Chap. XIV. EECAPITULATION. 461 



varieties which have been experimented on have been 

 produced under domestication ; and as domestication 

 apparently tends to eliminate sterility, we ought not to 

 expect it also to produce sterility. 



The sterility of hybrids is a very different case from 

 that of first crosses, for their reproductive organs are 

 more or less functionally impotent; whereas in first 

 crosses the organs on both sides are in a perfect con- 

 dition. As we continually see that organisms of all 

 kinds are rendered in some degree sterile from their 

 constitutions having been disturbed by slightly dif- 

 ferent and new conditions of life, we need not feel 

 surprise at hybrids being in some degree sterile, for 

 their constitutions can hardly fail to have been dis- 

 turbed from being compounded of two distinct organisa- 

 tions. This parallelism is supported by another parallel, 

 but directly opposite, class of facts ; namely, that the 

 vigour and fertility of all organic beings are increased 

 by slight changes in their conditions of life, and that 

 the offspring of slightly modified forms or varieties ac- 

 quire from being crossed increased vigour and fertility. 

 So that, on the one hand, considerable changes in the 

 conditions of life and crosses between greatly modified 

 forms, lessen fertility ; and on the other hand, lesser 

 changes in the conditions of life and crosses between 

 less modified forms, increase fertility. 



Turning to geographical distribution, the difficulties 

 encountered on the theory of descent with modification 

 are grave enough. All the individuals of the same 

 species, and all the species of the same genus, or even 

 higher group, must have descended from common 

 parents ; and therefore, in however distant and isolated 

 parts of the world they are now found, they must in the 

 course of successive generations have passed from some 

 one part to the others. We are often wholly unable 



