416 DARWINISM chap. 



birds whose powers of flight were abeady 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 which they never fly ; for, the Avings 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 completely covered by a pro- 

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

 us back to a remote past, of whose conditions we 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 which the 

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



On a review of the various examples that have 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 were 

 the only or chief efficient cause, and a cause cajjable of pro- 

 ducing a decided eff"ect during the comparatively short period 



^ Tlie idea of the uon -heredity of acquired variations was suggested by 

 the summary of Professor Weismanu's views, iu Nature, referred to later on. 

 But since this chajjter was %vritten I have, through the kindness of Mr. E. B. 

 Poultou, seen some of the proofs of tlie 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 parts in 

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

 nearly always attended by the 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 accompanied by a cor- 



