XIV FUNDAMENTAL PROBLEMS 415 



reduction of those muscles. The amount of reduction observed 

 by Darwin in the -wdng-bones of domestic ducks and poultry, 

 and in the hind legs of tame rabbits, is very small, and is 

 certainly no greater than the above causes will well account 

 for; while so many of the external characters of all our 

 domestic animals have been subject to long-continued artificial 

 selection, and we are so ignorant of the possible correlations 

 of different parts, that the phenomena presented by them 

 seem sufficiently explained without recurrence to the assump- 

 tion that any changes in the individual, due to disuse, are 

 inherited by the offspring. 



Supposed Effects of Disuse among Wild Animals. 



It may be urged, however, that among wild animals we have 

 many undoubted results of disuse much more pronounced than 

 those among domestic kinds, results which cannot be explained 

 by the causes already adduced. Such are the reduced size of 

 the wings of many birds on oceanic islands ; the abortion of 

 the eyes in many cave animals, and in some which live under- 

 ground ; and the loss of the hind limbs in whales and in some 

 lizards. These cases differ greatly in the amount of the re 

 duction of parts which has taken place, and may be due to 

 different causes. It is remarkable that in some of the bii-ds 

 of oceanic islands the reduction is little if any greater than in 

 domestic birds, as in the water-hen of Tristan d'Acunha. Now 

 if the reduction of wing were due to the hereditary effects of 

 disuse, we should expect a very much greater effect in a bird 

 inhabiting an oceanic island than in a domestic bird, Avhere the 

 disuse has been in action for an indefinitely shorter period. 

 In the case of many other birds, however — as some of the New 

 Zealand rails and the extinct dodo of Mauritius — the wings 

 have been reduced to a much more rudimentary condition, 

 though it is still obvious that they were once organs of flight ; 

 and in these cases we certainly require some other causes than 

 those which have reduced the wings of our domestic fowls. 

 One such cause may have been of the same nature as that 

 which has been so efficient in reducing the ^vings of the insects 

 of oceanic islands — the destruction of those Avhich, during the 

 occasional use of their \Wngs, were carried out to sea. This 

 form of natural selection may well have acted in the case of 



