PAET III. 



VAKIATION IN ITS BELATION TO 

 EVOLUTION. 



CHAPTER XI. 



THE ACTION OF NATUKAL SELECTION ON 



VAKIATIONS. 



"Selection does nothing without variability, and this depends in some 

 manner on the action of the surrounding circumstances on the 

 organism" (Darwin, " Animals and Plants," i. p. 7). 



1 ' The foundation of the Darwinian theory is the variability of species " 

 (Wallace, "Darwinism" p. 41). 



" What forms the basis of the constant 'individual variations' which, 

 after the precedent of Darwin and Wallace, we regard as the foun- 

 dation of all processes of natural selection?" (Weismann, "Germ- 

 Plasm," p. 410.) 



Proof of Natural Selection in the crab and in the sparrow Selection 

 in man Evolution of the mouse Inheritance of acquired char- 

 acters seems to be shown by cumulative effects of conditions of 

 life, as European climate acting on American maize; domestica- 

 tion acting on wild turkeys and ducks ; changed climate acting on 

 sheep and dogs Environment may act on germ- plasm through 

 specific excretions and secretions Cases of inherited effects of use 

 and disuse, and of epilepsy, accounted for Somatic variations 

 may increase variability, and so afford Natural Selection a better 

 handle to work upon. 



THE contents of this chapter are well summarised in 

 the three quotations given at its head. It deals with 

 variations in their relation to Natural Selection, and 



335 



