

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. To give one example for the sake of clearness: The frog, in its early stages of embryonic development, is essentially a fish, and if it stopped at this stage would be so called and classed. But it does not stop; for this is a temporary stage, not a permanent condition. It passes through the fish stage and through several other temporary stages, which we shall explain hereafter, and onward to the highest condition attained by amphibians. Now, if we could trace perfectly the successive forms of amphibians, back through the geological epochs to their origin in the Carboniferous, the resemblance of this series to the stages of the development of a frog would doubtless be still closer. Surely this fact, if it be a 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.

We will now illustrate by a few striking examples.

We can not do better than to take, again, as our first example, the development of tailless amphibians, and dwell a little more upon it:

=1. Ontogeny of Tailless Amphibians.=--It is well known 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. This is shown in the diagram, which must be read upward, line by line.

Now, this is undoubtedly the order of succession of forms in geological times--i. e., in the phylogenic series. This series is indicated by the arrows in the diagram. 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. If we read the diagram horizontally, we have the ontogenic series; if diagonally with the arrows, we have both the taxonomic and the phylogenic series.

=2. Aortic Arches.=--But some will, perhaps, say that these stages in the ontogeny are only examples of adaptive modifications--like modifications for like conditions of life--and had better be accounted for in this way, without reference to family history. We will, therefore, take another example, which can not be thus accounted for--an example in which there is no possible use _now_ for the peculiar form or structure which we find. For this purpose we take the case of the _course of circulation in vertebrates_.

If one examines the large vessels _going out_ from the heart of a lizard, he will find _six aortic arches_--i. e., three on each side. These all unite below to form the one descending abdominal aorta. This is shown in the accompanying figure (Fig. 36), in which _a a′ a″_ and _b b′ b″_ are the six arches. Now, there is no conceivable use in having so many aortic arches. We know this, because there is but one in birds and mammals, and the circulation is as effective, nay, much more effective in these than in reptiles. The explanation of this anomaly is revealed at once as soon as we examine the circulation of a _fish_, which is shown in the accompanying figure (Fig. 37). The multiplication of the aortic arches is here, of course, necessary, for they are the _gill-arches_. The whole of the blood passes through these arches, to be aërated in the gill-fringes. The use of this peculiar structure is here obvious enough. If a lizard were ever a fish, and afterward turned into a lizard, changing its gill-respiration for lung-respiration, then, of course, the useless gill-arches would remain to tell the story. Now, although a lizard never was a fish, in its _individual_ history or ontogeny, it was a fish in its family history or phylogeny, and therefore it yet retains, by heredity, this curious and _useless_ structure as evidence of its ancestry.

That this is the true explanation is demonstrated by the fact that in amphibians this very change actually takes place before our eyes in the _individual history_. We have already seen that the individual frog, in its tadpole state, is a gill-breather. It has therefore its gill-arches (Fig. 38), three on each side, like a fish, and for the same reason, viz., the aëration of the blood. But when its gills dry up and lung-respiration is established, its now useless gill-arches still remain as aortic arches, to attest their previous condition (Fig. 39). Now, the lizard undoubtedly came from an air-breathing, tailed amphibian, and therefore inherited this form of arterial distribution. In both lizard and amphibian the ultimate cause is an origin from fishes, in which such arches are obviously necessary. The diagrams, Figs. 38 and 39, are illustrations somewhat idealized, showing the manner in which the change actually takes place in air-breathing amphibians. Fig. 38 represents the tadpole stage, and Fig. 39 the mature condition. In the former the gills are mostly external, G G′, etc., but also internal, _g g′_, as in the fish. Observe in this condition the small connecting vessels, _c c′_. When the external gills dry up, these are enlarged, and the whole of the blood passes through them, as shown in Fig. 39. It is seen, also, in Fig. 38, that a small branch, _p_, goes from the lower gill-arches to the yet rudimentary lung, _l_. When the gill-fringes have disappeared, the whole of the blood of the lower arch goes through the now enlarged pulmonary branch to the lungs, L, now in full activity, and the remainder of this arch disappears, as shown by the dotted lines in Fig. 39.

The change which actually took place in the family history of the lizard probably differed from the above only in being more simple, the gills being only internal like the fish. The external gills complicate the process a little in the case of the frog, but the principle is precisely the same.

