Organic Evolution. 205 



any effect in furthering a differentiation of form or colour. 

 He admits that so soon as a slight differentiation of form 

 or colour has been effected, segregation will arise by the 

 selective association of the animals themselves ; but he 

 does not admit that such selective association can carry 

 the differentiation further. 



Now, it is clear that mating preferences must be either 

 fixed or variable. If fixed, how can differentiation occur 

 in the same flock or herd ? And how can selective asso- 

 ciation be a means of isolation? Or, granting that dif- 

 ferentiation has occurred, if the mating preferences are 

 then stereotyped, all further differentiation, so far as colour 

 and form are concerned, will be rendered impossible ; for 

 divergent modifications, not meeting the stereotyped 

 standard of taste, will for that reason fail to be perpetuated. 

 We must admit, then, that these mating preferences are 

 subject to variation. And now we come to the central 

 question with regard to sexual selection by means of 

 preferential mating. What guides the variation along 

 special lines leading to heightened beauty ? This, I take 

 it, is the heart and centre of Mr. Wallace's criticism of 

 Darwin's hypothesis. Sexual selection of preferential 

 mating involves a standard of taste ; that standard has 

 advanced from what we consider a lower to what we con- 

 sider a higher sesthetic level, not along one line, but along 

 many lines. What has guided it along these lines ? 



Not as in any sense affording a direct answer to this 

 question, but for illustrative purposes, we may here draw 

 attention to what seems to be a somewhat parallel case, 

 namely, the development of flowers through insect agency. 

 In his " Origin of Species," Darwin contended that flowers 

 had been rendered conspicuous and beautiful in order to 

 attract insects, adding, " Hence we may conclude that, if 

 insects had not been developed on the earth, our plants 

 would not have been decked with beautiful flowers, but 

 would have produced only such poor flowers as we see on 

 our fir, oak, nut, and ash trees, on grasses, docks, and 

 nettles, which are all fertilized through the agency of the 



