RECAPITULATION AND CONCLUSION 415 



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 differ- 

 ent 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 or- 

 ganisations. 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 acquire 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 

 even to conjecture how this could have been effected. 

 Yet, as we have reason to believe that some species 

 have retained the same specific form for very long 

 periods, enormously long as measured by years, too 

 much stress ought not to be laid on the occasional wide 

 diffusion of the same species ; for during very long 

 periods of time there will always have been a good 

 chance for wide migration by many means. A broken 

 or interrupted range may often be accounted for bv 



