II.] SPECIFIC STABILITY. 137 



variability, while the 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 long periods indeed. 



Yet some of the instances given as probable or possible 

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

 cites, for example, the occasional presence of supernumerary 

 digits in man. 14 For this notion, however, he is not re- 

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

 sage published by Prof. Owen. Again, he refers 1B 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 affinity. 

 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 Ungulata. But of the two, their affinities with 

 the odd-toed division are more marked. 16 



Another argument in favor of the, at least intermitting, 

 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. 



14 "Animals and Plants under Domestication," vol. ii., p. 16. 



15 Ibid., vol. ii., p. 57. 



16 This has been shown by my late friend Mr. H. N. Turner, Jr., in 

 an excellent paper by him in the u Proceedings of the Zoological Society 

 for 1849," p. 147. The untimely death, through a dissecting wound, of 

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



