416 ON THE ORIGIN OF SPECIES 



the extinction of the species in the intermediate 

 regions. It cannot be denied 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 ob- 

 viously have greatly facilitated migration. As an 

 example, I have attempted to show how potent has 

 been the influence of the Glacial period on the dis- 

 tribution both of the same and of representative 

 species throughout the world. We are as yet pro- 

 foundly ignorant of the many occasional means of 

 transport. 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 

 daring 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 

 (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 climate and other conditions of life 

 change insensibly in going 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 intermediate varieties in the intermediate zone. 

 For we have reason to believe that only a few species 

 are undergoing change at any one period ; and all 

 changes are slowly effected. I have also shown that 

 the intermediate varieties which will at first probably 

 exist in the intermediate zones, will be liable to be 



