Darwin, and After Darwin Volume I Part 7

[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.

On looking, then, at the skeleton of a bear (Fig. 74), the first thing to observe is that there is a perfect serial h.o.m.ology between the bones of the hind legs and of the fore legs. The thigh-bone, or femur, corresponds to the shoulder-bone, or humerus; the two shank bones (tibia and fibula) correspond to the two arm-bones (radius and ulna); the many little ankle-bones (tarsals) correspond to the many little wrist-bones (carpals); the foot-bones (meta-tarsals) correspond to the hand-bones (meta-carpals); and, lastly, the bones of each of the toes correspond to those of each of the fingers.

[Ill.u.s.tration: FIG. 74.--Skeleton of Polar Bear, drawn from nature (_Brit. Mus._).]

The next thing to observe is, that the disposition of bones in the case of the bear is such that the animal walks in the way that has been called plantigrade. That is to say, all the bones of the fingers, as well as those of the toes, feet, and ankles, rest upon the ground, or help to const.i.tute the "soles." Our own feet are constructed on a closely similar pattern. But in the majority of living mammalian forms this is not the case. For the majority of mammals are what has been called digitigrade. That is to say, the bones of the limb are so disposed that both the foot and hand bones, and therefore also the ankle and wrist, are removed from the ground altogether, so that the animal walks exclusively upon its toes and fingers--as in the case of this skeleton (Fig. 75), which is the skeleton of a lion. The next figures display a series of limbs, showing the progressive pa.s.sage of a completely plantigrade into a highly digitigrade type--the curved lines of connexion serving to indicate the h.o.m.ologous bones (Figs. 76, 77).

[Ill.u.s.tration: _Fig_. 75.--Skeleton of Lion. (After Huxley.)]

[Ill.u.s.tration: FIG. 76.--Anterior limb of Man, Dog, Hog, Sheep, and Horse. (After Le Conte.) _Sc_, shoulder-blade; _c_, coracoid; _a_, _b_, bones of fore-arm; 5, bones of the wrist; 6, bones of the hand; 7, bones of the fingers.]

[Ill.u.s.tration: FIG. 77.--Posterior limb of Man, Monkey, Dog, Sheep and Horse. (After Le Conte.) 1, Hip-joint; 2, thigh-bone; 3, knee-joint; 4, bones of leg; 5, ankle-joint; 6, bones of foot; 7, bones of toes.]

I will now proceed to detail the history of mammalian limbs, as this has been recorded for us in fossil remains.

The most generalized or primitive types of limb hitherto discovered in any vertebrated animal above the cla.s.s of fishes, are those which are met with in some of the extinct aquatic reptiles. Here, for instance, is a diagram of the left hind limb of _Baptanodon discus_ (Fig. 78). It has six rows of little symmetrical bones springing from a leg-like origin.

But the whole structure resembles the fin of a fish about as nearly as it does the leg of a mammal. For not only are there six rows of bones, instead of five, suggestive of the numerous rays which characterise the fin of a fish; but the structure as a whole, having been covered over with blubber and skin, was throughout flexible and unjointed--thus in function, even more than in structure, resembling a fin. In this respect, also, it must have resembled the paddle of a whale (see Fig.

79); but of course the great difference will be noted, that the paddle of a whale reveals the dwindled though still clearly typical bones of a true mammalian limb; so that although in outward form and function these two paddles are alike, their inward structure clearly shows that while the one testifies to the absence of evolution, the other testifies to the presence of degeneration. If the paddle of _Baptanodon_ had occurred in a whale, or the paddle of a whale had occurred in _Baptanodon_, either fact would in itself have been well-nigh destructive of the whole theory of evolution.

[Ill.u.s.tration: FIG. 78.--A, posterior limb of _Baptanodon discus_.

(After Marsh.) F, thigh-bone; I to VI, undifferentiated bones of the leg and foot. B, anterior limb of _Chelydra serpentina_. (After Gegenbaur.) U and R, bones of the fore-arm; I to V, fully differentiated bones of the hand, following those of the wrist.]

[Ill.u.s.tration: FIG. 79.--Paddle of a Whale.]

Such, then, is the most generalized as it is the most ancient type of vertebrate limb above the cla.s.s of fishes. Obviously it is a type suited only to aquatic life. Consequently, when aquatic Vertebrata began to become terrestrial, the type would have needed modification in order to serve for terrestrial locomotion. In particular, it would have needed to gain in consolidation and in firmness, which means that it would have needed also to become jointed. Accordingly, we find that this archaic type gave place in land-reptiles to the exigencies of these requirements. Here for example is a diagram, copied from Gegenbaur, of the right fore-foot of _Chelydra serpentina_ (Fig. 78). As compared with the h.o.m.ologous limb of its purely aquatic predecessor, there is to be noticed the disappearance of one of the six rows of small bones, a confluence of some of the remainder in the other five rows, a duplication of the arm-bone into a radius and ulna, in order to admit of jointed rotation of the hand, and a general disposition of the small bones below these arm-bones, which clearly foreshadows the joint of the wrist. Indeed, in this fore-foot of _Chelydra_, a child could trace all the princ.i.p.al h.o.m.ologies of the mammalian counterpart, growing, like the next stage in a dissolving view, out of the primitive paddle of _Baptanodon_--namely, first the radius and ulna, next the carpals, then the meta-carpals, and, lastly, the three phalanges in each of the five digits.

