502 ORIGIN OF SPECIES 



cated varieties when crossed and their mongrel offspring are 

 perfectly fertile. 



Turning to geographical distribution, the difficulties en- 

 countered on the theory of descent with modification are 

 serious enough. All the individuals of the same species, and 

 all the species of the same genus, or even higher group, are 

 descended from common parents ; and therefore, in however 

 distant and isolated parts of the world they may now be found, 

 they must in the course of successive generations have 

 travelled from some one point to all 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 of time, immensely long as measured by years, too 

 much stress ought not to be laid on the occasional wide dif- 

 fusion of the same species; for during very long periods 

 there will always have been a good chance for wide migra- 

 tion by many means. A broken or interrupted range may 

 often be accounted for by the extinction of the species in the 

 intermediate regions. It cannot be denied that we are as yet 

 very ignorant as to the full extent of the various climatal 

 and geographical changes which have affected the earth dur- 

 ing modern periods; and such changes will often have facili- 

 tated migration. As an example, I have attempted to show 

 how potent has been the influence of the Glacial period on 

 the distribution of the same and of allied species throughout 

 the world. We are as yet profoundly ignorant of the many 

 occasional means of transport. With respect to distinct 

 species of the same genus inhabiting distant and isolated 

 regions, as the process of modification has necessarily been 

 slow, all the means of migration will have been possible dur- 

 ing a very long period ; and consequently the difficulty of the 

 wide diffusion of the species of the same genus is in some 

 degree lessened. 



As according to the theory of natural selection an inter- 

 minable number of intermediate forms must have existed, 

 linking together all the species in each group by gradations 

 as fine as are our existing varieties, it may be asked, Why do 

 we not see these linking forms all around us ? Why are not all 

 organic beings blended together in an inextricable chaos? 



