196 THE DESCENT OF MAN. 



the degrees of difference between the parents in external 

 structure or habits of life. Man in many respects may be 

 compared with those animals which have long been domes- 

 ticated, 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. From these sev- 

 eral considerations, it may be justly urged that the perfect 

 fertility of the intercrossed races of man, if established, 

 would not absolutely preclude us from ranking them as 

 distinct species. 



Independently of fertility, the characters presented by 

 the offspring from a cross have been thought to indicate 

 whether or not the parent-forms ought to be ranked as 



* '* 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 other 

 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 steril- 

 ity 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 aug- 

 mented by the preservation or survival of the more and more sterile 

 individuals; for, as the sterility increases, fewer and fewer offspring 

 will be produced from which to breed, and at last only single indi- 

 viduals will be produced at the rarest intervals. But there is even a 

 higher grade of sterility than this. Both Gartner and Kol renter 

 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 sterility, when the germen alone is 

 affected, cannot have been gained through selection. This acme, 

 and no doubt the other grades of sterility, are the incidental results 

 of certain unknown differences in the constitution of the reproductive 

 system of the species which are crossed. 



