416 DARWINISM chap. 



birds whose powers of flight were ah^eady somewhat reduced, 

 and to whom, there being no enemies to escape from, their use 

 was only a source of danger. We may thus, perhaps, account 

 for the fact that many of these birds retain small but useless 

 wings with Avhich they never fly ; for, the wings having been 

 reduced to this functionless condition, no power could reduce 

 them further except correlation, of growth or economy of 

 nutrition, causes which only rarely come into play. 



The complete loss of eyes in some cave animals may, 

 perhaps, be explained in a somewhat similar way. When- 

 ever, owing to the total darkness, they became useless, they 

 might also become injurious, on account of their delicacy of 

 organisation and liability to accidents and disease ; in which 

 case natural selection would begin to act to reduce, and finally 

 abort them ; and this explains why, in some cases, the rudi- 

 mentary eye remains, although com23letely covered by a pro- 

 tective outer skin. AYhales, like moas and cassowaries, carry 

 us back to a remote past, of Avhose conditions Ave know too 

 little for safe speculation. We are quite ignorant of the ances- 

 tral forms of either of these groups, and are therefore without 

 the materials needful for determining the steps by Avhich the 

 change took place, or the causes which brought it about. ^ 



On a review of the various examples that haA^e been given 

 by Mr. Darwin and others of organs that have been reduced 

 or aborted, there seems too much diversity in the results for 

 all to be due to so direct and uniform a cause as the individual 

 effects of disuse accumulated by heredity. For if that Avere 

 the only or chief efhcient cause, and a cause capable of pro- 

 ducing a decided effect during the comparatively short period 



^ The idea of the non-heredity of acquired variations was suggested by 

 the summary of Professor Weismann's views, in Nature, referred to later on. 

 But since tliis chaj^ter was written I have, through the kindness of Mr. E. B. 

 Poulton, seen some of the proofs of the forthcoming translation of Weismann's 

 Essays on Heredity, in which he sets forth an explanation very similar 

 to that here given. On the difficult question of the almost entire disap- 

 pearance of organs, as in the limbs of snakes and of some lizards, he adduces 

 " a certain form of correlation, which Roux calls ' the struggle of the jiarts in 

 the organism,' " as playing an important part. Atrophy following disuse is 

 nearly always attended by tlie corresponding increase of other organs : blind 

 animals possess more developed organs of touch, hearing, and smell ; the loss 

 of power in the wings is accompanied by increased strength of the legs, etc. 

 Now as these latter characters, being useful, will be selected, it is easy to 

 understand that a congenital increase of these will be accomiianied by a cor- 



