or at least nearly all, that is essential, but am rather convinced that
whole regions of phenomena are still sealed to us, and I consider it
probable that the nervous system in particular is not yet exhaustively
known to us, either in regard to its functioning or in regard to its
finest structural architecture, although I gratefully recognize the
advances in this domain that the last decades have brought about. In
any case, such assumptions as I have just indicated, or similar ones,
seem to me quite too improbable to furnish any foothold for progress.
Yet we must always remain conscious that we cannot decide as to the
possibility or impossibility of any biological process whatever from
a purely theoretical standpoint, because we can only guess at, not
discern, the fundamental nature of biological processes. At the close
of this lecture I shall return to the question of the theoretical
conceivability of an inheritance of functional adaptations; but first
of all we must consider the facts and be guided by them alone. If they
prove, or even make it seem probable, that such inheritance exists,
then it must be possible, and our task is no longer to deny it, but to
find out how it can come about.
Let us therefore investigate the question whether an inheritance
of acquired characters, that is, in the first place, of functional
adaptations, is demonstrable from experience. We shall speak later on
of the effect of climatic and similar influences in causing variation;
the case in regard to them is quite different, because they undoubtedly
affect not only the parts of the body but the germ-cells as well.
When we inquire into the facts which have been brought forward by
the modern adherents of the Lamarckian principle as proofs of the
inheritance of acquired characters in this restricted sense, we shall
find that none of them can withstand criticism.
First, there are the numerous reputed cases of the inheritance of
mutilations and losses of whole parts of the body.
It is not without interest to note here how opinion in regard to this
point has altered in the course of the debate.
At the beginning of the discussion they were all brought forward as
evidence of undoubted value for the Lamarckian principle.
At the Naturalists' Congress in Wiesbaden in 1887, kittens with only
stumps of tails were exhibited, and they were said to have inherited
this peculiarity from their mother, whose tail, it was asserted, had
been accidentally amputated. The newspapers reported that the case
excited great interest, and biologists of the standing of Rudolf
Virchow declared it to be noteworthy, and regarded it as a proof, if
all the details of it were correct. From many sides similar cases were
brought forward, intended to prove that the amputation of the tail
in cats and dogs could give rise to hereditary degeneration of this
part; even students' fencing-scars were said to have been occasionally
transmitted to their sons (happily not to the daughters); a mutilated
or torn ear-lobe in the mother was said to have given rise to deformity
of the ear in a son; an injury to a father's eye was said to have
caused complete degeneration of the eyes in his children; and deformity
of a father's thumb, due to frostbite, was said to have produced
misshapen thumbs in the children and grandchildren. A multitude of
cases of this kind are to be found in the older textbooks of physiology
by Burdach, and above all by Blumenbach, and the majority have no
more than an anecdotal value, for they are not only related without
any adequate guarantee, but even without the details indispensable to
criticism.
As far back as the eighteenth century the great philosopher Kant, and
in our own day the anatomist Wilhelm His, gave their verdict decidedly
against such allegations, and absolutely denied any inheritance of
mutilations; and now, after a decade or more of lively debate over
the pros and cons, combined with detailed anatomical investigations,
careful testing of individual cases, and experiment, we are in a
position to give a decided negative and say _there is no inheritance of
mutilations_.
Let me briefly explain how this result has been reached.
In the first place, the assertion that congenital stump-tails in dogs
and cats depended on inherited mutilation proved to be unfounded. In
none of the cases of stump-tails brought forward could it even be
proved that the tail of the relevant parent had been torn or cut off,
much less that the occurrence, in parents or grandparents, of short
tails from internal causes was excluded. At the same time anatomical
investigation of such stump-tails as occur in cats in the Isle of Man,
and in many Japanese cats, and are frequently found in the most diverse
breeds of dogs, showed that these had, in their structure, nothing
in common with the remains of a tail that had been cut off, but were
spontaneous degenerations of the whole tail, and are thus deformed
tails, not shortened ones (Bonnet).
