lateral increase in weight, the leg-bones
should have lengthened considerably so that their total deficiency in
proportional length is 17 per cent.,--a changed proportion which being
_linear_ is more excessive than the increase of weight by 28 per cent.
So marked is the effect of the combined thickening and shortening that
in the Aylesbury breed--which is the most typically representative
one--the leg-bones have become 70 per cent. heavier than they should be
if their thickness had continued to be proportional to their length.

[25] This excessive thickening under disuse appears to be due partly to
a positive lateral enlargement or increase of proportional weight of
about 7-1/2 per cent., and partly to a shortening of about 15 per cent.
Carefully calculated, the reduction of the weight of the wing-bones in
this breed is only 8.3 per cent. relatively to the whole skeleton, or
only 5 per cent. relatively to the skeleton _minus_ legs and wings. The
latter method is the more correct, since the excessive weight of the
leg-bones increases the weight of the skeleton more than the diminished
weight of the wing-bones reduces it.

[26] _Variation of Animals and Plants under Domestication_, i. 284.

[27] _Variation of Animals and Plants under Domestication_, i. 184, 185.

[28] _Ibid._, i. 144, 145.

[29] _Ibid._, i. 185.

[30] _Variation of Animals and Plants under Domestication_, i. 175.

[31] _Variation of Animals and Plants under Domestication_, i. 184. I
suspect that Darwin was in poor health when he wrote this page. He nods
at least four times in it. Twice he speaks of "twelve" breeds where he
obviously should have said eleven.

[32] If a prominent breast is admired and selected by fanciers, the
sternum might shorten in assuming a more forward and vertical position.
If the shortening of the sternum is entirely due to disuse, it seems
strange that Darwin has not noticed any similar shortening in the
sternum of the duck. But selection has not tended to make the duck
elegant, or "pigeon-breasted"; it has enlarged the abdominal sack
instead, besides allowing the addition of an extra rib in various cases.

[33] _Variation of Animals and Plants under Domestication_, 144, 175.

[34] _Variation of Animals and Plants under Domestication_, i. 179.

[35] In the six largest breeds the shortening of the sternum is nearly
twice as great as in the three smaller breeds which remain nearest the
rock-pigeon in size. We can hardly s