462 RECAPITULATION. . Chap. XIV. 



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 be 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 de- 

 nied that we are as yet very ignorant of the full extent 

 of the various climatal and geographical changes which 

 have affected the earth during modern periods ; and 

 such changes will obviously have greatly facilitated mi- 

 gration. As an example, I have attempted to show 

 how potent has been the influence of the Glacial period 

 on the distribution both of the same and of representa- 

 tive species throughout the world. We are as yet pro- 

 foundly ignorant of the many occasional means of trans- 

 port. With respect to distinct species of the same 

 genus inhabiting very 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 species of the same genus is in some 

 degree lessened. 



As on the theory of natural selection an interminable 

 number of intermediate forms must have existed, linking 

 together all the species in each group by gradations as 

 fine as our present 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 inex- 

 tricable chaos ? With respect to existing forms, we 

 should remember that we have no right to expect (ex- 

 cepting in rare cases) to discover directly connecting 