As already explained (pages 82-85), the large gap between fishes and reptiles, as regards mode of respiration, is completely filled both in the taxonomic series--i. e., in ganoids, dipnoi, and the mature condition of the different orders of amphibians--and in the ontogeny of the higher amphibians. Now, we add that the same is true of the arterial distribution. We have just traced the change in the ontogeny of the frog, but the steps of the same change are traceable in passing from the typical fish (teleosts), through dipnoi and amphibians to reptiles. Thus, again, the phylogeny, the taxonomy, and the ontogeny, are in complete accord.

But the argument for evolution does not stop here. If birds and mammals have come from reptiles, and therefore from fishes, we may expect to find some evidences of the same kind still lingering in the great arteries. And such we do find. It is a most curious and significant fact that, in the early embryonic condition of birds and mammals, including man himself, we find on each side of the neck several gill-slits, each with its gill-arch, and therefore _several aortic arches on each side_, precisely similar to what we have already described. These arches are subsequently, some of them, obliterated; some modified to form the one aortic arch, and some of them still more modified to form the other great arteries coming from the heart to supply the head and forelimbs.

This is so beautiful and convincing an example, and one so generally unfamiliar, to even intelligent persons, not especially acquainted with biology, that it is best to explain it more fully. In Fig. 40 we give a mammalian heart and outgoing vessels, very slightly modified, so as to suggest the process of change. In Fig. 41 we give an ideal diagram representing the primitive aortic arches as they exist in the embryo of mammals, birds, and reptiles. It represents, also, substantially, the arches as they exist in the _mature_ condition in the most reptilian fishes (dipnoi) and in some sharks, except that in these the arches are of course furnished with gill-fringes. We will use this figure, therefore, to represent both the embryonic condition of air-breathing vertebrates and the mature condition of some fishes. The place of the heart is indicated by the dotted circle. Fig. 36, on page 134, shows what these arches become in reptiles (lizard). It is seen that the two upper arches on each side are obliterated, as indeed they already are in some teleost fishes. Fig. 42 shows what they become in birds. The two upper arches are, of course, obliterated. The others are all modified, each in a manner which may be readily understood by comparison with Fig. 41. Finally, Fig. 43 shows what they become in mammals and in man. In the bird (Fig. 42) the first pair of arches become the two pulmonary arteries as they do also in the lizard. The second pair become on the right side (left of the diagram) the aortic arch, on the left side (right of the diagram) the left subclavian, _s′c′_ (the right subclavian, _sc_, is a branch of the aortic arch). The third pair become carotids, _cc_, while the fourth and fifth, as already said, are aborted. In the mammal (Fig. 43), on the left side (right of the diagram) the first arch becomes the pulmonary artery, _p_. In the fœtus the continuation of this arch forms the ductus arteriosus, which is afterward obliterated, as shown in the dotted line. The second arch becomes the aortic arch, the third the left exterior carotid. On the right side (left of the diagram) the first arch becomes aborted; the second, the right subclavian, _sc_ (the _left_ subclavian, _s′c′_, is a branch of the aortic arch); and the third, the right carotid. Nos. 4 and 5, on both sides, as usual, are aborted.

See, then, the gradual process of change through the whole vertebrate department. In the lowest of all vertebrates, if vertebrate it may be called (for what corresponds to its backbone is an unjointed, fibrous cord), the amphioxus or lancelet (Fig. 44), there are about forty gill-arches on each side. As we rise in the scale of fishes these are reduced in number. In the lamprey, there are seven; in the sharks, usually five; in ordinary fishes (teleosts), there are four or sometimes only three on each side, the others being aborted. Thus far the change is only by diminution of number in accordance with a law universal in biology, that decrease in the number of identical organs is evidence of advance in the grade of organization, provided that it be associated with more perfect structure of the organ. The further change is one of adaptive modification. In some reptiles (lizard) the three gill-arches on each side all retain the form of aortic arches; in some reptiles only two retain this form. In birds and mammals only one arch is retained, in the form of aortic arch, the others being modified to form the great outgoing vessels of the heart, or else aborted. It may be well to observe that in birds the one aortic arch turns to the right, while in mammals it turns to the left. This is positive evidence that mammals could not have come from birds, nor _vice versa_. They both came from reptiles, and, of the many reptilian arches, a right one was retained by the bird branch, and a left one by the mammalian.

In all the figures illustrating this subject, we have left out the great _incoming_ vessels or veins, because we are not here concerned with them, they not being transformed gill-arches.

