12 SEXUAL DIMORPHISM 



unity of plan is due to heredity, the divergence of detail 

 to adaptation under different conditions. But we must 

 pursue the investigation further and endeavour to discover 

 how the modification is effected. It may be asked, since we 

 admit that adaptation is such a prevalent phenomenon in 

 the animal kingdom, even if it is not universal and exclusive, 

 what other explanation of it is required than natural selec- 

 tion ? In reply to this I would urge in the first place that 

 natural selection implies and assumes the appearance of 

 variations, of slight modifications, by virtue of which certain 

 individuals differ from their brethren and from their parents, 

 in fact from all pre-existing individuals. The real explana- 

 tion of evolution therefore lies in the explanation of in- 

 dividual variations. We admit that they occur, but how and 

 why ? 



Darwin held that the use or disuse of organs and the 

 direct action of conditions caused modifications of individuals 

 in definite directions, and that these modifications were 

 hereditary in some degree. Now, if once we admit this, 

 selection becomes a secondary and subordinate factor. For 

 if a new set of conditions, or a change of habits, caused a 

 hereditary change of structure in all the individuals exposed 

 to it, continuous modification would take place even if all the 

 individuals generated survived, or if those which were killed 

 were taken at random without selection. A later school of 

 evolutionists have maintained that the effects of habits or 

 conditions on the individual are not inherited, and therefore 

 not cumulative. According to this view only those varia- 

 tions are hereditary which arise in the germ, in the internal 

 constitution of the egg ; such variations are supposed to be 

 numerous and to take place in all possible directions, and 

 natural selection is supposed to pick out from among them 

 those which are advantageous, and so accumulate them. I 

 do not propose here to discuss the various theories of 



