168 THE DESCENT OF MAN. 



The degrees of sterility do not coincide strictly with the degrees 

 of difference between the parents in external structure or habits 

 of life. Man in many respects may be compared with those ani- 

 mals which have long been domesticated, and a large body of 

 evidence can be advanced in favor of the Pallasian doctrine," 

 that domestication tends to eliminate the sterility which is so 

 general a result of the crossing of species in a state of nature. 

 Prom these several considerations, it may be justly urged that 

 the perfect fertility of the intercrossed races of man, if estab- 

 lished, would not absolutely preclude us from ranking them as 

 distinct species. 



Independently of fertility, the characters presented by the off- 

 spring from a cross have been thought to indicate whether or not 

 the parent-forms ought to be ranked as species or varieties; but 

 after carefully studying the evidence, I have come to the con- 

 clusion that no general rules of this kind can be trusted. The 



" 'The Variation of Animals and Plants under Domestication,' vol. ii. 

 p. 109. I may here remind the reader that the sterility of species 

 when crossed is not a specially-acquired quality, but, like the inca- 

 pacity of certain trees to be grafted together, is incidental on oth«r 

 acquired differences. The nature of these differences is unknown, 

 but they relate more especially to the reproductive system, and much 

 less so to external structure or to ordinary differences in constitution. 

 One important element in the sterility of crossed species apparently 

 lies in one or both having been long habituated to fixed conditions; 

 for we know that changed conditions have a special influence on the 

 reproductive system, and we have good reason to believe (as before 

 remarked) that the fluctuating conditions of domestication tend to 

 eliminate that sterility which is so general with species, in a natural 

 state, when crossed. It has elsewhere been shown by me (ibid. vol. ii. 

 p. 185, and 'Origin of Species' 5th edit., p. 317), that the sterility of 

 crossed species has not been acquired through natural selection: we 

 can see that when two forms have already been rendered very sterile, 

 it is scarcely possible that their sterility should be augmented by the 

 preservation or survival of the more and more sterile individuals; 

 for as the sterility increases, fewer and fewer offspring will be pro- 

 duced from which to breed, and at last only single individuals will 

 be produced at the rarest intervals. But there is even a higher grade 

 of sterility than this. Both Gartner and Kolreuter have proved that 

 in genera of plants including many species, a, series can be formed 

 from species which when crossed yield fewer and fewer seeds, to 

 species which never produce a single seed, but yet are affected by the 

 pollen of the other species as shown by the swelling of the germen. 

 It is here manifestly impossible to select the more sterile individuals, 

 which have already ceased to yield seeds; so that the acme of ster- 

 ility, when the germen alone is affected cannot have been gained 

 through selection. This acme, and no doubt the other grades of ster- 

 ility, are the incidental results of certain unknown differences in the 

 constitution of the reproductive system of the species which are 

 crossed. 



