230 THE DESCENT OF MAN 



nate the sterility which is so general a result of the crossing 

 of species in a state of nature. From these several con- 

 siderations 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 spe- 

 cies or varieties; but, after carefully studying the evidence, 

 I have come to the conclusion that no general rules of this 

 kind can be trusted. The ordinary result of a cross is the 

 production of a blended or intermediate form; but in certain 

 cases some of the offspring take closely after one parent- 

 form, and some after the other. This is especially apt to 

 occur when the parents differ in characters which first 

 appeared as sudden variations or monstrosities." I refer 

 to this point because Dr. Rohlfs informs me that he has 

 frequently seen in Africa the offspring of negroes crossed 

 with members of other races, either completely black or 

 completely white, or rarely piebald. On the other hand, it 

 is notorious that in America mulattoes commonly present 

 an intermediate appearance. 



We have now seen that a naturalist might feel himself 



been shown by me (ibid., toI. ii. p. 185, and "Origin of Species," 5th edit, 

 p. 311) 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 sterUity 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 produced from which to 

 breed, and at last only single individuals will be produced, at the rarest inter- 

 vals. 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 

 poUen 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, ard no doubt 

 the other grades of sterihty, are tlie incidental results of certain unknown 

 differences in the constitution of the reproductive system of the species which 

 are crossed. 



16 oTjjg Yariation of Animals," etc., vol ii. p. 92. 



