fully intelligible--whether or not a true--explanation. For those
nerve-centres in the spinal cord which co-ordinate the muscles required
for retracting the feet are the centres used by the will for this
purpose. And, by hypothesis, the frequent use of them for this purpose
under circumstances of stimulation which render the muscular response
appropriate, will eventually establish an organic connexion between such
response and the kind of stimulation to which it is appropriate--even
though there be no utilitarian reason for its establishment[43]. To
invert a phrase of Aristotle, we do not frequently use this mechanism
because we have it (seeing that in our normal condition there is no
necessity for such use); but, by hypothesis, we have it because we have
frequently used its several elements in appropriate combination.
[43] It may be said, with regard to this particular reflex, that it
may perhaps be, so to speak, a mechanical accident, arising
from the contiguity of the sensory and motor roots in the cord.
But as this suggestion cannot apply to other reflexes presently
to be adduced, it need not be considered.
I will adduce but one further example in illustration of these general
principles--passing at once from the foregoing case of comparative
simplicity to one of extreme complexity.
There is a well-known experiment on a brainless frog, which reveals a
beautiful reflex mechanism in the animal, whereby the whole body is
enabled continually to readjust its balance on a book (or any other
plane surface), as this is slowly rotated on a horizontal axis. So long
as the book is lying flat, the frog remains motionless; but as soon as
the book is tilted a little, so that the frog is in danger of slipping
off, all the four feet begin to crawl up the hill; and the steeper the
hill becomes, the faster they crawl. When the book is vertical, the frog
has reached the now horizontal back, and so on. Such being the facts,
the question is--How can the complicated piece of machinery thus implied
have been developed by natural selection? Obviously it cannot have been
so by any of the parts concerned having been originally distributed
among different individuals, and afterwards united in single individuals
by survival (i.e. free intercrossing) of the fittest. In other words,
the case is obviously one of co-adaptation, and not one of the blending
of adaptations. Again, and no less obviously, it is impossible that the
co-adaptation can have been _gradually developed_ by natural selection,
because, in order to have been so, it must by hypothesis have been of
some degree of use in every one of its stages; yet it plainly cannot
have been until it had been fully perfected in all its astonishing
complexity[44].
[44] Of course it will be observed that the question is not with
regard to the development of all the nerves and muscles
concerned in this particular process. It is as to the
development of the co-ordinating centres, which thus so
delicately respond to the special stimuli furnished by
variations of angle to the horizon. And it is as inconceivable
in this case of reflex action, as it is in almost every other
case of reflex action, that the highly specialized machinery
required for performing the adaptive function can ever have had
its origin in the performance of any other function. Indeed, a
noticeable peculiarity of reflex mechanisms as a class is the
highly specialized character of the functions which their
highly organized structures subserve.
Lastly, not only does it thus appear impossible that during all stages
of its development--or while as yet incapable of performing its
intricate function--this nascent mechanism can have had any adaptive
value; but even as now fully developed, who will venture to maintain
that it presents any selective value? As long as the animal preserves
its brain, it will likewise preserve its balance, by the exercise of its
intelligent volition. And, if the brain were in some way destroyed, the
animal would be unable to breed, or even to feed; so that natural
selection can never have had any _opportunity_, so to speak, of
developing this reflex mechanism in brainless frogs. On the other hand,
as we have just seen, we cannot perceive how there can ever have been
any _raison d'etre_ for its development in normal frogs--even if its
development were conceivably possible by means of this agency. But if
practice makes perfect in the race, as it does in the individual, we can
immediately perceive that the constant habit of correctly adjusting its
balance may have gradually developed, in the batrachian organization,
this non-necessary reflex[45].
[45] We meet with a closely analogous reflex mechanism in brainless
vertebrata of other kinds; but these do not furnish such good
test cases, because the possibility of natural selection cannot
be so efficiently attenuated. The perching of brainless birds,
for instance, at once refers us to the roosting of sleeping
birds, where the reflex mechanism concerned is clearly of high
adaptive value. Therefore such a case is not available as a
test, although the probability is that birds have inherited
their balancing mechanisms from their sauropsidian ancestors,
where it would have been of no such adaptive importance.
And, of course, this example--like that of withdrawing the feet from a
source of stimulation, which a frog will do as well as a man--does not
stand alone. Without going further a-field than this same animal, any
one who reads, from our present point of view, Goltz's work on the
reflex actions of the frog, will find that the great majority of
them--complex and refined though most of them are--cannot conceivably
have ever been of any use to any frog that was in undisturbed possession
of its brain.
Hence, not to occupy space with a reiteration of facts all more or less
of the same general kind, and therefore all presenting identical
difficulties to ultra-Darwinian theory, I shall proceed to give two
others which appear to me of particular interest in the present
connexion, because they furnish illustrations of reflex actions in a
state of only partial development, and are therefore at the present
moment demonstrably useless to the animal which displays them.
Many of our domesticated dogs, when we gently scratch their sides and
certain other parts of the body, will themselves perform scratching
movements with the hind leg of the same side as that upon which the
irritation is being supplied. According to Goltz[46], this action is a
true reflex; for he found that it is performed equally well in a dog
which has been deprived of its cerebral hemispheres, and therefore of
its normal volition. Again, according to Haycraft[47], this reflex is
congenital, or not acquired during the life-time of each individual dog.
