1 86 THE DOCTRINE OF DESCENT. 



again, disuse and adaptation coincide as well as 

 differentiation. 



In the class of birds is repeated the spectacle we 

 have just witnessed in beetles and reptiles. In some 

 few families and smaller groups, individual species are 

 deprived of the power of flight, and one whole large 

 systematic group is characterized by the incapacity of 

 flying. In our opinion, there was a direct connection 

 between the inducements to disuse and its conse- 

 quences in the case of the dodo, which, with its few 

 congeners, so promptly fell a sacrifice to its helplessness 

 on the discovery of the lonely islands which they had 

 probably inhabited for thousands of years without dis- 

 turbance. 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 nume- 

 rous wingless ancestry made less use of their pinions, and 

 natural selection endowed them with greater strength 

 and nimbleness of leg. The effects of disuse of the 

 organs 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 especiall}^ each sex possesses such 

 distinct traces of the reproductive apparatus character- 

 istic of the other, that even antiquity assumed herma- 

 phroditism as a natural primaeval condition of mankind. 

 The technical proofs of the homologies concerning these 

 partly manifest, partly internal and hidden relations, are 



