[15] Weismann's _Essays on Heredity_, &c. Clarendon Press, 1889.
[16] _Life and Letters_, i. p. 16. Darwin's reverence for his father
"was boundless and most touching. He would have wished to judge
everything else in the world dispassionately, but anything his father
had said was received with almost implicit faith; ... he hoped none of
his sons would ever believe anything because he said it, unless they
were themselves convinced of its truth--a feeling in striking contrast
with his own manner of faith" (_Life and Letters_, i. pp. 10, 11).
[17] _Ibid._, i. p. 38.
[18] _Life and Letters_, ii. p. 14.
[19] _Origin of Species_, pp. 117, 118.
[20] _Ibid._, p. 180.
[21] _Contemporary Review_, December, 1875, pp. 89, 93.
[22] _Variation of Animals and Plants under Domestication_, i. 292.
[23] _Variation of Animals and Plants under Domestication_, i. 299-301.
[24] To keep pace with this 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 suppose that use-inheritance
especially affects the eight breeds that have varied most in size. If we
exclude these, there is only a total shortening of 7 per cent. to be
accounted for.
[36] _Variation of Animals and Plants under Domestication_, i. 183, 186.
[37] _Variation of Animals and Plants under Domestication_, i. 130, 135;
ii. 288.
[38] _Encyclopaedia Britannica_, article "Zoology."
[39] _Variation of Animals and Plants under Domestication_, ii. 367.
[40] _Variation of Animals and Plants under Domestication_, ii. 367. Why
then does the cheetah inherit ancestral habits so inadequately that it
is useless for the chase unless it has first learned to hunt for itself
before being captured? (ii. 133).
[41] _Descent of Man_, p. 33.
[42] _Origin of Species_, pp. 210, 211.
[43] E. S. Delamer on _Pigeons and Rabbits_, pp. 132, 103. For other
points referred to, see pages 133, 102, 100, 95, 131.
[44] _Origin of Species_, pp. 188, 110; _Descent of Man_, pp. 32-35;
_Variation of Animals and Plants under Domestication_, ii. 289, 293. Use
or disuse during lifetime of course co-operates, and in some cases, as
in that of the canoe Indians, may be the principal or even perhaps the
_sole_ cause of the change.
[45] For the importance of panmixia as invalidating Darwin's strongest
evidence for use-inheritance--namely, that drawn from the effects of
disuse in highly-fed domestic animals where there is supposed to be no
economy of growth--see Professor Romanes on Panmixia, _Nature_, April 3,
1890.
[46] _Descent of Man_, p. 33.
[47] _Descent of Man_, p. 33.
[48] _Variation of Animals and Plants under Domestication_, i., 453.
[49] _Descent of Man_, p. 33.
[50] _Descent of Man_, p. 33.
[51] Wallace shows that the changes in our domestic animals, if spread
over the thousands of years since the animals were first tamed, must be
extremely insignificant in each generation, and he concludes that such
infinitesimal effects of use and disuse would be swallowed up by the far
greater effects of variation and selection (_Darwinism_, p. 436).
Professor Romanes has replied to him in the _Contemporary Review_
(August 1889), showing that this is no disproof of the existence of the
minor factor, inasmuch as slight changes in each generation need not
necessarily be matters of life and death to the individual, although
their cumulative development by use-inheritance might eventually become
of much service. But selection would favour spontaneous variations of a
similarly serviceable character. The slightest tendency to eliminate the
extreme variations in either direction would proportionally modify the
average in a breed. Use-inheritance appears to be so relatively weak a
factor that probably neither proof nor disproof of its existence can
ever be given, owing to the practical impossibility of disentangling its
effects (if any) from the effects of admittedly far more powerful
factors which often act in unsuspected ways. Thus wild ducklings, which
can easily be reared by themselves, invariably "die off" if reared with
tame ones (_Variation_, &c., i. 292, ii. 219). They cannot get their
fair share in the competition for food, and are completely eliminated.
Professor Romanes fully acknowledges that there is the "gravest possible
doubt" as to the transmission of the effects of disuse (Letter on
Panmixia, _Nature_, March 13, 1890).
[52] _Variation of Animals and Plants under Domestication_, ii. 287-289.
[53] _Descent of Man_, pp. 612, 131.