DARWIN AXD HIS CRITICS. 61 



ranean oyster at once.* And on several different 

 occasions, the black- shouldered peacock has 

 appeared suddenly in different flocks in England, 

 sometimes increasing so rapidly as completely 

 to supplant the common kind.f The black- 

 shouldered peacock is unknown in a wild 

 state, yet it is so unlike the common form, 

 that some of our most eminent ornithologists 

 have not hesitated to regard it as a distinct 

 species, reappearing in consequence of the 

 breed having been formerly crossed with it. 

 ]>ut most naturalists are now of opinion 

 that this form is no more an independent 

 species than any of the breeds of our domestic 

 animals. 



Andrew Murray J was also of opinion that 

 species might arise by sudden variation. He 

 considered that a species might remain tolerably 

 stable during a long period, from the absence 

 of any special stimulus to variation, till an 

 impetus was given to it by some change in 

 surrounding conditions, and that a species 



* Costa, " Bull, de la Soc. Imp. d' Acclimatisation," tome viii. p. 351, 

 quoted l>y Darwin. 



t Darwin, " Variation of Animals and Plants under Domestication," 

 vol. i. p. 291. 



} " Geographical Distribution of Mammalia," ch. ii. 



