"I believe that heredity depends upon the fact that a small portion of
the effective substance of the germ, the germ-plasm, remains unchanged
during the development of the ovum into an organism, and that this part
of the germ-plasm serves as a foundation from which the germ-cells of the
new organism are produced. There is, therefore, continuity of the germ-
plasm from one generation to another. One might represent the germ-plasm
by the metaphor of a long creeping root-stock from which plants arise at
intervals, these latter representing the individuals of successive
generations." {31}
Mr. Wallace, who does not appear to have read Professor Weismann's essays
themselves, but whose remarks are, no doubt, ultimately derived from the
sequel to the passage just quoted from page 266 of Professor Weismann's
book, contends that the impossibility of the transmission of acquired
characters follows as a logical result from Professor Weismann's theory,
inasmuch as the molecular structure of the germ-plasm that will go to
form any succeeding generation is already predetermined within the still
unformed embryo of its predecessor; "and Weismann," continues Mr.
Wallace, "holds that there are no facts which really prove that acquired
characters can be inherited, although their inheritance has, by most
writers, been considered so probable as hardly to stand in need of direct
proof." {32}
Professor Weismann, in passages too numerous to quote, shows that he
recognises this necessity, and acknowledges that the non-transmission of
acquired characters "forms the foundation of the views" set forth in his
book, p. 291.
Professor Ray Lankester does not commit himself absolutely to this view,
but lends it support by saying (_Nature_, December 12, 1889): "It is
hardly necessary to say that it has never yet been shown experimentally
that _anything_ acquired by one generation is transmitted to the next
(putting aside diseases)."
Mr. Romanes, writing in _Nature_, March 18, 1890, and opposing certain
details of Professor Weismann's theory, so far supports it as to say that
"there is the gravest possible doubt lying against the supposition that
any really inherited decrease is due to the inherited effects of disuse."
The "gravest possible doubt" should mean that Mr. Romanes regards it as a
moral certainty that disuse has no transmitted effect in reducing an
organ, and it should follow that he holds use to have no transmitted
effect in its development. The sequel, however, makes me uncertain how
far Mr. Romanes intends this, and I would refer the reader to the article
which Mr. Romanes has just published on Weismann in the _Contemporary
Review_ for this current month.
The burden of Mr. Thiselton Dyer's controversy with the Duke of Argyll
(see _Nature_, January 16, 1890, _et seq._) was that there was no
evidence in support of the transmission of any acquired modification. The
orthodoxy of science, therefore, must be held as giving at any rate a
provisional support to Professor Weismann, but all of them, including
even Professor Weismann himself, shrink from committing themselves to the
opinion that the germ-cells of any organisms remain in all cases
unaffected by the events that occur to the other cells of the same
organism, and until they do this they have knocked the bottom out of
their case.
From among the passages in which Professor Weismann himself shows a
desire to hedge I may take the following from page 170 of his book:--
"I am also far from asserting that the germ-plasm which, as I hold, is
transmitted as the basis of heredity from one generation to another, is
absolutely unchangeable or totally uninfluenced by forces residing in the
organism within which it is transformed into germ-cells. I am also
compelled to admit it as conceivable that organisms may exert a modifying
influence upon their germ-cells, and even that such a process is to a
certain extent inevitable. The nutrition and growth of the individual
must exercise some influence upon its germ-cells . . . "
Professor Weismann does indeed go on to say that this influence must be
extremely slight, but we do not care how slight the changes produced may
be provided they exist and can be transmitted. On an earlier page (p.
101) he said in regard to variations generally that we should not expect
to find them conspicuous; their frequency would be enough, if they could
be accumulated. The same applies here, if stirring events that occur to
the somatic cells can produce any effect at all on offspring. A very
small effect, provided it can be repeated and accumulated in successive
generations, is all that even the most exacting Lamarckian will ask for.
Having now made the reader acquainted with the position taken by the
leading Charles-Darwinian authorities, I will return to Professor
Weismann himself, who declares that the transmission of acquired
characters "at first sight certainly seems necessary," and that "it
appears rash to attempt to dispense with its aid." He continues:--
"Many phenomena only appear to be intelligible if we assume the
hereditary transmission of such acquired characters as the changes which
we ascribe to the use or disuse of particular organs, or to the direct
influence of climate. Furthermore, how can we explain instinct as
hereditary habit, unless it has gradually arisen by the accumulation,
through heredity, of habits which were practised in succeeding
generations?" {33}
I may say in passing that Professor Weismann appears to suppose that the
view of instinct just given is part of the Charles-Darwinian system, for
on page 889 of his book he says "that many observers had followed Darwin
in explaining them [instincts] as inherited habits." This was not Mr.
