INTRODUCTION 31 



tion, or variability, on which selection acts. Now there are 

 two almost universal peculiarities of secondary sexual char- 

 acters on which the theory of sexual selection throws no 

 light whatever : (1) the characters do not begin to appear in 

 the individual until it is nearly adult and sexually mature, 

 in other words they appear when, or a little before, the animal 

 begins to breed ; (2) they are inherited only by the sex which 

 possesses them. 



If it were a mere question of the occurrence of individual 

 variations, and a preference by the female sex, there would 

 be no reason whatever why variations occurring at the 

 beginning of life should not be selected. The theory merely 

 supposes that each female has a number of candidates for her 

 favour which are not all alike. Now let us suppose that a 

 single cock-bird, a peacock for example, began to develop 

 its special tail as soon as it developed its other feathers, and 

 had a well-developed tail of the characteristic kind as soon 

 as it was fledged. When it became sexually mature its 

 special plumage would be perfect, while that of its fellows of 

 the same age was only partially developed, for in the ordinary 

 case males are mature before the development of their 

 secondary characters is completed. According to the theory 

 of sexual selection, then, the young male in which precocious 

 development of the special plumage occurred would be pre- 

 ferred to others of the same age in which the development 

 was later, and the result in a few generations would be that 

 the peculiarities of the males would appear as soon as the 

 permanent feathers appeared. The selection of course does 

 not take place until maturity is reached and mating occurs, 

 but the selection has nothing to do with the production of 

 the variations, and the time at which selection takes place 

 cannot determine the time at which the modifications occur 

 in the individual. 



Secondly, why should the individual peculiarities in the 