Experiments on mice also showed that the cutting off of the tail,
even when performed on both parents, does not bring about the
slightest diminution in the length of tail in the descendants. I
have myself instituted experiments of this kind, and carried them
out through twenty-two successive generations, without any positive
result. Corroborative results of these experiments on mice have
been communicated by Ritzema Bos and, independently, by Rosenthal,
and a corresponding series of experiments on rats, which these two
investigators carried out, yielded the same negative results.
When we remember that all the cases which have been brought forward in
support of an inheritance of mutilations refer to a _single_ injury
to one parent, while, in the experiments, the same mutilation was
inflicted on both parents through numerous generations, we must regard
these experiments as a proof that all earlier statements were based
either on a fallacy or on fortuitous coincidence. This conclusion is
confirmed by all that we know otherwise of the effects of oft-repeated
mutilations, as for instance the well-known mutilations and distortions
which many peoples have practised for long, sometimes inconceivably
long, ages on their children, especially circumcision, the breaking
of the incisors, the boring of holes in lip, ear, or nose, and so
forth. No child of any of these races has ever been brought into the
world with one of these marks: they have to be re-impressed on every
generation.
The experience of breeders agrees with this, and they therefore, as
Wilckens remarks, have long regarded the non-inheritance of mutilations
as an established fact. Thus there are breeds of sheep in which, for
purely practical reasons, the tails have been curtailed quite regularly
for about a century (Kühn); but no sheep with a stump-tail has ever
been born in this breed. This is all the more important because there
are other breeds of sheep (fat-rumped sheep) in which the lack of
the tail is a breed character; it is thus not the case that there is
anything in the intrinsic nature of the tail of the sheep to prevent
it becoming rudimentary. The artificially rounded ear of fox-terriers,
too, though cut for generations, never occurs hereditarily. Mr. Postans
of Eastbourne informs me that the cocks which are to be used for
cock-fighting are docked of their combs and wattles beforehand, and
that this had been done for at least a century, but that no fighting
cock without comb and wattles has been reared. In the same way various
breeds of dog, such as the spaniels, have had their tails cut to half
their length regularly and in both sexes for more than a century, yet
in this case there is no hereditary diminution of the length of tail.
Deformed stump-tails do indeed occur in most breeds of dog, but, as I
said before, their anatomical character is quite different from that of
artificially shortened tails, moreover they may occur in breeds whose
tails have not suffered from the fashion of docking, as, for instance,
in the Dachshund.
We may therefore affirm that an inheritance of artificially produced
defects and mutilations is quite unproved, and in no way bears out the
supposed inheritance of functional changes.
This is now admitted by the great majority of the adherents of the
Lamarckian principle, and we may now regard this kind of 'proof' as
disposed of.
In addition to the above, various sets of facts have been brought
forward as proofs, and in particular the much discussed experiments of
Brown-Séquard on guinea-pigs, from which it was inferred that epilepsy
artificially induced could be transmitted. But these experiments do
not really prove anything in regard to the question at issue, because
epileptic-like convulsions may have very various causes, and these
are, for the most part, quite unknown. Since artificial epilepsy
can be induced in guinea-pigs by the most diverse injuries to the
central or peripheral parts of the nervous system, this of itself
points to the fact that it is not a question of the mere lesion of
anatomical structure, I mean, of the breaking of the continuity of
a definite part, and of its transmission. The result would, in any
case, differ according to whether certain centres of the brain, or
half the spinal cord, or the main nerve-trunks were cut through. There
must, therefore, be something more needed to produce the appearance of
epilepsy--some morbid process which may arise at different parts of
the nervous system, and be continued from them to the brain-centres.
This is corroborated by the fact that it takes at least fourteen days,
and often from six to eight weeks, for epilepsy to develop after the
operation, and that in many cases it does not develop at all. I have
made the suggestion that, during or after the operation, some kind of
pathogenic micro-organism might easily reach the wounded parts, and
there excite inflammation, which may extend centripetally to the brain.
