Darwin, and After Darwin Volume I Part 8

Two kinds of reduction began. One leading to the existing perissodactyl foot, and the other, apparently later, resulting in the artiodactyl type. In the former the axis of the foot remained in the middle of the third digit, as in the pentadactyl foot. [See Fig. 81.] In the latter, it shifted to the outer side of this digit, or between the third and fourth toe. [See Fig. 82.]

In the further reduction of the perissodactyl foot, the fifth digit, being shorter than the remaining three, next left the ground, and gradually disappeared. [Fig. 81 B.] Of the three remaining toes, the middle or axial one was the longest, and retaining its supremacy as greater strength and speed were required, finally a.s.sumed the chief support of the foot [Fig. 81 C], while the outer digits left the ground, ceased to be of use, and were lost, except as splint-bones [Fig. 81 D]. The feet of the existing horse shows the best example of this reduction in the Perissodactyls, as it is the most specialized known in the Ungulates [Fig. 81 D].

In the artiodactyl foot, the reduction resulted in the gradual diminution of the two outer of the four remaining toes, the third and fourth doing all the work, and thus increasing in size and power. The fifth digit, for the same reasons as in the perissodactyl foot, first left the ground and became smaller. Next, the second soon followed, and these two gradually ceased to be functional, [and eventually disappeared altogether, as shown in the accompanying drawing of the feet of still existing animals, Fig. 82 B, C, D].

The limb of the modern race-horse is a nearly perfect piece of machinery, especially adapted to great speed on dry, level ground.

The limb of an antelope, or deer, is likewise well fitted for rapid motion on a plain, but the foot itself is adapted to rough mountain work as well, and it is to this advantage, in part, that the Artiodactyls owe their present supremacy. The plantigrade pentadactyl foot of the primitive Ungulate--and even the perissodactyl foot that succeeded it--both belong to the past humid period of the world's history. As the surface of the earth slowly dried up, in the gradual desiccation still in progress, new types of feet became a necessity, and the horse, antelope, and camel were gradually developed, to meet the altered conditions.

The best instance of such progressive modifications in the case of perissodactyl feet is furnished by the fossil pedigree of the existing horse, because here, within the limits of the same continuous family line, we have presented the entire series of modifications.

There are now known over thirty species of horse-like creatures, beginning from the size of a fox, then progressively increasing in bulk, and all standing in linear series in structure as in time. Confining attention to the teeth and feet, it will be seen from the wood-cut on page 189 that the former grow progressively longer in their sockets, and also more complex in the patterns of their crowns. On the other hand, the latter exhibit a gradual diminution of their lateral toes, together with a gradual strengthening of the middle one. (See Fig. 83.) So that in the particular case of the horse-ancestry we have a practically complete chain of what only a few years ago were "missing links." And this now practically completed chain shows us the entire history of what happens to be the most peculiar, or highly specialized, limb in the whole mammalian cla.s.s--namely, that of the existing horse. Of the other two wood-cuts, the former (Fig. 84) shows the skeleton of a very early and highly generalized ancestor, while the other is a partial restoration of a much more recent and specialized one. (Fig. 85.)

[Ill.u.s.tration: FIG. 83.--Feet and teeth in fossil pedigree of the Horse. (After Marsh.) _a_, bones of the fore-foot; _b_, bones of the hind-foot; _c_, radius and ulna; _d_, tibia and fibula; _e_, roots of a tooth; _f_ and _g_, crowns of upper and lower molar teeth.]

[Ill.u.s.tration: FIG. 84.--_Palaeotherium_. (Lower Tertiary of Paris Basin.)]

[Ill.u.s.tration: FIG. 85.--_Hipparion_. (New World Pliocene.)]

On the other hand, progressive modifications of the artiodactyl feet may be traced geologically up to the different stages presented by living ruminants, in some of which it has proceeded further than in others. For instance, if we compare the pig, the deer, and the camel (Fig. 82), we immediately perceive that the dwindling of the two rudimentary digits has proceeded much further in the case of the deer than in that of the pig, and yet not so far as in that of the camel, seeing that here they have wholly disappeared. Moreover, complementary differences are to be observed in the degree of consolidation presented by the two useful digits. For while in the pig the two foot-bones are still clearly distinguishable throughout their entire length, in the deer, and still more in the camel, their union is more complete, so that they go to const.i.tute a single bone, whose double or compound character is indicated externally only by a slight bifurcation at the base.

Nevertheless, if we examine the state of matters in the unborn young of these animals, we find that the two bones in question are still separated throughout their length, and thus precisely resemble what used to be their permanent condition in some of the now fossil species of hoofed mammalia.

Turning next from bones of the limb to other parts of the mammalian skeleton, let us briefly consider the evidence of evolution that is here likewise presented by the vertebral column, the skull, and the teeth.

