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 réle ; 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 
