=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. [Illustration: FIG. 61.--Vertebrated but symmetrical fin. A, form; B, structure.] 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. [Illustration: FIG. 62.--Tail of the Archæopteryx.] [Illustration: FIG. 63.--Tail of a modern bird.]