186 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 especially, each sex possesses such 
distinct traces of the reproductive apparatus character- 
istic of the other, that even antiquity assumed herma- 
phroditism as a natural primeval condition of mankind. 
The technical proofs of the homologies concerning these 
partly manifest, partly internal and hidden relations, are 
