At the close of my article in last month's number of the Universal
Review, I said I would in this month's issue show why the opponents
of Charles-Darwinism believe the effects of habits acquired during
the lifetime of a parent to produce an effect on their subsequent
offspring, in spite of the fact that we can rarely find the effect
in any one generation, or even in several, sufficiently marked to
arrest our attention.
I will now show that offspring can be, and not very infrequently is,
affected by occurrences that have produced a deep impression on the
parent organism--the effect produced on the offspring being such as
leaves no doubt that it is to be connected with the impression
produced on the parent. Having thus established the general
proposition, I will proceed to the more particular one--that habits,
involving use and disuse of special organs, with the modifications
of structure thereby engendered, produce also an effect upon
offspring, which, though seldom perceptible as regards structure in
a single, or even in several generations, is nevertheless capable of
being accumulated in successive generations till it amounts to
specific and generic difference. I have found the first point as
much as I can treat within the limits of this present article, and
will avail myself of the hospitality of the Universal Review next
month to deal with the second.
The proposition which I have to defend is one which no one till
recently would have questioned, and even now those who look most
askance at it do not venture to dispute it unreservedly; they every
now and then admit it as conceivable, and even in some cases
probable; nevertheless they seek to minimize it, and to make out
that there is little or no connection between the great mass of the
cells of which the body is composed, and those cells that are alone
capable of reproducing the entire organism. The tendency is to
assign to these last a life of their own, apart from, and
unconnected with that of the other cells of the body, and to cheapen
all evidence that tends to prove any response on their part to the
past history of the individual, and hence ultimately of the race.
Professor Weismann is the foremost exponent of those who take this
line. He has naturally been welcomed by English Charles-Darwinians;
for if his view can be sustained, then it can be contended that use
and disuse produce no transmissible effect, and the ground is cut
from under Lamarck's feet; if, on the other hand, his view is
unfounded, the Lamarckian reaction, already strong, will gain still
further strength. The issue, therefore, is important, and is being
fiercely contested by those who have invested their all of
reputation for discernment in Charles-Darwinian securities.
Professor Weismann's theory is, that at every new birth a part of
the substance which proceeds from parents and which goes to form the
new embryo is not used up in forming the new animal, but remains
apart to generate the germ-cells--or perhaps I should say "germ-
plasm"--which the new animal itself will in due course issue.
Contrasting the generally received view with his own, Professor
Weismann says that according to the first of these "the organism
produces germ-cells afresh again and again, and that it produces
them entirely from its own substance." While by the second "the
germ-cells are no longer looked upon as the product of the parent's
body, at least as far as their essential part--the specific germ-
plasm--is concerned; they are rather considered as something which
is to be placed in contrast with the tout ensemble of the cells
which make up the parent's body, and the germ-cells of succeeding
generations stand in a similar relation to one another as a series
of generations of unicellular organisms arising by a continued
process of cell-division." {274a} On another page he writes:--
"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."
{274b}
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."
{275}
Professor Weismann, in passages too numerous to quote, shows that he
recognizes 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 13, 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?" {277}
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 389 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.
233.]
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 off-hand, 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, etc., 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. {279} 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 had 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, etc.
"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."