Similar processes have been observed in connexion with lymph-vessels,
and why should they not occur in connexion with nerves?
It has been objected to this that the guinea-pig's epilepsy may be
produced by blows on the skull, and also by a destructive compression
of the _nervus ischiadicus_ through the skin, and that in both cases
the epilepsy may reappear in the following generation; and this, it
is supposed, shows that the intrusion of microbes is excluded. If
this were so beyond a doubt, and if we could exclude the possibility
that there were previously various microbes within the body, which
could only penetrate into the nervous substance after the cutting
or destruction of the neurilemma, nothing would be gained that
would in any way support the Lamarckian principle. One could only
say: Certain injuries to the nervous system give rise secondarily
in guinea-pigs to morbid phenomena like epilepsy, and all sorts of
functional disturbances of the nervous system often appear in the next
generation, including in rare cases even the phenomena of epileptic
convulsions. That this is a case of the transmission of an acquired
anatomical modification brought about by the injury is not only
unproved, but is decidedly negatived, for the injuries themselves are
never transmitted. Thus what is transmitted must be quite different
from what was acquired, for no one has ever detected in the offspring
the lesion of the nerve-trunk which was cut through in the parent, or
any other result except the disease to which the original injury gives
rise. Moreover, the inheritance of these morbid phenomena has been
again brought into dispute quite recently owing to the investigations
of such experts in nervous diseases as Sommer and Binswanger, and the
correctness of Brown-Séquard's results, which have dragged through the
literature of the subject for so long, has been emphatically denied[11].
[11] See H. E. Ziegler's report in _Zool. Centralblatt_, 1900, Nos. 12
and 13.
Clearly formulated problems, like that of the inheritance of acquired
characters, should not be confused by bringing into them phenomena
whose causes are quite unknown. What do we know of the real causes
of those central brain-irritations which give rise to the phenomena
of epilepsy? It is certain enough that there are diseases which are
acquired and are yet 'inherited,' but that has nothing to do with
the Lamarckian principle, because it is a question of _infection_
of the germ, not of a definite variation in the constitution of the
germ. We know this with certainty in regard to the so-called Pebrine,
the silkworm disease which wrought such devastation in its time; the
germs of the pebrine organism have been demonstrated _in the egg_ of
the silk-moth; they multiply, not at once but later, in the young
caterpillar, and it is the half-grown caterpillar, or even the moth,
that succumbs to the disease.
Whether in this case also the disease germs are transmitted through
the male sex-cells is not proved, as far as I am aware, but that this
can happen is shown by the transmission of syphilis from father to
child. That in this case, also, the exciting cause of the disease is a
micro-organism cannot be doubted, although it has not yet been proved.
Thus even the minute spermatozoon of Man can contain microbes, and
transmit them to the germ of a new individual.
This discussion of scientific questions ought not to be brought down
to the level of a play upon words, by bringing forward cases like the
above as evidence for the inheritance of 'acquired characters,' as was
done, for instance, by M. Nussbaum, who cited as a proof of this the
migration of the alga-cells which live in the endoderm of the green
freshwater Hydra into the ovum, which is originally colourless, and
originates in the ectoderm of the animal (Fig. 35_B_, p. 169, vol.
i). It seems to me better to make a precise distinction between the
transmission of extraneous micro-organisms through the germ-cells and
the handing on of the germ-plasm with the characters inherent in its
structure. Only the latter is inheritance in the strict scientific
sense, the former is infection of the germ.
