138 THE GKXESrS OF SPECIES. [Chap. 



Santo raliLit brouu:ht to Eii'j^liind reverted iu a iiianner tlie 

 most striking, recovering the proper colour of it.s fui' " in 

 rather less than four years." ^ Again, the wliite silk fowl, 

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

 mon fuwl in its skin and bones, due care having been taken 

 to prevent any cross."- Tliis reversion taking place in 

 spite of careful selection is very remarkable. 



Xumcrous other instances of reversion are given by ^Ir. 

 Darwin, both as regards plants and animals ; amongst 

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

 recurring development of black sheep, in spite of tlie 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, wliile 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 Avell 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 possil)le 

 causes of reversi(jn by ^Ir. Darwin, can hardly be such. 

 He cites, for example, the occasional presence of super- 

 numerary digits in man."* 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^ 



' "Animals and Plants iukUt Domestication," vol. i, ]>. 114. 



- Iliid. vol. i. ]). 243. 3 ji,i,i yt)l. ii. p. 361. 



* Ibid. vol. ii. j>. 16. Since the jmlilioation of the first edition of the 

 *' Genesis of Species," Mr. Darwin lias admitted his mistake in this 

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

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



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



