172 



The Descent of Man. 



Part 1, 



tertile. 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 animals which have long been domesticated, and a large 

 body of evidence can be adyanced in favour 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 several 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 oif- 

 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 

 ordinary result of a cross is the production of a blended or 



'* 'The Variation of Animals and sterile, it is scarcely possible that 



Plants under DoiuesticatioD,' 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 in- 

 capacity of certain trees to be graft- 

 ed together, is incidental on other 

 acquired differences. The nature 

 of these differences is unknown, but 

 they relate more especially to the re- 

 productive 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 habi- 



their sterility should be augmented 

 by the preservation or survival of 

 the more and more sterile indi- 

 viduals ; for as the sterility in- 

 creases, fewer and fewer offspring 

 wil 1 be produced from w hich to 

 breed, and at last only single in- 

 dividuals will be produced, at the 

 rarest intervals. But there is even 

 a higher grade of sterility than 

 this. Both Gartner and Kblreuter 

 have proved that in genera of plant," 

 including many species, a series 

 can be formed from species \\4iich 

 when crossed yield fewer and fewei 

 seeds, to species which never pi-o- 



tuated to fixed conditions ; for we ^ duce a single seed, but yet are 



know that changed conditions have 

 a special influence on the repro- 

 ductive system, and we have good 

 reason to believe (as before re- 

 marked) that the fluctuating con- 

 ditions of domestication tend to 

 eliminate that sterility which is so 

 general with species, in a natural 

 Btate, when crossed. It has else- 

 where been shewn by me (ibid. vol. 

 ii, p. ]85, and * Origin of Species' 

 5th edit. p. 317), that the sterility 

 of ( rossed species has not been ac- 

 quiied through natural selection : 

 we can see that when two forms 

 & ive already ])een rcndeied very 



affected by the pollen of the othe: 

 species, as shewn by the swelling 

 of the germen. It is here mani- 

 festly impossible to select the more 

 sterile individuals, which have al- 

 ready 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 differ- 

 ences in the constitution of the re- 

 productive system of the spoci<'i 

 which are crossed 



