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.

INHERITED INJURIES.

INHERITED MUTILATIONS.

The almost universal _non-inheritance_ of mutilations seems to me a far
more valid argument _against_ a general law of modification-inheritance
than the few doubtful or abnormal cases of such inheritance can furnish
in its favour. No inherited effect has been produced by the docking of
horses' tails for many generations, or by a well-known mutilation which
has been practised by the Hebrew race from time immemorial. As lost or
mutilated parts are reproduced in offspring independently of the
existence of those parts in the parent, there is the less reason to
suppose that the particular condition of parental parts transmits
itself, or tends to transmit itself, to the offspring. So unsatisfactory
is the argument derivable from inherited mutilations that Mr. Spencer
does not mention them at all, and Darwin has to attribute them to a
special cause which is independent of any general theory of
use-inheritance.[54]

Darwin's most striking case--and to my mind the only case of any
importance--is that of Brown-Sequard's epileptic guinea-pigs, which
inherited the mutilated condition of parents who had gnawed off their
own gangrenous toes when anaesthetic through the sciatic nerve having
been divided.[55] Darwin also mentions a cow that lost a horn by
accident, followed by suppuration, and subsequently produced three
calves which had on the same side of t