416 
DARWINISM 
CHAP. 
birds whose powers of flight were already 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 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 Anally 
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. 1 
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 capable of pro¬ 
ducing a decided effect during the comparatively short period 
1 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 this chapter 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 parts in 
the organism,’ ” as playing an important part. Atrophy 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- 