Still less than the cases of inherited traumatic epilepsy can the
morbid constitution of the children of drunkards be regarded as a
proof of the inheritance of somatogenic characters, though this has
often been maintained. I will not lay any stress on the fact that the
allegation itself is, according to the most competent observers, such
as Dr. Thomas Morton[12], far from being established. But even if it
were quite certain that the numerous diseases of the nervous system,
amounting sometimes to mania, which are frequently observed in the
children of drunkards, were really _caused_ by the drinking of the
parents, it ought not to be overlooked that we have here to do not with
the hereditary transmission of somatic variations, but of variations
_directly_ induced in the germ-plasm of the reproductive cells, for
these are exposed to the influence of the alcohol circulating in the
blood, just as any other part of the body is. That by this means
variations in the germ-plasm can be brought about, and that these
may lead to morbid conditions in the children cannot be denied, and
ought not on _a priori_ grounds to be called in question. For we are
acquainted with many other influences--climatic, for instance--which
directly affect and cause variation in the germ-plasm. Whether this
is so in the case of drunkenness, and in what manner it comes about,
whether through direct action of the alcohol, or through infection of
the germ with some microbe, we must leave to the future to decide; the
whole question is out of place here; it can in no way help us to clear
up the problem with which we are now occupied.
[12] Morton, 'The Problem of Heredity in Reference to Inebriety,'
Proceed. Soc. for the Study of Inebriety, No. 42, Nov. 1894.
But even if there were not a trace of proof of the transmissibility of
functional modifications, that alone would not justify us in concluding
that the transmission is impossible, for many things may happen that we
are not in a position to prove at present. If it could be shown that
there was a whole group of phenomena that could not be explained in any
other way than on the hypothesis of such inheritance, then we should
be obliged to assume that it really occurred, although it was not
demonstrable, and, indeed, not even theoretically conceivable. This is
the standpoint of the adherents of the Lamarckian principle at present.
They say there are a great number of transformations which are simply
and easily explained, if we regard them as the effects of inherited
use or disuse, but which admit only of a strained explanation, and
sometimes of none at all, on the basis of natural selection, and these
are not a few isolated cases, but whole categories of them.
I will submit a few of these, and show at the same time why I cannot
regard them as convincing, even if it be the case that we are not at
present in a position to explain them without the aid of the Lamarckian
principle. But let me hasten to add that it is my belief that we can do
this, although certainly not without first giving a somewhat extended
application to the principle of selection.
It has often been maintained that the existence of animal instincts is
in itself enough to prove that the Lamarckian principle is operative.
In one of the earlier lectures I showed that at least the greater
number of instincts must have originated in purely reflex actions,
and therefore, like these actions themselves, can only be explained
through natural selection. A reflex action, such as coughing, sneezing,
shutting of the eyelids, and so on, differs from an instinctive
action in the lesser complexity and shorter duration of the series
of movements liberated by a sense-impression, and also in that it
does not require to enter into consciousness at all; but no very
precise boundary can be drawn between the two, and, in any case, both
depend, as we have already seen, on a quite analogous anatomical
basis. It is only a difference in degree whether, at the sight of a
rapidly approaching object, the muscles of the eyelids contract, and
by shutting the lids, protect the eye, or whether the fly, which we
intend to seize with our hand, is impelled by the sight of the rapidly
approaching shadow of the hand to fly quickly up. The action of the fly
may be regarded as reflex, or equally well as instinctive. But there
is also only a difference in degree, not in kind, between this simple
action and the complex and protracted behaviour of a mason-bee, the
sight of whose colony impels her to fly out and fetch clay, with it
gradually to build a neat cell, to fill this with honey, to lay an egg
in it, and finally to furnish the cell with a roof of clay. Since all
reflex mechanisms, and all the natural instincts of animals, contribute
to the maintenance of the species, and are therefore useful, their
origins must be referable to natural selection, and we have only to ask
whether they must be referred to it always, and to it alone.
