Palaeontology. 



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 non- 

 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. 



FIG. 71. Tail of Archaopteryx. 

 A indicates origin of simply- 

 jointed tail. 



FIG. 73. Tail of modern Bird. 

 The numerals indicate the fore- 

 shortened, enlarged, and con- 

 solidated joints; f t terminal 

 segment of the vertebral column; 

 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 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 