Darwin's own view of the matter. He wrote:--
"If we suppose any habitual action to become inherited--and I think it
can be shown that this does sometimes happen--then the resemblance
between what originally was a habit and an instinct becomes so close as
not to be distinguished. . . But it would be the most serious error to
suppose that the greater number of instincts have been acquired by habit
in one generation, and then transmitted by inheritance to succeeding
generations. It can be clearly shown that the most wonderful instincts
with which we are acquainted, namely, those of the hive-bee and of many
ants, could not possibly have been thus acquired."--["Origin of Species,"
ed., 1859, p. 209.]
Again we read: "Domestic instincts are sometimes spoken of as actions
which have become inherited solely from long-continued and compulsory
habit, but this, I think, is not true."--_Ibid._, p. 214.
Again: "I am surprised that no one has advanced this demonstrative case
of neuter insects, against the well-known doctrine of inherited habit, as
advanced by Lamarck."--["Origin of Species," ed. 1872, p. 283.]
I am not aware that Lamarck advanced the doctrine that instinct is
inherited habit, but he may have done so in some work that I have not
seen.
It is true, as I have more than once pointed out, that in the later
editions of the "Origin of Species" it is no longer "the _most_ serious"
error to refer instincts generally to inherited habit, but it still
remains "a serious error," and this slight relaxation of severity does
not warrant Professor Weismann in ascribing to Mr. Darwin an opinion
which he emphatically condemned. His tone, however, is so offhand, that
those who have little acquaintance with the literature of evolution would
hardly guess that he is not much better informed on this subject than
themselves.
Returning to the inheritance of acquired characters, Professor Weismann
says that this has never been proved either by means of direct
observation or by experiment. "It must be admitted," he writes, "that
there are in existence numerous descriptions of cases which tend to prove
that such mutilations as the loss of fingers, the scars of wounds, &c.,
are inherited by the offspring, but in these descriptions the previous
history is invariably obscure, and hence the evidence loses all
scientific value."
The experiments of M. Brown-Sequard throw so much light upon the
question at issue that I will quote at some length from the summary given
by Mr. Darwin in his "Variation of Animals and Plants under
Domestication." {34} Mr. Darwin writes:--
"With respect to the inheritance of structures mutilated by injuries or
altered by disease, it was until lately difficult to come to any definite
conclusion." [Then follow several cases in which mutilations practised
for many generations are not found to be transmitted.] "Notwithstanding,"
continues Mr. Darwin, "the above several negative cases, we now possess
conclusive evidence that the effects of operations are sometimes
inherited. Dr. Brown-Sequard gives the following summary of his
observations on guinea-pigs, and this summary is so important that I will
quote the whole:--
"'1st. Appearance of epilepsy in animals born of parents having been
rendered epileptic by an injury to the spinal cord.
"'2nd. Appearance of epilepsy also in animals born of parents having
been rendered epileptic by the section of the sciatic nerve.
"'3rd. A change in the shape of the ear in animals born of parents in
which such a change was the effect of a division of the cervical
sympathetic nerve.
"'4th. Partial closure of the eyelids in animals born of parents in
which that state of the eyelids had been caused either by the section of
the cervical sympathetic nerve or the removal of the superior cervical
ganglion.
"'5th. Exophthalmia in animals born of parents in which an injury to the
restiform body had produced that protrusion of the eyeball. This
interesting fact I have witnessed a good many times, and I have seen the
transmission of the morbid state of the eye continue through four
generations. In these animals modified by heredity, the two eyes
generally protruded, although in the parents usually only one showed
exophthalmia, the lesion having been made in most cases only on one of
the corpora restiformia.
"'6th. Haematoma and dry gangrene of the ears in animals born of parents
in which these ear-alterations had been caused by an injury to the
restiform body near the nib of the calamus.
"'7th. Absence of two toes out of the three of the hind leg, and
sometimes of the three, in animals whose parents had eaten up their hind-
leg toes which had become anaesthetic from a section of the sciatic nerve
alone, or of that nerve and also of the crural. Sometimes, instead of
complete absence of the toes, only a part of one or two or three was
missing in the young, although in the parent not only the toes but the
whole foot was absent (partly eaten off, partly destroyed by
inflammation, ulceration, or gangrene).
