Chap. XV.] RECAPITULATION. 271 



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 diffu- 

 sion of the same species ; for during very long periods 

 there will always have been a good chance for wide 

 migration by many means. A broken or interrupted 

 range may often be accounted for by the extinction of 

 the species in the intermediate regions, lit 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 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 dis- 

 tribution 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 during a very long period; and 

 consequently the diflficulty 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 

 interminable number of intermediate forms must have 

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