Such a type of foot no doubt admirably meets the requirements of slow reptilian locomotion over swampy ground. But for anything like rapid locomotion over hard and uneven ground, greater modifications would be needed. Such modifications, however, need not be other in kind: it is enough that they should continue in the same line of advance, so as to reach a higher degree of firmness, combined with better joints.

Accordingly we find that this took place, not indeed among reptiles, whose habits of cold-blooded life have not changed, but among their warm-blooded descendants, the mammals. Moreover, when we examine the whole mammalian series, we find that the required modifications must have taken place in slightly different ways in three lines of descent simultaneously. We have first the plantigrade and digitigrade modifications already mentioned (pp. 178, 179) Of these the plantigrade walking entailed least change, because most resembling the ancestral or lizard-like mode of progression. All that was here needed was a general improvement as to relative lengths of bones, with greater consolidation and greater flexibility of joints. Therefore I need not say anything more about the plantigrade division. But the digitigrade modification necessitated a change of structural plan, to the extent of raising the wrist and ankle joints off the ground, so as to make the quadruped walk on its fingers and toes. We meet with an interesting case of this transition in the existing hare, which while at rest supports itself on the whole hind foot after the manner of a plantigrade animal, but when running does so upon the ends of its toes, after the manner of a digitigrade animal.

It is of importance for us to note that this transition from the original plantigrade to the more recent digitigrade type, has been carried out on two slightly different plans in two different lines of mammalian descent. The hoofed mammals--which are all digitigrade--are sub-cla.s.sified as artiodactyls and perissodactyls, i. e. even-toed and odd-toed. Now, whether an animal has an even or an odd number of toes may seem a curiously artificial distinction on which to found so important a cla.s.sification of the mammalian group. But if we look at the matter from a less empirical and more intelligent point of view, we shall see that the alternative of having an even or an odd number of toes carries with it alternative consequences of a practically important kind to any animal of the digitigrade type. For suppose an aboriginal five-toed animal, walking on the ends of its five toes, to be called upon to resign some of his toes. If he is left with an even number, it must be two or four; and in either case the animal would gain the firmest support by so disposing his toes as to admit of the axis of his foot pa.s.sing between an equal number of them--whether it be one or two toes on each side. On the other hand, if our early mammal were called upon to retain an odd number of toes, he would gain best support by adjusting matters so that the axis of his foot should be coincident with his middle toe, whether this were his only toe, or whether he had one on either side of it. This consideration shows that the cla.s.sification into even-toed and odd-toed is not so artificial as it no doubt at first sight appears. Let us, then, consider the stages in the evolution of both these types of feet.

Going back to the reptile _Chelydra_, it will be observed that the axis of the foot pa.s.ses down the middle toe, which is therefore supported by two toes on either side (Fig. 78). It may also be noticed that the wrist or ankle bones do not interlock, either with one another or with the bones of the hand or foot below them. This, of course, would give a weak foot, suited to slow progression over marshy ground--which, as we have seen, was no doubt the origin of the mammalian plantigrade foot.

Here, for instance, to all intents and purposes, is a similar type of foot, which belonged to a very early mammal, antecedent to the elephant series, the horse series, the rhinoceros, the hog, and, in short, all the known hoofed mammalia (Fig. 80). It was presumably an inhabitant of swampy ground, slow in its movements, and low in its intelligence.

[Ill.u.s.tration: FIG. 80.--Fossil skeleton of _Phenacodus primavus_.

(After Cope.)]

But now, as we have seen, for more rapid progression on hard uneven ground, a stronger and better jointed foot would be needed. Therefore we find the bones of the wrist and ankle beginning to interlock, both among themselves and also with those of the foot and hand immediately below them. Such a stage of evolution is still apparent in the now existing elephant. (See Fig. 81.)

[Ill.u.s.tration: FIG. 81.--Bones of the foot of four different forms of the perissodactyl type, showing gradual reduction in the number of digits, coupled with a greater consolidation of the bones above the digits. The series reads from right to left. Drawn from nature (_Brit. Mus._).]

Next, however, a still stronger foot was made by the still further interlocking of the wrist and ankle bones, so that both the first and second rows of them were thus fitted into each other, as well as into the bones of the hand and foot beneath. This further modification is clearly traceable in some of the earlier perissodactyls, and occurs in the majority at the present time. Compare, for example, the greater interlocking and consolidation of these small bones in the Rhinoceros as contrasted with the Elephant (Fig. 81). Moreover, simultaneously with these consolidating improvements in the mechanism of the wrist and ankle joints, or possibly at a somewhat later period, a reduction in the number of digits began to take place. This was a continuation of the policy of consolidating the foot, a.n.a.logous to the dropping out of the sixth row of small bones in the paddle of _Baptanodon_. (Fig. 78.) In the pentadactyl plantigrade foot of the early mammals, the first digit, being the shortest, was the first to leave the ground, to dwindle, and finally to disappear. More work being thus thrown on the remaining four, they were strengthened by interlocking with the wrist (or ankle) bones above them, as just mentioned; and also by being brought closer together.

[Ill.u.s.tration: FIG. 82.--Bones of the foot of four different forms of the artiodactyl type, showing gradual reduction of the number of digits, coupled with a greater consolidation of the bones above the digits. The series reads from right to left. Drawn from nature (_Brit. Mus._).]

The changes which followed I will render in the words of Professor Marsh.