"'8th. Appearance of various morbid states of the skin and hair of the
neck and face in animals born of parents having had similar alterations
in the same parts, as effects of an injury to the sciatic nerve.'
"It should be especially observed that Brown-Sequard has bred during
thirty years many thousand guinea-pigs from animals which had not been
operated upon, and not one of these manifested the epileptic tendency.
Nor has he ever seen a guinea-pig born without toes, which was not the
offspring of parents which had gnawed off their own toes owing to the
sciatic nerve having been divided. Of this latter fact thirteen
instances were carefully recorded, and a greater number were seen; yet
Brown-Sequard speaks of such cases as one of the rarer forms of
inheritance. It is a still more interesting fact, 'that the sciatic
nerve in the congenitally toeless animal has inherited the power of
passing through all the different morbid states which have occurred in
one of its parents from the time of the division till after its reunion
with the peripheric end. It is not, therefore, simply the power of
performing an action which is inherited, but the power of performing a
whole series of actions, in a certain order.'
"In most of the cases of inheritance recorded by Brown-Sequard only one
of the two parents had been operated upon and was affected. He concludes
by expressing his belief that 'what is transmitted is the morbid state of
the nervous system,' due to the operation performed on the parents."
Mr. Darwin proceeds to give other instances of inherited effects of
mutilations:--
"With the horse there seems hardly a doubt that exostoses on the legs,
caused by too much travelling on hard roads, are inherited. Blumenbach
records the case of a man who had his little finger on the right hand
almost cut off, and which in consequence grew crooked, and his sons had
the same finger on the same hand similarly crooked. A soldier, fifteen
years before his marriage, lost his left eye from purulent ophthalmia,
and his two sons were microphthalmic on the same side."
The late Professor Rolleston, whose competence as an observer no one is
likely to dispute, gave Mr. Darwin two cases as having fallen under his
own notice, one of a man whose knee had been severely wounded, and whose
child was born with the same spot marked or scarred, and the other of one
who was severely cut upon the cheek, and whose child was born scarred in
the same place. Mr. Darwin's conclusion was that "the effects of
injuries, especially when followed by disease, or perhaps exclusively
when thus followed, are occasionally inherited."
Let us now see what Professor Weismann has to say against this. He
writes:--
"The only cases worthy of discussion are the well-known experiments upon
guinea-pigs conducted by the French physiologist, Brown-Sequard. But the
explanation of his results is, in my opinion, open to discussion. In
these cases we have to do with the apparent transmission of artificially
produced malformations . . . All these effects were said to be
transmitted to descendants as far as the fifth or sixth generation.
"But we must inquire whether these cases are really due to heredity, and
not to simple infection. In the case of epilepsy, at any rate, it is
easy to imagine that the passage of some specific organism through the
reproductive cells may take place, as in the case of syphilis. We are,
however, entirely ignorant of the nature of the former disease. This
suggested explanation may not perhaps apply to the other cases; but we
must remember that animals which have been subjected to such severe
operations upon the nervous system have sustained a great shock, and if
they are capable of breeding, it is only probable that they will produce
weak descendants, and such as are easily affected by disease. Such a
result does not, however, explain why the offspring should suffer from
the same disease as that which was artificially induced in the parents.
But this does not appear to have been by any means invariably the case.
Brown-Sequard himself says: 'The changes in the eye of the offspring were
of a very variable nature, and were only occasionally exactly similar to
those observed in the parents.'
"There is no doubt, however, that these experiments demand careful
consideration, but before they can claim scientific recognition, they
must be subjected to rigid criticism as to the precautions taken, the
nature and number of the control experiments, &c.
"Up to the present time such necessary conditions have not been
sufficiently observed. The recent experiments themselves are only
described in short preliminary notices, which, as regards their accuracy,
the possibility of mistake, the precautions taken, and the exact
succession of individuals affected, afford no data on which a scientific
opinion can be founded" (pp. 81, 82).
The line Professor Weismann takes, therefore, is to discredit the facts;
yet on a later page we find that the experiments have since been repeated
by Obersteiner, "who has described them in a very exact and unprejudiced
manner," and that "the fact"--(I imagine that Professor Weismann intends
"the facts")--"cannot be doubted."