Last of all, it may be well to stop a moment to show the cogency of this evidence. If it were a question of the origin of some structure not only useful (for all structures selected by Nature must be useful) but the _best imaginable_, like the eye or the ear, for example; then, if _we examined only the highest form_ or _the finished article_, there are two ways in which it is possible to explain the adaptive structure. We may either suppose that it was made at once out of hand, by some intelligent contriver; or else that it was slowly made by a process of evolution, becoming more and more perfect by a selection of only the most perfect from generation to generation. But in the case of the six aortic arches of the lizard, we are shut up to the one explanation only, viz., by slow process of evolution. One arch is all that is necessary, as is plainly shown by the use of only one in the more perfect circulation of birds and mammals. If the thing were done out of hand, unconditioned by the previous structure in fishes, to have made six was surely but a bungling piece of work.

=3. Vertebrate Brain.=--Another excellent example is the structure of the vertebrate brain. The brain of an average fish is represented in Fig. 45. It consists of four or five swellings, or ganglia, strung along, one beyond another. Commencing behind, these are, first, the medulla, _m_; then the cerebellum, _cb_; then the optic lobes, _ol_; then the cerebrum and thalamus combined, _cr_; and last, the olfactive lobes, _of_. Of these, it will be observed, the optic lobe is the largest in the brain of the fish (Fig. 45). In the brain of the reptile (Fig. 46) we have the same serial arrangement, of the same parts, only that the cerebrum has now become the dominant part instead of the optic lobes. In the average bird (Fig. 47) the cerebrum has grown so large that it extends backward, and partly covers the optic lobes. In the lower mammals (marsupials), the brain is much the same in this respect, as in birds--i. e., the cerebrum only partly covers the optic lobes, so that, looked at from above, the whole series of ganglia are still visible. But in the average mammal (Fig. 48) the cerebrum is so enlarged that it covers entirely the optic lobes and encroaches on the cerebellum behind and the olfactive lobes in front. In some monkeys, indeed, the cerebellum is nearly or even quite covered. Finally, in man (Fig. 49), the cerebrum has grown so enormously that it covers every other part and completely conceals them from view when the brain is looked at from above. In front it not only covers but has grown far beyond the olfactive lobes; behind it extends beyond and overhangs the cerebellum; on the sides it overhangs and covers all. Looked at from above, nothing is seen but this great ganglion. The ideal section (Fig. 50) represents all these stages diagrammatically in one figure. After what has been said, the figure will be readily understood.

Now, it is a most remarkable fact that substantially these same stages, which are permanent conditions in the taxonomic series, are passed through as transient stages in the embryonic development of the human brain, and in the order given above. The very early condition of the human brain is represented in Fig. 51. It is evidently nothing more than the intercranial continuation of the spinal cord, enlarged a little into three swellings or ganglia. These are the early representatives of the medulla, the optic lobes, and the thalamus; which last may be regarded as the basal and most fundamental part of the cerebrum. This stage may be regarded as lower than that of the ordinary fish. I have called it, therefore, the _sub-fish stage_. The cerebellum is a subsequent outgrowth from the medulla, as is the cerebrum and olfactive lobes from the thalamus. Fig. 52 may be said, therefore, to represent fairly the fish-stage. Henceforward the principal growth is in the cerebrum and cerebellum, both of which are subsequent outgrowths of the original simple ganglia, the medulla, and the thalamus. The cerebrum especially increases steadily in relative size, first becoming larger than but not covering the optic lobes (Fig. 53). This represents the reptilian stage. Next, by further growth, it covers partly the optic lobes (Fig. 54). This may be called the bird-stage. Then it covers wholly the optic lobes, and encroaches on the cerebellum behind and olfactive lobes in front (Fig. 55). This is the mammalian stage. Finally, it covers and overhangs all, and thus assumes the human stage (Fig. 56).

We have spoken thus far only of relative _size_; but progressive changes take place also in complexity of structure--i. e., in the depth and number of convolutions of the cerebrum and cerebellum. The cerebrums of fish, of reptile, bird, and lower mammals are smooth. About the middle of the mammalian series it begins to be convoluted. These convolutions become deeper and more numerous as we go upward in the scale, until they reach the highest degree in the human brain. The object of these inequalities is to increase the surface of gray matter--i. e., the extent of the force-generating as compared with the force-transmitting part of the brain, or battery as compared with conducting-wire. Now, in embryonic development the human brain passes also through these stages of increasing complexity of organization. Here also the ontogenic is similar to the taxonomic series.

