Darwin's Artificial and Natural Selection 105 



that this had appeared as a monstrosity ; but I have as yet 

 failed to find, after diligent search, cases of monstrosities 

 resembling normal structures in nearly allied forms, and 

 these alone bear on the question. If monstrous forms of this 

 kind ever do appear in a state of nature and are capable of 

 reproduction (which is not always the case), as they occur 

 rarely and singly, their preservation would depend on unusu- 

 ally favorable circumstances. They would, also, during the 

 first and succeeding generations cross with the ordinary form, 

 and thus their abnormal character would almost inevitably be 

 lost." 



It is clear that Darwin does not think that the sudden and 

 large variations that sometimes occur furnish the basis for 

 natural selection, and the final statement in the last citation 

 (which was added in later editions of the " Origin of Species "), 

 to the effect that if such monstrous variations appeared as 

 single or occasional variations they would be lost by intercross- 

 ing implies that, in general, single variations would likewise 

 be lost unless they appeared in a sufficient number of indi- 

 viduals to maintain themselves against the swamping effects 

 of intercrossing. 



It is necessary to quote again, in order to show that, in 

 some cases at least, Darwin believed selection plays little or 

 no part in the origin and maintenance of certain peculiarities 

 that are of no use to the species. " There is one point con- 

 nected with individual differences, which is extremely per- 

 plexing : I refer to those genera which have been called 

 protean or ' polymorphic,' in which the species present an 

 inordinate amount of variation. With respect to many of 

 these forms, hardly two naturalists agree, whether to rank 

 them as species or as varieties. We may instance Rubus, 

 Rosa, and Hieracium amongst plants, several genera of in- 

 sects and of Brachiopod shells. In most polymorphic genera 

 some of the species have fixed and definite characters. Gen- 

 era which are polymorphic in one country seem to be, with 



