II-l SPECIFIC STABILITY. I37 



variabilify, while tlic cases of non*reversion do not contra- 

 dict it, as it is not contended that all species have the same 

 tendency to revert, but rather that their capacities in this 

 respect, as well as for change, are different in different 

 kinds, so that often reversion may only show itself at the 

 end of very Ion*];- periods indeed. 



Yet some of the instances given as probable or possible 

 causes of reversion by Mr. Darwin, can hardlj' be such. He 

 cites, for examjile, the occasional presence of supernumerary 

 dig-its in man.'* For this notion, however, he is not re- 

 sponsible, as he rests his remark on the autliority of a pas- 

 sage published by Prof. Owen. Again, he refers '* to "the 

 greater frequency of a monster proboscis in the pig than 

 in any other animal." But with the exception of the pe- 

 culiar muzzle of the Saiga (or European antelope), the 

 only known proboscidian Ungulates are the elephants and 

 tapirs, and to neither of these has the pig any close aninit3\ 

 It is rather in the horse than in the pig that we might look 

 for the appearance of a reversionary proboscis, as both the 

 elephants and the tapirs have the toes of the hind-foot of 

 an odd number. It is true that the elephants are generally 

 considered to form a group apart from both the odd and the 

 even toed Ungiilata. But of the two, their affinities with 

 the odd-toed division are more marked. " 



Another argument in favor of the, at least intermiiting, 

 constancy of specific forms and of sudden modification, may 

 be drawn from the absence of minute transitional forms, but 

 this will be considered in the next chapter. 



''• "Animals and Tlants under Domestication," vol. ii., p. IC. 



'5 Ibid., vol. ii., p. 57. 



'" This 1ms boon shown by my lato friend Mr. II. N. Turner, Jr., in 

 an excellent paper by him in the " rroceedings of the Zoological Society 

 for 184f>," p. 147. The untimely death, through a dis5ecting wound, of 

 this most promising young naturalist, was a very great loss to zoological 

 science. 



