50 PARALLELISM IN DEVELOPMENT. 



ruminants is, however, comparatively remote, but in the 

 latter group and the camels these teeth are so alike as to 

 require an expert to distinguish between them. Never- 

 theless, there is good evidence to show that the camels and 

 the ruminants, if not also the chevrotains, have acquired 

 their crescent-like molar teeth quite independently of one 

 another, and it therefore yet remains for those writers 

 who explain evolution by some mode of what they are 

 pleased to call natural selection to account adequately for 

 the similarity thus existing between structures of such 

 totally different origin, when they could have been made 

 equally efficient if unlike. 



Passing from the consideration of teeth to that of limbs, 

 we may mention the remarkable similarity displayed in the 

 mode by which the lower segments of the limbs of the 

 even-toed and odd -toed hoofed mammals have been gradu- 

 ally elongated by the formation of a cannon-bone and the 

 disappearance of either three or four of the lateral digits ; 

 the cannon-bone in the horse consisting of but a single 

 element carrying one digit, while in the ruminants it 

 comprises two united elements supporting a pair of toes. 

 This is clearly a case of parallelism in development 

 attained by a slightly different modification of principle. 

 The parallelism does not, however, stop here, since an 

 essentially similar type of cannon-bone has been produced 

 in birds ; only that in that group (with the exception of the 

 ostrich) three long bones enter into its composition, which 

 is further complicated by the addition of a bone from 

 the ankle above. Seeing that in all these groups the 

 parallelism has been arrived at by a different structural 

 modification, the explanation of its mode of evolution is 

 much less difficult than in the case of the molars of the 

 camels and ruminants, where, as we have seen, the struc- 

 ture is practically identical. 



Recent discoveries in North America have brought to 

 light the existence of a kind of secondary parallelism 

 among certain peculiar mammals which may be included 

 among the hoofed or ungulate division of that class. In 

 the even-toed group of that division, as exemplified by 

 the pigs and ruminants, it is the third and fourth digits 



