450 RECAPITULATION. 



long periods of time, immensely 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 

 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 climati- 

 cal 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 through- 

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

 many occasional means of transport. With respect to dis- 

 tinct 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 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 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 

 discover directly connecting links between them, but only 

 between each and some extinct and supplanted form. Even 

 on a wide area, which has during a long period remained 

 continuous, and of which the climatic and other conditions 

 of life change insensibly in proceeding from a district occu- 

 pied by one species into another district occupied by a 

 closely allied species, we have no just right to expect often 

 to find intermediate varieties in the intermediate zones. For 

 we have reason to believe that only a few species of a genus 

 ever undergo change ; the other species becoming utterly 

 extinct and leaving no modified progeny. Of the species 

 which do change, only a few within the same country change 

 at the same time ; and all modifications are slowly effected. 

 I have also shown that the intermediate varieties which 

 probably at first existed in the intermediate zones, would be 