As regards the vertebral column, if we examine this structure in any of the existing hoofed animals, we find that the bony processes called zygapophyses, which belong to each of the const.i.tuent vertebrae, are so arranged that the anterior pair belonging to each vertebra interlocks with the posterior pair belonging to the next vertebra. In this way the whole series of vertebrae are connected together in the form of a chain, which, while admitting of considerable movement laterally, is everywhere guarded against dislocation. But if we examine the skeletons of any ungulates from the lower Eocene deposits, we find that in no case is there any such arrangement to secure interlocking. In all the hoofed mammals of this period the zygapophyses are flat. Now, from this flat condition to the present condition of full interlocking we obtain a complete series of connecting links. In the middle Miocene period we find a group of hoofed animals in which the articulation begins by a slight rounding of the previously flat surfaces: later on this rounding progressively increases, until eventually we get the complete interlocking of the present time.

As regards teeth, and still confining attention to the hoofed mammals, we find that low down in the geological series the teeth present on their grinding surfaces only three simple tubercles. Later on a fourth tubercle is added, and later still there is developed that complicated system of ridges and furrows which is characteristic of these teeth at the present time, and which was produced by manifold and various involutions of the three or four simple tubercles of Eocene and lower Miocene times. In other words, the principle of gradual improvement in the construction of teeth, which has already been depicted as regards the particular case of the Horse-family (Fig. 83), is no less apparent in the pedigree of all the other mammalia, wherever the palaeontological history is sufficiently intact to serve as a record at all.

Lastly, as regards the skull, casts of the interior show that all the earlier mammals had small brains with comparatively smooth or unconvoluted surfaces; and that as time went on the mammalian brain gradually advanced in size and complexity. Indeed so small were the cerebral hemispheres of the primitive mammals that they did not overlap the cerebellum, while their smoothness must have been such as in this respect to have resembled the brain of a bird or reptile. This, of course, is just as it ought to be, if the brain, which the skull has to accommodate, has been gradually evolved into larger and larger proportions in respect of its cerebral hemispheres, or the upper ma.s.ses of it which const.i.tute the seat of intelligence. Thus, if we look at the above series of wood-cuts, which represents the comparative structure of the brain in the existing cla.s.ses of the Vertebrata, we can immediately understand why the fossil skulls of Mammalia should present a gradual increase in size and furrowing, so as to accommodate the general increase of the brain in both these respects between the level marked "maml" and that marked "man," in the last of the diagrams. (Fig. 87.)

[Ill.u.s.tration: FIG. 86.--Comparative series of Brains. (After Le Conte.) The series reads from above downwards, and represents diagrammatically the brain of a Fish, a Reptile, a Bird, a Mammal, and a Man. In each case the letter A marks a side view, and the letter B a top view. The small italics throughout signify the following h.o.m.ologous parts: _m_, medulla; _cb_, cerebellum; _op_, optic lobes; _cr_, cerebrum and thalamus; _ol_, olfactory lobes. The series shows a progressive consolidation and enlargement of the brain in general, and of the cerebrum and cerebellum in particular, which likewise exhibit continually advancing structure in respect of convolution. In the case of Man, these two parts of the brain have grown to so great a size that they conceal all the other parts from the superficial points of view represented in the diagram.]

[Ill.u.s.tration: FIG. 87.--Ideal section through all the above stages.

(After Le Conte.])

The tabular statement on the following diagram, which I borrow from Prof. Cope, will serve at a glance to reveal the combined significance of so many lines of evidence, united within the limits of the same group of animals.

To give only one special ill.u.s.tration of the principle of evolution as regards the skull, here is one of the most recent instances that has occurred of the discovery of a missing link, or connecting form (see Fig. 88). The fossil (B), which was found in New Jersey, stands in an intermediate position between the stag and the elk. In the stag (A) the skull is high, showing but little of that anterior attenuation which is such a distinctive feature of the skull of the elk (C). The nasal bones (N) of the former, again, are remarkably long when compared with the similar bones of the latter, and the premaxillaries (PMX), instead of being projected forward along the horizontal plane of the base of the skull, are deflected sharply downward. In all these points, it will be seen, the newly discovered form (_Cervalces_) holds an intermediate position (B). "The skull exhibits a partial attenuation anteriorly, the premaxillaries are directed about equally downward and forward, and the nasal bones are measurably contracted in size. The horns likewise furnish characters which further serve to establish this dual relationship[18]."

[18] Heilprin, _Geological Evidences of Evolution_, pp. 73-4 (1888).

[Ill.u.s.tration: FIG. 88.--Skulls of--A, Canadian Stag; B, _Cervalces America.n.u.s_; and C, Elk. (After Heilprin.)]

Formation.

No. of toes Feet Astragalus.

Carpus and tarsus.

Ulno-radius.

Superior molars.

Zygapophyses.

Brain.

Pliocene.

1-1, 2-2 Digitigrade. (Plantigrade.) Grooved. (Flat.) Interlocking. (Opposite.) Faceted.

