138 THE GENESIS OF SPECIES. [CHAP. 



Santo rabbit brought to England reverted in a manner the 

 most striking, recovering the proper colour of its fur " in 

 rather less than four years." 1 Again, the white silk fowl, 

 in our climate, " reverts to the ordinary colour of the com- 

 mon fowl in its skin and bones, clue care having been taken 

 to prevent any cross." 2 This reversion taking place in 

 spite of careful selection is very remarkable. 



Numerous other instances of reversion are given by Mr. 

 Darwin, both as regards plants and animals ; amongst 

 others, the singular fact of bud reversion. 3 The curiously 

 recurring development of black sheep, in spite of the most 

 careful breeding, may also be mentioned, though, perhaps, 

 reversion has no part in the phenomenon. 



These facts seem certainly to tell in favour of limited 

 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. 



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 super- 

 numerary digits in man. 4 For this notion, however, he is 

 not responsible, as he rests his remark on the authority of 

 a passage published by Professor Owen. Again, he refers 5 



1 "Animals and Plants under Domestication," vol. i. p. 114. 



2 Ibid. vol. i. p. 243. 3 Ibid. vol. ii. p. 361. 



4 Ibid. vol. ii. p. 16. Since the publication of the first edition of the 

 "Genesis of Species," Mr. Darwin has admitted his mistake in this 

 matter, as also in the attribution by him of supernumerary mammse to 

 reversion. See "Descent of Man," vol. i. p. 125. 



5 "Animals and Plants under Domestication," vol. ii. p. 57. 



