8 HEREDITY [ch. 



originally created, and divergences from this type 

 were regarded as imperfections. But it was obvious 

 that there was always more or less fluctuation about 

 the type in different individuals, and breeders of 

 plants and animals made use of this want of uni- 

 formity to select the best specimens and so to 

 improve the race. The Natural Selection theory 

 of Darwin and Wallace supposes that a process 

 comparable with this takes place in nature, and so 

 brings about the adaptations of natural species. 



Of the causes which induce variation nothing 

 definite was known, but Darwin's belief was generally 

 accepted that it is due to changes in environment 

 acting directly or indirectly on the organism. He 

 regarded the action of such changes as cumulative 

 through a number of generations, so that its effect 

 in producing variation might not be visible until the 

 change had acted on several generations. This belief 

 was founded on the observation that animals bred 

 in captivity appear to be much more variable than in 

 the wild condition, and the changed conditions of 

 life are supposed to induce the variation. But species 

 in nature are not by any means subject to uniform 

 environment, and thus their variability was ascribed 

 to similar causes. 



Darwin and Wallace pointed out that variation 

 occurs in all parts of every species, that it appears 

 to occur in every possible direction, and to every 



