1 86 THE DOCTRINE OF DESCEXT. 



tematic group is characterized by the incapacity of fly- 

 ing. In our opinion, there was a direct connection be- 

 tween the inducements to disuse and its consequences 

 in the case of the dodo, which, with its few con- 

 geners, so promptly fell a sacrifice to its helplessness on 

 the discovery of the lonely islands which they had prob- 

 ably inhabited for thousands of years without disturb- 

 ance. In no other way has the northern penguin (Alca 

 impennis) at some time obtained the curtailment of its 

 wings; and the scanty but wide-spread remains of the 

 order of flightless birds indicate a period at which, in a 

 more peaceful environment, their far more numerous 

 wingless ancestry made less use of their pinions, and 

 natural selection endowed them with greater strength 

 and nimbleness of leg. The eiTects of disuse of the or- 

 gans of locomotion are likewise directly exhibited by 

 artificial selection. 



Use and disuse, combined with selection, elucidate 

 the separation of the sexes, and the existence, otherwise 

 totally incomprehensible, of rudimentary sexual organs. 

 In the Vertebrata especially, each sex possesses such dis- 

 tinct traces of the reproductive apparatus characteristic 

 of the other, that even antiquity assumed hermaphrodit- 

 ism as a natural primseval condition of mankind. The 

 technical proofs of the homologies concerning these 

 partly manifest, partly internal and hidden relations, are 

 given in the manuals of comparative anatomy. We shall 

 merely indicate the manner in which the theory of selec- 

 tion is here borne out. It is self-evident that in her- 

 maphrodite animals, fluctuations in the sexual sphere 

 must take place, in which one half or the other will 

 predominate. Should these fluctuations be sufficiently 



