Chap. yij. 
THE RACES OP MAN. 
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, 
Vv '°uld 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 
a nord evidence whether the parent-forms ought to be 
j'anked as species or varieties ; but after carefully study- 
the evidence, I have come to the conclusion that no 
S e neral rules of this kind can be trusted. Thus with 
mankind the offspring of distinct races resemble in all 
r® 8 pects the offsj wing of true species and of varieties, 
^ds i s shewn, for instance, by the manner in which 
characters of both parents are blended, and by 
one form absorbing another through repeated crosses. 
11 this latter case the progeny both of crossed species 
a nd varieties retain for a long period a tendency to 
y e vert to their ancestors, especially to that one which 
ls prepotent in transmission. When any character has 
Suddenly appeared in a race or species as the result of a 
t ) °ssi'bl e that their sterility should be augmented by the preservation or 
? Ur vival of the more and more slerile individuals; for as the sterility 
greases fewer and fewer offspring will be produced from which to 
. re ed,and at last only single individuals will be produced, at tire rarest 
Q.. ei vitls. But there is even a higher grade of sterility than this. Both 
^ ar frier and Ivdlreuter have proved that in genera of plants including 
^ Onerous species, a series can be formed from species which when 
s |°S3ed ybdd fewer and fewer seeds, to species which never produce a 
t , n = le seed, but yot are affected by the pollen of the other species, for 
G german swells, It is here manifestly impossible to select the more 
ti^ r *k individuals, which have already ceased to yield seeds; so that 
„ ? acme of sterility, when the ganiicn alone is affected, cannot be 
Mined through selection. This acme, and no doubt the other grades 
in are the incidental results of certain unknown differences 
c Jle const itution of the reproductive system of the species which are 
