Fig. 69.—Heterocercal Tail, showing (A) external form and (B) internal structure.
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 Agassiz, the tail-fin is first like Fig. 70; then becomes heterocercal, like Fig. 69; and, finally, becomes homocercal 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 (Jurassic 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.
| Fig. 71.—Tail of Archæopteryx. A indicates origin of simply-jointed tail. | 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. |
Fig. 73.—Archæopteryx macura, restored, ½ nat. size. (After Flower.) The section of the tail is copied from Owen, nat. size.
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. 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 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 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 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 extinct Archæopteryx above alluded to presents throughout its whole organization a most interesting assemblage 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 palæontological 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.