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 the head, instead of a horn, a bony
lump attached merely to the skin. Such cases may seem to prove that
mutilation _associated with morbid action_ is occasionally inherited or
repeated with a promptitude and thoroughness that contrast most
strikingly with the imperceptible nature of the immediate inheritance of
the effects of use and disuse; but they by no means prove that
mutilation in general is inheritable, and they are absolutely no proof
whatever of a _normal_ and non-pathological tendency to the inheritance
of acquired characters. Those who accept Darwin's special explanation
of the supposed inheritance of mutilations, ought to notice that his
explanation applies equally well under a theory which is strongly
adverse to use-inheritance--namely, Galton's idea of the sterilization
and complete "using up" of otherwise reproductive matter in the growth
and maintenance of the personal structure.
Darwin's explanation of inherited mutilations--which, as he notes, occur
"especially or perhaps exclusively" when the injury has been followed by
disease[56]--is that all the representative gemmules which would develop
or repair or reproduce the injured part are attracted to the diseased
surface during the reparative process and are there destroyed by the
morbid action.[57] Hence they cannot reproduce the part in offspring.
This explanation by no means implies that mutilation would _usually_
affect the offspring. On the contrary, in all ordinary cases of
mutilation the purely atavistic elements or gemmules would be set free
from any modifying influence of the non-existent or mutilated part. The
gemmules--as in Galton's theory of heredity and with neuter
insects--might be perfectly independent of pangenesis and the normal
inheritance of acquired characters. Such self-multiplying gemmules
without pangenesis would enable us to understand both the excessive
weakness or non-existence of normal use-inheritance, and the excessive
strength and abruptness of the effect of their partial destruction under
special pathological conditions.
The series of epileptic phenomena that can be excited by tickling a
certain part of the cheek and neck of the adult guinea-pig during the
growth and rejoining of the ends of the severed nerve, are said to be
repeated with striking accuracy of detail in the young who inherit
mutilated toes; but as epilepsy is often due to some _one_ exciting
cause or morbid condition, the single transmission of a highly morbid
condition of the system might easily reproduce the whole chain of
consequences and might also have caused the loss of toes.
The particulars of the guinea-pig cases are very inadequately
recorded,[58] but the results are so anomalous[59] that Brown-Sequard's
own conclusion is that the epilepsy and the inherited injuries are
_not_ directly transmitted, but that "what is transmitted is the morbid
state of the nervous system." He thinks that the missing toes may
"possibly" be exceptions to this conclusion, "but the other facts only
imply the transmission of a morbid state of the sympathetic or sciatic
nerve or of a part of the medulla oblongata." Until we can tell what is
transmitted, we are not in a position to determine whether there is any
true inheritance or only an exaggerated simulation of it under peculiar
circumstances. When the actual observers believe that the mutilations
and epilepsy are not the cause of their own repetition, and when these
observers guard themselves by such phrases as, "if any conclusion can at
present be drawn from those facts," we who have only incomplete reports
to guide us may well be excused if we preserve an even more pronounced
attitude of caution and reserve.[60] The morbid state of the system may
be wholly due to general injury of the germs rather than to specific
inheritance.
Weismann suggests that the morbid condition of the nervous system may be
due to some infection such as might arise from microbes, which find a
home in the mutilated and disordered nervous system in the parent, and
subsequently transmit themselves to the offspring through the
reproductive elements, as the infections of various diseases appear to
do--the muscardine silkworm disease in particular being known to be
conveyed to offspring in this manner.
But whether we can discover the true explanation or not, inherited
mutilations can hardly be accounted for as the result of a general
tendency to inherit acquired modifications. How could a factor which
seems to be totally inoperative in cases of ordinary mutilation, and
only infinitesimally operative in transmitting the normal effects of use
and disuse, suddenly become so powerful as to completely overthrow
atavism, and its own tendency to transmit the non-mutilated type of one
of the parents and of the non-mutilated type presented by the injured
parent in earlier life? Does not so striking and abrupt an
intensification of its usually insignificant power demand an explanation
widely different from that which might account for the extremely slow
and slight inheritance of the normal effects of use and disuse? Surely
it would be better to suspend one's judgment as to the true explanation
of highly exceptional and purely pathological cases rather than resort
to an hypothesis that creates more difficulties than it solves.