1/2 The Descent of Man. Part I. 



fertile. 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 advanced in favour of the Pallasian 

 doctrine, 14 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 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 

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



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

 Plants under Domestication,' vol. ii. their sterility should be augmented 

 p. 109. I may here remind the by the preservation or survival of 

 reader that the sterility of species the more and more sterile ind-i- 

 when crossed is not a specially- viduals ; for as the sterility in- 

 acquired quality, but, like the in- creases, fewer and fewer offspring 

 capacity of certain trees to be graft- will be produced from which to 

 ed together, is incidental on other breed, and at last only single in- 

 acquired differences. The nature dividuals will be produced, at the 

 of these differences is unknown, but rarest intervals. But there is even 

 they relate more especially to the re- a higher grade of sterility than 

 productive system, and much less so this. Both Gartner and Kolreuter 

 to external structure or to ordinary have proved that in genera of plant? 

 differences in constitution. One including many species, a series 

 important element in the sterility can be formed from species which 

 of crossed species apparently lies in when crossed yield fewer and fewei 

 one or both having been long habi- seeds, to species which never pro- 

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

 know that changed conditions have affected by the pollen of the othei 

 a special influence on the repro- species, as shewn by the swelling 

 ductive system, and we have good of the germen. It is here mani- 

 reason to believe (as before re- festly impossible to select the more 

 marked) that the fluctuating con- sterile individuals, which have al- 

 ditions of domestication tend to ready ceased to yield seeds ; so that, 

 eliminate that sterility which is so the acme of sterility, when the 

 general with species, in a natural germen alone is affected, cannot 

 state, when crossed. It has else- have been gained through selection, 

 where been shewn by me (ibid. vol. This acme, and no doubt the other 

 ii. p. 185, and 'Origin of Species' grades of sterility, are the incidental 

 5th edit. p. 317), that the sterility results of certain unknown differ- 

 of crossed species has not been ac- ences in the constitution of the re- 

 quired through natural selection : productive system of the speciei 

 we can see that when two forms which are crossed 

 have already been rendered very 



