Volume I Part 7 (1/2)

[Ill.u.s.tration: FIG. 59.--Diagram of Geological Succession of the Cla.s.ses of the Animal Kingdom. (After Le Conte.)]

I will here leave the evidence which is thus yielded by the most general principles that have been established by the science of palaeontology; and I will devote the rest of this chapter to a detailed consideration of a few highly special lines of evidence. By thus suddenly pa.s.sing from one extreme to the other, I hope to convey the best idea that can be conveyed within a brief compa.s.s of the minuteness, as well as the extent, of the testimony which is furnished by the rocks.

When Darwin first published his _Origin of Species_, adverse critics fastened upon the ”missing-link” argument as the strongest that they could bring against the theory of descent. Although Darwin had himself strongly insisted on the imperfection of the geological record, and the consequent precariousness of any negative conclusions raised upon it, these critics maintained that he was making too great a demand upon the argument from ignorance--that, even allowing for the imperfection of the record, they would certainly have expected at least a few cases of testimony to _specific_ trans.m.u.tation. For, they urged in effect, looking to the enormous profusion of the extinct species on the one hand, and to the immense number of known fossils on the other, it was incredible that no satisfactory instances of specific trans.m.u.tation should ever have been brought to light, if such trans.m.u.tation had ever occurred in the universal manner which the theory was bound to suppose.

But since Darwin first published his great work palaeontologists have been very active in discovering and exploring fossiliferous beds in sundry parts of the world; and the result of their labours has been to supply so many of the previously missing links that the voice of competent criticism in this matter has now been well-nigh silenced.

Indeed, the material thus furnished to an advocate of evolution at the present time is so abundant that his princ.i.p.al difficulty is to select his samples. I think, however, that the most satisfactory result will be gained if I restrict my exposition to a minute account of some few series of connecting links, rather than if I were to take a more general survey of a larger number. I will, therefore, confine the survey to the animal kingdom, and there mention only some of the cases which have yielded well-detailed proof of continuous differentiation.

It is obvious that the parts of animals most likely to have been preserved in such a continuous series of fossils as the present line of evidence requires, would have been the hard parts. These are horns, bones, teeth, and sh.e.l.ls. Therefore I will consider each of these four cla.s.ses of structures separately.

Horns wherever they occur, are found to be of high importance for purposes of cla.s.sification. They are restricted to the Ruminants, and appear under three different forms or types--namely solid, as in antelopes; hollow, as in sheep; and deciduous, as in deer. Now, in each of these divisions we have a tolerably complete palaeontological history of the evolution of horns. The early ruminants were altogether hornless (Fig. 60). Then, in the middle Miocene, the first antelopes appeared with tiny horns, which progressively increased in size among the ever-multiplying species of antelopes until the present day. But it is in the deer tribe that we meet with even better evidence touching the progressive evolution of horns; because here not only size, but shape, is concerned. For deer's horns, or antlers, are arborescent; and hence in their case we have an opportunity of reading the history, not only of a progressive growth in size, but also of an increasing development of form. Among the older members of the tribe, in the lower Miocene, there are no horns at all. In the mid-Miocene we meet with two-p.r.o.nged horns (_Cervus dicrocerus_, Figs. 61, 62, 1/5 nat. size). Next, in the upper Miocene (_C. matheronis_, Fig. 63, 1/8 nat. size), and extending into the Pliocene (_C. pardinensis_, Fig. 64, 1/18 nat. size), we meet with three-p.r.o.nged horns. Then, in the Pliocene we find also four-p.r.o.nged horns (_C. issiodorensis_, Fig. 65, 1/16 nat. size), leading us to five-p.r.o.nged (_C. tetraceros_). Lastly, in the Forest-bed of Norfolk we meet with arborescent horns (_C. Sedgwickii_, Fig. 66, 1/35 nat. size).

The life-history of existing stags furnishes a parallel development (Fig. 67), beginning with a single horn (which has not yet been found palaeontologically), going on to two p.r.o.ngs, three p.r.o.ngs, four p.r.o.ngs, and afterwards branching.

[Ill.u.s.tration: FIG. 60.--Skull of _Oreodon Culbertsoni_. (After Leidy.)]

