272 BBCAPITULATION. [Chap. XV. 



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 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 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 be- 

 coming 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 modi- 

 fications 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 sup- 

 planted by the allied forms on either hand; for the lat- 

 ter, 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 infini- 

 tude of connecting 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 



