416 THE ORIGIN OF SPECIES 



the various climatical and geographical changes which have af- 

 fected 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 occa- 

 sional means of transport. With respect to distinct species of the 

 same genus, inhabiting distant and isolated regions, as the process 

 of modification has necessarily been slow, all the means of migra- 

 tion will have been possible during a very long period; and con- 

 sequently 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 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 con- 

 tinuous, 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 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 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 

 older species, why is not every geological formation charged with 

 such links? Why does not every collection of fossil remains afford 



