THE CLASSIFICATION OF THE VERTEBEATA. 185 



is the character common to hairs and feathers, and distin- 

 guishes both distinctly from other horny growths. But it 

 will be said that certain fossil reptiles, such as the Iguanodon 

 and the Pterodactyle, make near approach to birds, and that 

 the early fossil birds, such as Hesperornis, make in important 

 respects an ajoproach to reptiles. This is undoubtedly true, 

 just as it is true that the Lepidosyren approaches the 

 Amphibia, and the Monotremata approach Birds; but just 

 as in n'-'ither of these cases is the approximation the remains 

 of a bridge of transition, so also there is no sufficient evidence 

 that birds are descended from allies of either Iguanodons or 

 Pterodactyles. In particular there does not ap])ear to have 

 been yet found a fossil reptile with a skull in the least like 

 that of a bird. 



The fishes Avhich appear to be nearest to any possible 

 genetic link with other vertebrates are the Elasmobranchs, 

 of Avliich the sharks furnish an example. Those on the oth^r 

 hand which are far more numerous at the present time and 

 may be said to exhibit distinct piscine character in the highest 

 degree, are the osseous fishes, which present remai-kable modi- 

 fications, sundering them far from any possible transition to 

 the other vertebrates. The restricted Keptilia most probably 

 took origin from Amphibia, in which case the Cuvierian Reptilia 

 may be looked on as consisting of a primitive batrachian stem, 

 breaking up into numerous branches of which the serpents, 

 turtles, lizards and crocodiles alone survive ; and I may be 

 allowed to doubt if, from any of the extinct branches known, 

 it is possible that birds any more than mammals have 

 descended. 



This isolation of mammals is not so remarkable if evolution 

 be determinate, like the development of an embryo into an 

 adult, since in that case the stages of evolution of an organ 

 are not necessarily advantageous. Like the stages of develop- 

 ment of such an organ as the eye in the vertebrate embryo, 

 they may be functionally useless. Therefore, one would 

 expect transitional forms to disappear, and also that the 

 character of the whole evolution would not be fully brought 

 out till it was complete or near completion ; and in favour of 

 this I have already said something in speaking of the circula- 

 tory organs. 



The restricted Rejjtilia, Aves and Mammalia may all three 

 have originated from the Amphibia, but the Amphibia do not 

 differ more from serpents, turtles, lizaixls and crocodiles than 

 these groups differ one from another. If that be so, there 