[Ill.u.s.tration: FIGS. 61-66. The series is reduced from Gaudry's ill.u.s.trations, after Farge, Croizet, Jobert and Boyd Dawkins.]

[Ill.u.s.tration: FIG. 67.--Successive stages in the development of an existing Deer's Antlers. (After Gaudry, but a better ill.u.s.tration has already been given on p. 100.)]

Coming now to bones, we have a singularly complete record of transition from one type or pattern of structure to another in the phylogenetic history of tails. This has been so clearly and so tersely conveyed by Prof. Le Conte, that I cannot do better than quote his statement.

It has long been noticed that there are among fishes two styles of tail-fins. These are the even-lobed, or h.o.m.ocercal (Fig. 68), and the uneven-lobed, or heterocercal (Fig. 69). 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. 68); 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. 69). This type of fin is, therefore, _vertebrated_, the other _non-vertebrated_.

Figs. 68 and 69 show these two types in form and structure. But there is still another type found only in the lowest and most generalized forms of fishes. In these the tail-fin is vertebrated and yet symmetrical. This type is shown in Fig. 70.

[Ill.u.s.tration: FIG. 68.--h.o.m.ocercal Tail, showing (A) external form and (B) internal structure.]

[Ill.u.s.tration: FIG. 69.--Heterocercal Tail, showing (A) external form and (B) internal structure.]

[Ill.u.s.tration: FIG. 70.--Vertebrated but symmetrical fin (diphycercal), showing (A) external form and (B) internal structure.]

Now, in the development of a teleost fish (Fig. 68), as has been shown by Alexander Aga.s.siz, the tail-fin is first like Fig. 70; then becomes heterocercal, like Fig. 69; and, finally, becomes h.o.m.ocercal like Fig. 68. 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 phylogenetic series_. The earliest fish-tails were either like Fig. 69 or Fig. 70; never like Fig. 68. The earliest of all were almost certainly like Fig. 70; then they became like Fig. 69; and, finally, only much later in geological history (Jura.s.sic or Cretaceous), they became like Fig. 68. 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 Jura.s.sic _Archaeopteryx_--had a long reptilian tail of twenty-one joints, each joint bearing a feather on each side, right and left (Fig.

71): [see also Fig. 73]. 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. 72). The _Archaeopteryx'_ 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 Jura.s.sic strata--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 _pa.s.ses 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 pa.s.ses through the stages of the phylogeny. The same is true of all tailless animals.

[Ill.u.s.tration: FIG. 71.--Tail of _Archaeopteryx_. A indicates origin of simply-jointed tail.]

[Ill.u.s.tration: FIG. 72.--Tail of modern Bird. The numerals indicate the foreshortened, enlarged, and consolidated joints; _f_, terminal segment of the vertebral column; D, shafts of feathers.]

[Ill.u.s.tration: FIG. 73.--_Archaeopteryx macura_, restored, 1/2 nat.

size. (After Flower.) The section of the tail is copied from Owen, nat. size.]

The extinct _Archaeopteryx_ above alluded to presents throughout its whole organization a most interesting a.s.semblage of ”generalized characters.” For example, its teeth, and its still unreduced digits of the wings (which, like those of the feet, are covered with scales), refer us, with almost as much force as does the vertebrated tail, to the Sauropsidian type--or the trunk from which birds and reptiles have diverged.

We will next consider the palaeontological evidence which we now possess of the evolution of mammalian limbs, with special reference to the hoofed animals, where this line of evidence happens to be most complete.

I may best begin by describing the bones as these occur in the sundry branches of the mammalian type now living. As we shall presently see, the modifications which the limbs have undergone in these sundry branches chiefly consist in the suppression of some parts and the exaggerated development of others. But, by comparing all mammalian limbs together, it is easy to obtain a generalized type of mammalian limb, which in actual life is perhaps most nearly conformed to in the case of bears. I will therefore choose the bear for the purpose of briefly expounding the bones of mammalian limbs in general--merely asking it to be understood, that although in the case of many other mammalia some of these bones may be dwindled or altogether absent, while others may be greatly exaggerated as to relative size, in no case do any _additional_ bones appear.