46 UNIVERSALITY OF DEGENERATIVE EVOLUTION 



velopment has reached its ultimate limit; the 

 limb has now to disappear altogether, or else to 

 provide itself with new elements. 



Judging from the embryology of the Urodele 

 Amphibia, the primitive terrestrial limb had only 

 one toe at first, and then a second appeared, then 

 a third, a fourth, and a fifth. In this way was 

 formed the primitive five-toed limb. 



Let us now trace the adaptation of this five-toed 

 limb to the principal functions it fulfils in the 

 animal kingdom walking upon two legs, running, 

 jumping, climbing, burrowing, swimming, and flying. 

 We shall find that the transformations it has under- 

 gone for all these functions have been accompanied 

 by degeneration. 



1. Adaptation to walking on two legs. Adapta- 

 tion to the upright position may be effected in two 

 different ways, by walking or jumping. We will 

 first investigate the former. 



This adaptation to bipedal progression may be 

 effected with or without the intervention of arboreal 

 life, as may be seen by the following examination, 

 the adaptations of man and of birds. 



(a) Man. The first sign, among mammals, of a 

 definite adaptation to arboreal life, is the oppos- 

 ability of the great toe. The great toes of human 

 beings are not opposable, the cases of " prehension " 

 exhibited among savage races being due to lateral 

 movements ; we shall see presently, however, that 

 this opposability of the great toe which is exhibited 



