CHAP. XV.] RECAPITULATION. 407 



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 distribution of 

 the same and of allied species throughout the world. We are as 

 yet profoundly ignorant of the many occasional means of trans- 

 port. With respect to distinct species of the same genus inha- 

 biting 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 interminable 

 number of intermediate forms must have existed, linking together 

 all the species in each group by gradations as fine as are 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 ex- 

 pect (excepting in rare cases) to discover directly connecting links 

 between them, but only between each and some extinct and sup- 

 planted 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 

 occupied by one species into another district occupied by a closely 

 allied species, we have no just right to expect often to find inter- 

 mediate 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, 

 Avould be liable to be supplanted by the allied forms on either 

 hand ; for the latter, from existing in greater numbers, would 

 generally be modified and improved at a quicker rate than the 

 intermediate varieties, which existed in lesser numbers ; so that 

 the intermediate varieties would, in the long run, be supplanted 

 and exterminated. 



On this doctrine of the extermination of an infinitude of con- 

 necting links, between the living and extinct inhabitants of the 

 world, and at each successive period between the extinct and still 