It cannot be doubted that, in Man, and in the higher animals voluntary
actions which are often repeated gradually acquire the character
of instinctive actions. The individual movements pertaining to the
particular action are no longer each guided by the will, but a single
exercise of will is enough to liberate the whole complex action, such
as writing, speaking, walking, or the playing of a whole piece of
music; frequently the will-impulse may be absent altogether, and the
action be set going simply by an adequate external stimulus, as in
the case of sleep-walking, which is observed in fatigued children and
soldiers, and in somnambulists. The external stimulus is transmitted
to the proper group of muscles as unfailingly as in the case of true
instincts, and this happens not only in regard to actions which,
like walking, are essential to the life of the species, but also in
regard to those which have arisen from chance habits or exercises.
Often a short practice is sufficient to make an action in this sense
instinctive, and the complexity of the instinct-mechanism gained by
such practice is often astounding. Under some circumstances a person
may play a piece on the piano from the score, and yet be thinking
intently of other things, and be quite unconscious of what is played.
In the same way it may happen that a person dominated by violent
emotion, when trying to free himself from it by reading, may read a
whole page, line by line, without understanding in the least what has
been read. In the last case it is not directly demonstrable that the
reader has made all the complex delicate eye-movements which would be
liberated by the sight of the words, but in the case of playing, the
listeners can perceive that the piece is correctly played, and thus
that the stimulus exercised by each note on the retina of the eye
is translated into the complex muscular movement of arm and finger,
corresponding both to the pitch and the duration of the note, and to
the simultaneousness of several notes.
In all these cases it is probably not always quite new paths which
are established in the brain, but use is made of particular tracks
in the innumerable nerve-paths already existing in the nerve-cells
(neurons) which are 'more thoroughly trodden' by practice, so that
the distribution of the nerve-current takes place more easily along
them than along others[13]. This much-used metaphor does not indicate
the actual structural changes which have taken place, but it serves
at least to indicate that we have to do with material changes in the
ultimate living elements of the nerve-substance (nerve-biophors)
whether these changes be in position or in quality. Now, if such
brain-structures and mechanisms acquired through exercise in the
individual life could be transmitted, new instincts would certainly
arise in this way, and many naturalists hold this view still.
[13] This, however, is by no means intended to cast doubt on the
possibility that quite new paths may arise during the individual life,
as is made probable by the recent investigations of Apáthy, Bethe, and
others.
If the inheritance of acquired characters had already been proved in
other ways, we could not refuse to admit that it might play a part in
the higher animals in the modification and new formation of instincts.
We should then have to admit that habits can be inherited, and that
instincts actually are or may be, as they have often been said to be,
inherited habits. But to make the converse conclusion, and to infer
from the result of the brain-exercise in the individual life and its
similarity to inborn instincts that the latter also depend on inherited
exercise, and that there must therefore be inheritance of acquired
characters, is hardly admissible.
It might be all very well if there were no other explanation! But as
instincts depend on material brain-mechanisms which are variable, like
every other part of the body, and as, furthermore, they are essential
to the existence of the species, and, down to the minutest detail, are
adapted to the circumstances of life, there is no obstacle in the way
of referring their origin and transformation to processes of selection.
It has been asserted that the results of training, for instance in
dogs, can be inherited, since the untaught young pointer points at
the game, and the young sheep-dog runs round and barks at the flock
of sheep without biting them. It is, however, often forgotten that,
not only have these breeds arisen under the influence of artificial
selection by Man, but that they are even now strictly selected. My
colleague and friend, Dr. Otto vom Rath, who unhappily died all too
soon for Science, and who was not only a capable investigator, but
an experienced sportsman, told me that huntsmen distinguish very
carefully between the better and the inferior young in a litter, and
that by no means every whelp of a pair of pointers can be used for
hunting game-birds. Lloyd Morgan points out the same thing, and he is
undoubtedly a competent judge in the domain of instinct; he confirms
the statement that the pointer 'often points at the quarry, it may be a
lark's nest, without instruction,' but he says at the same time, that
the power is inborn in very varying degrees, and that, in his opinion,
selection undoubtedly plays a part.