276 TOWARDS THE HUMAN FORM 



characters known in fossil and living forms. He supposes that 

 in this imaginary animal each of the body segments except 

 those of the anterior half of the neck and posterior half of the 

 tail, carried a transverse series of dermal bones, covered by 

 horny shields whose relative position and dimensions were 

 subsequently modified by the manner of growth of the trunk, 

 which took place by a rapid diminution of the parts nearest 

 the neck and tail. 



The order of Chelonians attained its maximum development 

 toward the end of the Secondary Epoch. The extant types are 

 but a remnant of those existing in Secondary times. It is 

 probable that the normally web-footed varieties inhabiting the 

 marshes were the ancestors of terrestrial forms whose mode of 

 progression is still reminiscent of a kind of swimming action 

 on resistant ground, and that marine Turtles, with their 

 feet transformed into paddles, were likewise derived from them, 

 the feet having gone through a modification analogous to 

 that which we have already noticed in Plesiosaurs and Ich- 

 thyosaurs, and on whose significance we have already insisted. 1 



At the close of the Secondar}^ Period, those prodigious 

 Reptiles whose history we have just narrated disappeared as 

 completely as the Ammonites had disappeared from the sea. 

 To what are we to attribute the world-wide extinction of such 

 puissant animals, whose vitality was manifested by their 

 extraordinary longevity ? We can scarcely believe that organic 

 types, like individuals, grow old and die. This oft-repeated 

 proposition has no other value than as a figure of speech. So 

 long as a given species has representatives capable of repro- 

 ducing their kind, and sufficiently numerous to carry on the 

 process, that species is no more likely to disappear spontaneously 

 than the type to which it belongs. Of course it is within the 

 bounds of possibility that some extraordinary modification 

 in environment may induce sterility in all the individuals of 

 the same organic group, but for this to come about the modifica- 

 tion must be so general that no species escapes it, and so sudden 

 as to make any adaptation out of the question. Both suppositions 

 are equally unlikely, and hence we are led to conclude that 

 things came to pass in those days very much as they do to-day, 

 when species do not disappear unless wiped out by their enemies 



1 Cf. pp. 193, 272. 



