Paleontology. 171 
backbone runs through the fin to its very point, growing slen- 
derer 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 xon- 
vertebrated. Figs. 68 and 69 show these two types in form and 
structure. But there is still another type found only in the low- 
est and most generalized forms of fishes. In these the tail-fin is 
vertebrated and yet symmetrical. This type is shown in Fig. 70. 
Sun} 
is 
KE 
Me Fe 
Yip 
i 
AN A. 
Aas i \ 
etl y 
| \\ 
LZ 
SSKER 
£ELASSSTY 
SK 
= 
AZZ 
ZZ, 
SEEK 
KES 
Fic. 72.—Tail of modern Tird. 
The numerals indicate the fore- 
shortened, enlarged, and con- 
Fic. 71.—Tail of Archaopleryx. solidated joints; 4 terminal 
A indicates origin of sim, ly- segment of the vertebral column; 
jointed tail. D, shafts of feathers. 
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, be- 
comes 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 em- 
bryonic states. The answer is found in the fact that hds zs the 
order of change in the phylogenetic series. The earliest fish-tails 
were either hke Fig. 69 or Fig. 70; never like Fig. 68. The 