4-tubercles, crested and cemented.

Doubly involute. Singly involute.

Hemispheres larger, convoluted.

Upper Miocene. (Loup Fork.) 3-3, 4-4, (5-5) Middle. (John Day.) 2-2, 3-3, 4-4 Digitigrade.

Grooved.

Interlocking.

Faceted. Smooth 4-tubercles, and crested Singly involute. Double involute.

Hemispheres larger, convoluted.

Lower (White River.) 3-3, 4-3 Digitigrade. Plantigrade.

Grooved.

Interlocking.

Smooth. Faceted.

4-tubercles, and crested ? Singly involute.

Hemispheres small, and largeer.

Eocene. Upper (Bridger.) 3-3, 4-3, 4-5 (Digitigrade.) Plantigrade.

Grooved. (Flat.) Opposite. Interlocking.

Smooth.

4-tubercles. 3-tubercles, and crested.

Singly involute. Plane Hemispheres small.

Middle. (Wasatch.) 4-3, 4-5, 5-5 Plantigrade. (Digitigrade.) Flat. (Grooved.) Opposite. Interlocking.

Smooth.

4-tubercles. 3-tubercles, a few crested.

Plane. Singly involute.

Hemispheres small; mesencephalon sometimes exposed Lower (Puerco.) 5-5 Plantigrade.

Flat.

Opposite.

Smooth.

3-tubercles. (4-tubercles), none crested.

Plane.

Mesencephalon exposed; hemisphere small and smoother.

The evidence, then, which is furnished by all parts of the vertebral skeleton--whether we have regard to Fishes, Reptiles, Birds, or Mammals--is c.u.mulative and consistent. Nowhere do we meet with any deviation or ambiguity, while everywhere we encounter similar proofs of continuous transformation--proofs which vary only with the varying amount of material which happens to be at our disposal, being most numerous and detailed in those cases where the greatest number of fossil forms has been preserved by the geological record. Here, therefore, we may leave the vertebral skeleton; and, having presented a sample of the evidence as yielded by horns and bones, I will conclude by glancing with similar brevity at the case of sh.e.l.ls--which, as before remarked, const.i.tute the only other sufficiently hard or permanent material to yield unbroken evidence touching the fossil ancestry of animals.

Of course it will be understood that I am everywhere giving merely samples of the now superabundant evidence which is yielded by palaeontology; and, as this chapter is already a long one, I must content myself with citing only the case of mollusk-sh.e.l.ls, although sh.e.l.ls of other cla.s.ses might be made to yield highly important additions to the testimony. Moreover, even as regards the one division of mollusk-sh.e.l.ls, I can afford to quote only a very few cases. These, however, are in my opinion the strongest single pieces of evidence in favour of trans.m.u.tation which have thus far been brought to light.

Near the village of Steinheim, in Wurtemberg, there is an ancient lake-basin, dating from Tertiary times. The lake has long ago dried up; but its aqueous deposits are extraordinarily rich in fossil sh.e.l.ls, especially of different species of the genus _Planorbis_. The following is an authoritative _resume_ of the facts.

As the deposits seem to have been continuous for ages, and the fossil sh.e.l.ls very abundant, this seemed to be an excellent opportunity to test the theory of derivation. With this end in view, they have been made the subject of exhaustive study by Hilgendorf in 1866, and by Hyatt in 1880. In pa.s.sing from the lowest to the highest strata the species change greatly and many times, the extreme forms being so different that, were it not for the intermediate forms, they would be called not only different species, but different genera. And yet the gradations are so insensible that the whole series is nothing less than a demonstration, in this case at least, of origin of species by derivation with modifications. The accompanying plate of successive forms (Fig. 89), which we take from Prof. Hyatt's admirable memoir, will show this better than any mere verbal explanation. It will be observed that, commencing with four slight varieties--probably s.e.xually isolated varieties--of one species, each series shows a gradual transformation as we go upward in the strata--i. e. onward in time. Series I branches into three sub-series, in two of which the change of form is extreme. Series IV is remarkable for great increase in size as well as change in form. In the plate we give only selected stages, but in the fuller plates of the memoir, and still more in the sh.e.l.ls themselves, the subtilest gradations are found[19].

[19] Le Conte, _loc. cit._, pp. 236-7.

[Ill.u.s.tration: FIG. 89.--Trans.m.u.tations of _Planorbis_. (After Hyatt.)]

Here is another and more recently observed case of trans.m.u.tation in the case of mollusks.

The recent species, _Strombus accipitrinus_, still inhabits the coasts of Florida. Its extinct prototype, _S. Leidy_, was discovered a few years ago by Prof. Heilprin in the Pliocene formations of the interior of Florida. The peculiar shape of the wing, and tuberculation of the whorl, are thus proved to have grown but of a previously more conical form of sh.e.l.l.