384 Darwin, and after Darwin. 



males rival one another to the best of their musical 

 ability. 



Thus there can be no question that the courtship 

 of birds is a highly elaborate business, in which the 

 males do their best to surpass one another in charming 

 the females. Obviously the inference is that the males 

 do not take all this trouble for nothing ; but that the 

 females give their consent to pair with the males 

 whose personal appearance, or whose voice, proves to 

 be the most attractive. But, if so, the young of the 

 male bird who is thus selected will inherit his superior 

 beauty ; and thus, in successive generations, a con- 

 tinuous advance will be made in the beauty of 

 plumage or of song, as the case may be, both the 

 origin and development of beauty in the animal world 

 being thus supposed due to the aesthetic taste of 

 animals themselves. 



Such is the theory of sexual selection in its main 

 outlines ; and with regard to it we must begin by 

 noting two things which are of most importance. In 

 the first place, it is a theory wholly and completely 

 distinct from the theory of natural selection ; so that 

 any truth or error in the one does not in the least 

 affect the other. The second point is, that there is 

 not so great a wealth of evidence in favour of sexual 

 selection as there is in favour of natural selection ; 

 and, therefore, that while all naturalists nowadays 

 accept natural selection as a (whether or not the) cause 

 of adaptive, useful, or life-preserving structures, there 

 is no such universal but only a very general agree- 

 ment with reference to sexual selection as a cause 

 of decorative, beautiful, or life-embellishing struc- 

 tures. Nevertheless, the evidence in favour of sexual 



