BIED SONG HAWKINS. 463 



The same principle of natural selection may be attributed to the 

 call of birds. "These are evidently a valuable addition to the 

 means of recognition of the two sexes, and are a further indication 

 that the pairing season has arrived; and the production, intensifica- 

 tion, and differentiation of these sounds and odors are clearly 

 within the power of natural selection. The same remark will apply 

 to the peculiar calls of birds, and even to the singing of the males. 

 These may well have originated merely as a means of recognition 

 between the two sexes of a species and as an invitation from the 

 male to the female bird. When the individuals of a species are 

 widely scattered, such a call must be of great importance in enabling 

 pairing to take place as easily as possible and thus the clearness, 

 loudness, and individuality of the song becomes a useful character, 

 and therefore the subject of natural selection." 



The increase and development of beautiful plumage is caused by 

 the superabundant energy of the male bird. 



During excitement and when the organism develops superabundant energy, 

 many animals find it pleasurable to exercise their various muscles, often in 

 fantastic ways, as seen in the gambols of kittens, lambs, and other young ani- 

 mals. But at the time of pairing male birds are in a state of the most perfect 

 development, and possess an enormous store of vitality, and under the excite- 

 ment of the sexual passion they perform strange antics or rapid flights, as much 

 probably from the internal impulse to motion and exertion as with any desire to 

 please their mates. 



So, also, "the act of singing is evidently a pleasurable one, and it 

 probably serves as an outlet for superabundant nervous energy and 

 excitement, just as dancing, singing, and field sports do with us." 

 If superabundant vigor can account for the songs and ornaments of 

 birds "then no other mode of selection is needed to account for the 

 presence of such ornament." 



Brooks attacks the theory of Wallace that the duller colors of 

 the female are acquired by natural selection. Thus there is found 

 a difference in the colors of lizards where the female does not 

 incubate and does not require the duller colors for the purpose of 

 protection. In domestic fowl where danger from natural enemies 

 is almost nothing the same difference in the color between the male 

 and female continues. Thus the explanation is more fundamental 

 than the one proposed by either Darwin or Wallace. Brooks bases 

 his explanation upon a theory of heredity which supposes that the 

 body gives off gemmules and that "the male reproductive cell has 

 gradually acquired, as its special and distinctive function, a peculiar 

 power to gather and store up these gemmules." The male cell, 

 therefore, has acquired the power to transmit variation while the 

 female cell keeps up the constancy of the species. "We thus look 

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



