12 THE EVOLUTION OF SEX. 



" The fact too that many structures, which are not at all con- 

 spicuous, are confined, like gay plumage, to male birds, also 

 indicates the existence of an explanation more fundamental 

 than the one proposed by Wallace, and the latter explanation 

 gives no reason why the females of allied species should often 

 be exactly alike when the males are very different." To the 

 explanation which Brooks proposes we must therefore pass. 



According to Darwin, Brooks says, the greater modification 

 of the males is due to their struggling with rivals, and to their 

 selection by the females, but " I do not believe that this goes 

 to the root of the matter." The study of domesticated pigeons, 

 for instance, shows that "something within the animal 

 determines that the male should lead and the female follow in 

 the evolution of new breeds. The same is true in other 

 domesticated animals, where, from the nature of the circum- 

 stances, it is inadmissible to explain this with Darwin, by 

 supposing that the male is more exposed than the female to 

 the action of selection, whether natural or sexual. Darwin 

 concludes, indeed, that the male is more variable than the 

 female, but he gives no satisfactory reason why female 

 variations should be less apt than male variations to become 

 hereditary, or, in other words, why the right of entail is so much 

 restricted to the male sex. Darwin merely attributes this to 

 the greater eagerness of the males, which " in almost all 

 animals have stronger passions than the females." The theory 

 which Brooks maintains, is bound up with an hypothesis of 

 heredity differing considerably from that held by Darwin. He 

 supposes that the cells of the body give off gemmules, chiefly 

 during change of function or of environment, and that " the 

 male reproductive cell has gradually acquired, as its especial and 

 distinctive function, a peculiar power to gather and store up 

 these gemmules." The female reproductive cells keep up the 

 general constancy of the species, the male cells transmit 

 variations. " A division of physiological labour has arisen 

 during the evolution of life, and the functions of the repro- 

 ductive elements have become specialised in different direc- 

 tions." " The male cell became adapted for storing up 

 gemmules " (the results of variations in the body), " and at the 

 same time gradually lost its unnecessary and useless jDOwer to 

 transmit hereditary characteristics." " We thus look to the 

 cells of the male body for the origin of most of the variations 

 through which the species has attained to its present organisa- 



