Chap. VII. 
THE EACES OF MAH. 
223 
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 ferti- 
lity of the intercrossed races of man, if established, 
would not absolutely preclude us from ranking them as 
distinct species. 
Independently of fertility, the character of the off- 
spring from a cross has sometimes been thought to 
afford evidence whether the parent-forms ought to be 
ranked as species or varieties ; but after carefully study- 
ing the evidence, I have come to the conclusion that no 
general rules of this kind can be trusted. Thus with 
mankind the offspring of distinct races resemble in all 
respects the offspring of true species and of varieties. 
This is shewn, for instance, by the manner in which 
the characters of both parents are blended, and by 
one form absorbing another through repeated crosses. 
In this latter case the progeny both of crossed species 
and varieties retain for a long period a tendency to 
revert to their ancestors, especially to that one which 
is prepotent in transmission. When any character has 
suddenly appeared in a race or species as the result of a 
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 produced 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 
numerous species, a series can be formed horn 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, for 
the germen swells. 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 be 
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. 
