136 THE GENESIS OF SPECIES. [CHAP. 



is no argument in favor of its existence, until it can be 

 shown that the causes of variability in the one case are the 

 same as in the other. An argument against it, however, 

 may be drawn from the fact that certain animals, though 

 placed under the influence of those exceptional causes of 

 variation to which domestic animals are subject, have yet 

 never been known to vary, even in a degree equal to that 

 in which certain wild kinds have been ascertained to vary. 



In addition to this immutability of character in some 

 animals, it is undeniable that domestic varieties have little 

 stability, and much tendency to reversion, whatever be the 

 true explanation of such phenomena. 



In controverting the generally received opinion as to 

 " reversion," Mr. Darwin has shown that it is not all breeds 

 which in a few years revert to the original form ; but he 

 has shown no more. Thus, the feral rabbits of Porto Santo, 

 Jamaica, and the Falkland Islands, have not yet so reverted 

 in those several localities. 10 Nevertheless, a Porto Santo 

 rabbit brought to England reverted in a manner the most 

 striking, recovering the proper color of its fur " in rather 

 less than four years." ] Again, the white silk fowl, in our 

 climate, " reverts to the ordinary color of the common fowl 

 in its skin and bones, due care having been taken to pre- 

 vent any cross." ] 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 ; among others, 

 the singular fact of bud reversion. 13 The curiously-recurr- 

 ring development of black sheep, in spite of the most care- 

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

 reversion has no part in the phenomenon. 



These facts seem certainly to tell in favor of limited 



10 " Animals and Plants under Domestication," vol. L, p. 115. 



11 Ibid., vol. L, p. 114. 12 Ibid., vol. i., p. 243 



13 Ibid., vol. ii., p. 361. 