Now, why should this peculiar order be observed in the building of the individual brain? We find the answer, the only conceivable scientific answer to this question, in the fact that _this is the order of the building of the vertebrate brain by evolution_ throughout geological history. We have already seen that fishes were the only vertebrates living in the Devonian times. The first form of brain, therefore, was that characteristic of that class. Then reptiles were introduced; then birds and marsupials; then true mammals; and, lastly, man. The different styles of brains characteristic of these classes were, therefore, successively made by evolution from earlier and simpler forms. In phylogeny this order was observed because these successive forms were necessary for perfect adaptation to the environment at each step. In taxonomy we find the same order, because, as already explained (page 11), every stage of advance in phylogeny is still represented in existing forms. In ontogeny we have still the same order, because ancestral characteristics are inherited, and family history recapitulated in the individual history.

But not only is this order found in the evolution of the whole vertebrate department, but something of the same kind is found also in the evolution of _each class_. The earliest reptiles, the earliest birds, and the earliest mammals had smaller and less perfectly organized brains than their nearest congeners of the present day. This is shown in the accompanying figures (Figs. 57 and 58). To carry out one example more perfectly: In the history of the horse family, in connection with the changes of skeletal structure already described (page 108), we have also corresponding changes in the size and structure of the brain; _pari passu_ with the improvement of the mechanism we have also increased engine-power and increased muscular energy and therefore increased activity and grace. The brain of a modern horse, though not very large, is remarkable for the complexity of its convolutions. The great energy, activity, and nervous excitability of the horse are the result of this structure.

=Cephalization.=--Thus, in going up the phylogenic, the taxonomic, or the ontogenic series, we find a gradual process of development headward, brainward, cerebrumward; or, more generally, we might say that in all organic evolution we find an increasing dominance of the higher over the lower, and of the highest over all. For example, in the lowest plane of either series we find first the different systems imperfectly or not at all differentiated. Then, as differentiation of these progress, we find an increased dominance of the highest system--the _nervous system_; then in the nervous system, the increasing dominance of its highest part--the _brain_; then in the brain the increasing dominance of its highest ganglion--the _cerebrum_; and, lastly, in the cerebrum the increasing dominance of its highest substance--the exterior gray matter--as shown by the increasing number and depth of the convolutions. This whole process may be called _cephalization_.

Shall the process stop here? When evolution is transferred from the animal to the human plane, from the physiological to the psychical, from the involuntary and necessary to the voluntary and free, shall not the same law hold good? Yes! all social evolution, all culture, all education, whether of the race or the individual, must follow the same law. All _psychical advance is a cephalization_--i. e., an increasing dominance of the higher over the lower and of the highest over all; of the mind over the body, and in the mind of the higher faculties over the lower; and, finally, the subordination of the whole to the highest moral purpose.

=4. Fish-Tails.=--Still another and last example: It has long been noticed that there are among fishes two styles of tail-fins. These are the even-lobed, or homocercal (Fig. 59), and the uneven-lobed, or heterocercal (Fig. 60). The one is characteristic of ordinary fishes (teleosts), the other of sharks and some other orders. In _structure_ the difference is even more fundamental than in _form_. In the former style the backbone stops abruptly in a series of short, enlarged joints, and thence sends off rays to form the tail-fin (Fig. 59, B); in the latter the backbone runs through the fin to its very point, growing slenderer by degrees, and giving off rays above and below from each joint, but the rays on the lower side are much longer (Fig. 60, B). This style of fin is, therefore, _vertebrated_, the other _non-vertebrated_. Figs. 59 and 60 show these two styles in form and structure. But there is still another style found only in the lowest and most generalized forms of fishes. In these the tail-fin is vertebrated and yet symmetrical. This style is shown in Fig. 61, A and B.

Now, in the development of a teleost fish (Fig. 58), as has been shown by Alexander Agassiz,[24] the tail-fin is first like Fig. 61; then becomes heterocercal, like Fig. 60; and, finally, becomes homocercal like Fig. 59. Why so? Not because there is any special advantage in this succession of forms; for the changes take place either in the egg or else in very early embryonic states. The answer is found in the fact that _this is the order of change in the phylogenic series_. The earliest fish-tails were either like Fig. 61 or Fig. 60; never like Fig. 59. The earliest of all were almost certainly like Fig. 61; then they became like Fig. 60; and, finally, only much later in geological history (Jurassic or Cretaceous), they became like Fig. 59. This order of change is still retained in the embryonic development of the last introduced and most specialized order of existing fishes. The family history is repeated in the individual history.

