ts, 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.

[Illustration: FIG. 68.--Homocercal Tail, showing (A) external form
and (B) internal structure.]

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

[Illustration: 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.

Similar changes have taken place in the form and structure of
birds' tails. The earliest bird known--the Jurassic
_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 fea