FLIGHTLESS BIRDS 397 



islands have lost the power of flight. Such are the kiwi, the 

 kakapo, the weka, the notornis and the already extinct gigantic 

 moas of New Zealand ; the dodo of Mauritius, and the solitaire 

 of Rodriguez. Although belonging to several very distinct 

 families of birds, including ratites, parrots, rails and pigeons, all 

 the forms enumerated have undergone the same curious modifi- 

 cation, resulting in the most extreme cases (the moas) in the 

 complete loss of the wings, and in others in the reduction of those 

 organs to a more or less vestigial condition. 1 



This convergence is clearly due to the similarity of the con- 

 ditions under which these birds have had to live. One of the 

 most characteristic features of oceanic islands is the absence 

 from them of predaceous mammals, the natural enemies of birds, 

 which have never been able to cross the great stretches of open 

 ocean which separate such islands from the continental areas on 

 which the Mammalia have been evolved. Birds, however, and 

 even land birds, by virtue of their powers of flight, have been 

 able to reach these islands at more or less frequent intervals 

 and to establish themselves there. Finding abundance of food, 

 which they could obtain near the ground, and finding themselves 

 no longer under the necessity of constantly using their wings in 

 order to escape from their enemies, some of these birds, though 

 by no means all, gradually gave up flying and their wings under- 

 went a slow process of degeneration in accordance with Lamarck's 

 principle of disuse. No doubt such disuse, if continued only 

 through a single lifetime, could scarcely produce a visible effect 

 upon the next generation, but continued under the same con- 

 ditions throughout thousands of generations it has brought about 

 a permanent deterioration which can no longer be retrieved. 



It is to be noted that this degeneration is the result of the 

 removal of the organism, to a certain extent, from the struggle 

 for existence. Natural selection can only act through the struggle 

 for existence and upon those organs which are of value in the 

 struggle. When the struggle ceases, natural selection ceases and 

 degeneration sets in, for there is no longer any reason why a 

 high standard of perfection should be maintained. All degrees of 

 imperfection now have equal opportunities of propagating them- 

 selves. The inferior individuals are no longer weeded out, and 

 the average condition of the species consequently deteriorates. 



But observe what happens when a degenerate organism is 



1 Compare Chapter XVII, Figs. Ill, 112. 