Similar changes have taken place in the form and structure of birds’ tails. The earliest bird known--the Jurassic Archæopteryx--had a long reptilian tail of twenty-one joints, each joint bearing a feather on each side, right and left (Fig. 62). In the typical modern bird, on the contrary, the tail-joints are diminished in number, shortened up, and enlarged, and give out long feathers, fan-like, to form the so-called tail (Fig. 63). The Archæopteryx’ tail is _vertebrated_, the typical bird’s _non-vertebrated_. This shortening up of the tail did not take place at once, but gradually. The Cretaceous birds, intermediate in time, had tails intermediate in structure. The Hesperornis of Marsh had twelve joints. At first--in Jurassic--the tail is fully a half of the whole vertebral column. It then gradually shortens up until it becomes the aborted organ of typical modern birds. Now, in embryonic development, the tail of the modern typical bird _passes through all these stages_. At first the tail is nearly one half the whole vertebral column; then, as development goes on, while the rest of the body grows, the growth of the tail stops, and thus finally becomes the aborted organ we now find. The ontogeny still passes through the stages of the phylogeny. The same is true of all tailless animals. The frog is tailed in the larval condition, because its ancestors were tailed amphibians. Even man himself is endowed with a much more considerable tail, viz., eight or nine joints, in his early embryonic condition.[25]

We have taken all our examples from vertebrates, but quite as many and as good examples might be found among articulates. Insects, in the larval state, are worm-like in form. Hence it is probable that the earliest progenitors of this class were worm-like. Again, some insects have aquatic larvæ. The progenitors of these--in fact, of all insects--were probably aquatic. Crabs, in a larval condition, are long-tailed, and we know that the long-tailed crustaceans (Macrourans) preceded the short-tailed (Brachyourans). Water-breathing animals preceded air-breathers; the same is true in the ontogeny of the frog, of many insects, and, we might add, even of mammals. For the breathing of the _fœtus in utero_ is essentially by exposure of fœtal blood to the oxygenated blood of the mother in a sort of _gill-fringes_ (placental tufts). But why should we multiply examples? The whole of embryology, in every department, is made up of examples of the same law.

=Illustration of the Differentiation of the Whole Animal Kingdom.=--Finally, the law of differentiation in the evolution of the whole animal kingdom may be well illustrated by means of the different directions taken in the development of the eggs of all the various kinds of animals. Suppose, then, we have one thousand eggs, representing all the different departments, classes, orders, families, etc., of animals. Many of these may doubtless be identified by form or size, or some other superficial character, as the eggs of this or that animal, _but structurally they are all alike_. At first, i. e., as germ-cells, they all represent the _earliest condition_ of life on the earth, and the _lowest forms_ of life _now_. If we now watch their development, we find that some remain in this first condition without further change. These we set aside. They are _Protozoa_. The remainder continue to develop, but at first it would be impossible to say to which of the several departments or primary groups they each belonged. Then, by cell-multiplication, the original single cell becomes a cell-aggregate. It may be compared now to a compound protozoan, such as Foraminifera. The cell-aggregate then differentiates into layers, and forms, in fact, a two-layered sac called a gastrula. This is the structure of some of the lowest cœlenterates, such as the hydra. Thus far all seem to go together. But now, for the first time, the primary groups are declared. If it be a vertebrate, for example, the most fundamental characters--the cerebro-spinal axis, the vertebral column, and the double cavity, neural and visceral, are outlined. Suppose, now, we set aside all other departments, and fix our attention on the vertebrates. At first we could not tell which were mammals, birds, reptiles, or fishes; but after a while the classes are declared. We now set aside all other classes and watch the mammals. After a while the order declares itself. We select the ungulates. Then the family is declared, say the _Equidæ_; then the genus, _Equus_; and, lastly, the species, _Caballus_.[26]

The same would be true if we followed any other line of development, whether in vertebrates or in any other department. Observe, then, that, in following any one line as we have done, there is an increasing specialization, and, if we followed all the lines, an increasing differentiation, like the branching and rebranching of a tree. Now, this is the type and illustration of what took place in the development of the animal kingdom. We conclude that the animal kingdom appeared first as Protozoa, then as living cell-aggregates or compound protozoans, then as gastrula or two-layered sacs with oral opening. Then the great primary departments, unless we except the vertebrates, commenced to separate. This took place before the primordial period; for in the primordial fauna we have all the departments, except vertebrates, already declared. This completely explains why it is that we are able to trace homology only within the limits of each primary group.

