liECAFITULATION. 479 



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 

 liigher 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 traveled from some one point 

 to all the others. We are often wholly unable even to con- 

 jecture 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 mnch stress ought not to be laid 

 on the occasional wide diffusion 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 ex- 

 tinction of the species in the intermediate regions. It can 

 not be denied that we are as yet very ignorant as to the full 

 extent of the various climatical 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 occasional means 

 of transport. With respect to distinct species of the same 

 genus, inhabiting distant and isolated regions, as the pro- 

 cess 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. 



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 fineas 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? 

 With respect to existing forms, we should remember that 

 we have no right to expect (excepting in rare cases) to dis- 

 cover directly connecting links between them, but only be- 



