280 THE ORIGIN OF SPECIES 



>^ " . are serious enough. -2VIJ the individmil s of the same spe- 

 ' I cies, and all t he s pecies^ ot" t"ric~l>Trntc gcuus^ pr, even 

 ; higher group, are descended from common parents; ^ ;iDd 

 therefore, in ho wever distant and isolated parts of the 

 world t hey may now be found, they must in th e course 

 •of successive oenerations have f-'^^P-^^pd 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 ver}^ long periods of 

 time, immensely long as measured b}^ 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 mi- 

 gration by many means. A broken or interrupted range 

 may often be accounted for by the extinction of the spe- 

 cies in the intermediate regions. It cannot be denied 

 that we are as j'et very ignorant as to the full extent of 

 the various climatal and geographical changes which have 

 affected the earth during modern periods; and such 

 changes will often have facilitated 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 occa- 

 sional means of transport. With respect to distinct spe- 

 cies 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 during a very long period; and consequently the 

 difficulty of the wide diffusion of the species of the same 

 genus is in some degree lessened. 



