vii ON THE INFERTILITY OF CROSSES 159 



own style, in any of the three forms, were either comparatively 

 or wholly sterile. 1 



We have here a wonderful amount of constitutional 

 difference of the reproductive organs within a single species, 

 greater than usually occurs within the numerous distinct 

 species of a genus or group of genera ; and all this diversity 

 appears to have arisen for a purpose which has been obtained 

 by many other, and apparently simpler, changes of structure 

 or of function, in other plants. This seems to show us, in the 

 first place, that variations in the mutual relations of the repro- 

 ductive organs of different individuals must be as frequent as 

 structural variations have been shown to be ; and, also, that 

 sterility in itself can be no test of specific distinctness. But 

 this point will be better considered when we have further 

 illustrated and discussed the complex phenomena of hybridity. 



Cases of the Fertility of Hybrids, and of the Infertility of Mongrels. 



I now propose to adduce a few cases in which it has been 

 proved, by experiment, that hybrids between two distinct 

 species are fertile inter se; and then to consider why it is that 

 such cases are so few in number. 



The common domestic goose (Anser ferus) and the Chinese 

 goose (A. cygnoides) are very distinct species, so distinct that 

 some naturalists have placed them in different genera ; yet they 

 have bred together, and Mr. Eyton raised from a pair of these 

 hybrids a brood of eight. This fact was confirmed by Mr. 

 Darwin himself, who raised several fine birds from a pair of 

 hybrids which were sent him. 2 In India, according to 

 Mr. Blyth and Captain Hutton, whole flocks of these hybrid 

 geese are kept in various parts of the country where neither 

 of the pure parent species exists, and as they are kept for 

 profit they must certainly be fully fertile. 



Another equally striking case is that of the Indian humped 

 and the common cattle, species which differ osteologically, and 

 also in habits, form, voice, and constitution, so that they are 

 by no means closely allied ; yet Mr. Darwin assures us that he 



1 For a full account of these interesting facts and of the various problems 

 to which they give rise, the reader must consult Darwin's volume on The 

 Different Forms of Flowers in Plants of the same Species, chaps, i.-iv. 



2 See Nature, vol. xxi. p. 207. 