But the question has doubtless already occurred to the thoughtful reader, “Why should the steps of the phylogeny be repeated in the ontogeny?” The general answer is doubtless to be found in the law of heredity--that wonderful 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.

=Proofs from Rudimentary and Useless Organs.=--These have to a large extent been anticipated under previous heads. The tails of birds and the gill-arches of reptiles are rudimentary. The finger-bones of a whale’s paddle or a turtle’s flipper may be regarded as useless, at least so far as the exact number of constituent pieces is concerned; for an extended surface, without visible joints or separate fingers, is all that is seen, and apparently all that is required. The splint-bones of a horse’s foot or the dew-claws of a dog’s foot are certainly useless. We have already, in speaking of modifications of structure and of embryonic conditions, given many examples of this kind, but it may be well to add some striking examples with this special point in view.

If different orders of existing mammals were indeed made by gradual modification of some generalized primal form, then it is evident that these useless remnants of once useful parts would be most common in the most highly modified forms. Now, of all mammals, the whales are perhaps the most modified or changed from the original mammalian form--so much modified, in fact, that the popular eye scarcely recognizes them as mammals at all. Here, then, we might expect, and do indeed find, many examples:

1. The baleen whales have no teeth, and no use for them. They have instead a wonderful armature of fringed whalebone plates (baleen), by means of which they gather their food.[27] Yet the embryo of the whale has a full set of rudimentary teeth deeply buried in the jawbone, and formed in the usual way characteristic of mammalian teeth--i. e., by an infolding of the epithelial surface of the gum--_but the teeth are never cut_; in fact, they reach their highest development in mid-embryonic life, and are again absorbed. Why, then, this waste of developmental energy? Why should teeth be formed only to be reabsorbed without being cut? The only conceivable answer is, because the ancestors of the whale, before the family of whales was fairly established, had teeth which were gradually, from generation to generation, aborted, because no longer used, the baleen plates having taken their place. If whales were made at once out of hand as we now see them, is it conceivable that these useless teeth would have been given them?

2. Again, many whales have rudimentary pelvic bones, but no hind-limbs. Why should there be pelvic bones, when the sole object of these bones is to act as a basis for hind-limbs? In some whales, for example the right whale, there are also rudiments of hind-legs, but these are buried beneath the skin and flesh, and therefore, of course, wholly useless. The only explanation of these facts is that the ancestors of all the whales before they had become whales were quadrupeds, which afterward took to the water, and little by little the hind-legs, for want of use, dwindled away to the useless remnants which we now find.

3. Again, whales seem to be hairless, yet rudimentary hairs are found in the skin. Their organs of smell are rudimentary, but made on the pattern of those of mammals, not of fishes--i. e., they are air-smelling, not water-smelling organs. From all these, as well as many other facts, it is evident that the whales descended in early Tertiary times from some marsh-loving, powerful-tailed, short-legged, scant-haired quadruped by modifications gradually induced by increasing aquatic habits.

Examples of such rudimentary organs might be multiplied without limit. As might be expected, some are found even in man. Such, for example, are the muscles for moving the ear, necessary in animals but useless in man, and therefore rudimentary. Similarly useless in man are the scalp-muscle, used by animals to erect the crest or bristles on the head, and the skin-muscle of the neck and chest, used by animals for shaking the skin of those parts. Most persons have lost the power of using these. For my part I can use them all--ear-muscles, scalp-muscle, skin-muscle--but they serve no useful purpose.

Again, and finally, in man and many mammals we find a slender, worm-like appendage about three inches long, attached to the cæcum of the large intestine. Anatomists and physiologists, under the influence of that philosophy which maintains that every part of the fearfully and wonderfully made human frame was _directly_ contrived to subserve some useful purpose, have puzzled themselves to find the use of this. It probably has no use; on the contrary, it is a continual source of danger. If the human body had been made at once out of hand, it would not have been there. How came it, then? It is the rudimentary remnant of an organ--a greatly enlarged cæcum--which has served, and in some mammals still serves, a useful purpose. All these cases are survivals; they are organs which, like many customs in society, have outlived their usefulness, but still continue by heredity.

But why multiply examples? All along the track of evolution organs become useless by changes in the habits of their possessors. They are not, however, shed or dropped bodily at once. No; they are _retained by heredity_, but _dwindle by disuse_, more and more, until they pass away entirely. But even when they are entirely gone in the adult, they are often found still lingering in the embryo. They are among the most obvious and convincing proofs of the origin of organic forms by derivation.