456 Evolution and Adaptation 



The difference between the two points of view that we 

 are contrasting can be best brought out by considering 

 the two other kinds of selection which Darwin supposes to 

 have been at work ; namely, artificial and sexual selection. 



Darwin thinks that the results of artificial selection are 

 brought about by the breeder picking out fluctuating varia- 

 tions. It appears that he has probably overestimated the 

 extent to which this process can be carried ; for while there 

 can be no doubt that a certain standard, or fixity of type, 

 can be obtained by selecting fluctuating variations, yet it 

 now seems quite certain that the extent to which this can be 

 carried is very limited. It appears that other factors have 

 also played an important role ; amongst these the occasional 

 appearance of discontinuous variation, also the bringing under 

 cultivation of the numerous "smaller species" of De Vries, 

 or the so-called " single variations " of Darwin. Further, 

 the effects of intercrossing in all combinations of the above 

 forms of variations, followed by the selection of certain of 

 the new forms obtained, has been largely employed, and 

 also the direct influence of food and of other external con- 

 ditions, which may be necessary to keep the race up to a 

 certain standard, have played a part in some cases. The 

 outcome is, therefore, by no means so simple as one might 

 infer from Darwin's treatment of the subject in his "Origin 

 of Species." For these reasons, as well as for others that 

 have been given, it will be evident that the process of arti- 

 ficial selection cannot be expected to give a very clear idea 

 of how natural selection could act. 



It is, however, the process of sexual selection that brings 

 out in the strongest contrast the difference between Darwin's 

 main idea of natural selection and the law_ of the survival 

 of species. In sexual selection the competition is supposed 

 to be always between the individuals of the same species 

 and of the same sex. There can be no doubt in one's 

 mind, after reading "The Descent of Man," that Darwin 



