DIFFICULTIES OF THE THEORY 279 



ful for beauty's sake; but this has been effected through 

 sexual selection, that is, by the more beautiful males hav- 

 ing been continually preferred by the females, and not 

 tor the delight of man. So it is with the music of birds. 

 We may infer from all this that a nearly similar taste 

 for beautiful colors and for musical sounds runs through 

 a large part of the animal kingdom. When the female 

 is as beautifully colored as the male, which is not rarely 

 the case with birds and butterflies, the cause apparently 

 lies in the colors acquired through sexual selection hav- 

 ing been transmitted to both sexes, instead of to the 

 males alone. How the sense of beauty in its simplest 

 form — that is, the reception of a peculiar kind of pleas- 

 ure from certain colors, forms, and sounds — was first de- 

 veloped in the mind of man and of the lower animals 

 is a very obscure subject. The same sort of difficulty is 

 presented, if we inquire how it is that certain flavors 

 and odors give pleasure, and others displeasure. Habit 

 in all these cases appears to have come to a certain 

 extent into play; but there must be some fundamental 

 cause in the constitution of the nervous system in each 

 species. 



Natural selection cannot possibly produce any modifi- 

 cation in a species exclusively for the good of another 

 species; though throughout nature one species mcessantly 

 takes advantage of, and profits by, the structures of oth- 

 ers. But natural selection can and does often produce 

 structures for the direct injury of other animals, as we 

 see in the fang of the adder, and in the ovipositor of the 

 icbneumon, by which its eggs are deposited in the living 

 bodies of other insects. If it could be proved that any 