Now, although the action of scratching is doubtless adaptive, it appears
to me incredible that it could ever have become organized into a
congenital reflex by natural selection. For, in order that it should,
the scratching away fleas would require to have been a function of
selective value. Yet, even if the irritation caused by fleas were
supposed to be so far fatal in the struggle for existence, it is certain
that they would always be scratched away by the conscious intelligence
of each individual dog; and, therefore, that no advantage could be
gained by organizing the action into a reflex. On the other hand, if
acquired characters are ever in any degree transmitted, it is easy to
understand how so frequently repeated an action should have become, in
numberless generations of dogs, congenitally automatic.
[46] _Pflueger's Archiv_, Bd. xx. s. 23 (1879).
[47] _Brain_, part xlviii, pp. 516-19 (1889).--There is still better
proof of this in the case of certain rodents. For instance,
observing that rats and mice are under the necessity of very
frequently scratching themselves with their hind-feet, I tried
the experiment of removing the latter from newly-born
individuals--i.e. before the animals were able to co-ordinate
their movements, and therefore before they had ever even
attempted to scratch themselves. Notwithstanding that they were
thus destitute of individual experience with regard to the
benefit of scratching, they began their scratching movements
with their stumps as soon as they were capable of executing
co-ordinated movements, and afterwards continued to do so till
the end of their lives with as much vigour and frequency as
unmutilated animals. Although the stumps could not reach the
seats of irritation which were bent towards them, they used to
move rapidly in the air for a time sufficient to have given the
itching part a good scratch, had the feet been present--after
which the animals would resume their sundry other avocations
with apparent satisfaction. These facts showed the hereditary
response to irritation by parasites to be so strong, that even
a whole life-time's experience of its futility made no
difference in the frequency or the vigour thereof.
So much for the general principle of selective value as applied to this
particular case. And similarly, of course, we might here repeat the
application of all the other general principles, which have just been
applied in the two preceding cases. But it is only one of these other
general principles which I desire in the present case specially to
consider, for the purpose of considering more closely than hitherto the
difficulty which this principle presents to ultra-Darwinian theory.
The difficulty to which I allude is that of understanding how all the
stages in the _development_ of a reflex action can have been due to
natural selection, seeing that, before the reflex mechanism has been
sufficiently elaborated to perform its function, it cannot have
presented any degree of utility. Now the particular force of the present
example, the action of scratching--as also of the one to
follow--consists in the fact that it is a case where a reflex action is
not yet completely organized. It appears to be only in course of
construction, so that it is neither invariably present, nor, when it is
present, is it ever fully adapted to the performance of its function.
That it is not invariably present (when the brain is so) may be proved
by trying the simple experiment on a number of puppies--and also of
full-grown dogs. Again, that even when it is present it is far from
being fully adapted to the performance of its function, may be proved by
observing that only in rare instances does the scratching leg succeed in
scratching the place which is being irritated. The movements are made
more or less at random, and as often as not the foot fails to touch the
body at any place at all. Hence, although we have a "prophecy" of a
reflex action well designed for the discharge of a particular function,
at present the machinery is not sufficiently perfected for the adequate
discharge of that function. In this important respect it differs from
the otherwise closely analogous reflex action of the frog, whereby the
foot of the hind leg is enabled to localize with precision a seat of
irritation on the side of the body. But this beautiful mechanism in the
frog cannot have sprung into existence ready formed at any historical
moment in the past history of the phyla. It must have been the subject
of a more or less prolonged evolution, in some stage of which it must
presumably have resembled the now nascent scratching reflex of the dog,
in making merely abortive attempts at localizing the seat of
irritation--supposing, of course, that some physiologist had been there
to try the experiment by first removing the brain. Now, even if one
could imagine it to be, either in the frog or in the dog, a matter of
selective importance that so exceedingly refined a mechanism should have
been developed for the sole purpose of inhibiting the bites of
parasites--which in every normal animal would certainly be discharged by
an _intentional_ performance of the movements in question,--even if, in
order to save an hypothesis at all costs, we make so violent a
supposition as this, still we should do so in vain. For it would still
remain undeniably certain that the reflex mechanism is _not_ of any
selective value. Even now the mechanism in the dog is not sufficiently
precise to subserve the only function which occasionally and abortively
it attempts to perform. Thus it has all the appearance of being but an
imitating shadow of certain neuro-muscular adjustments, which have been
habitually performed in the canine phyla by a volitional response to
cutaneous irritation. Were it necessary, this argument might be
strengthened by observing that the reflex action is positively
_improved_ by removal of the brain.
The second example of a nascent reflex in dogs which I have to mention
is as follows.
Goltz found that his brainless dogs, when wetted with water, would shake
themselves as dry as possible, in just the same way as normal dogs will
do under similar circumstances. This, of course, proves that the shaking
movements may be performed by a reflex mechanism, which can have no
other function to perform in the organization of a dog, and which,
besides being of a highly elaborate character, will respond only to a
very special kind of stimulation. Now, here also I find that the
mechanism is congenital, or not acquired by individual experience. For
the puppies on which I experimented were kept indoors from the time of
their birth--so as never to have had any experience of being wetted by
rain, &c.--till they were old enough to run about with a full power of
co-ordinating their general movements. If these young animals were
suddenly plunged into water, the shock proved too great: they would
merely lie and shiver. But if their feet alone were wetted, by being
dipped in a basin of water, the puppies would soon afterwards shake
their heads in the peculiar manner which is required for shaking water
off the ears, and which in adult dogs constitutes the first phase of a
general shaking of the whole body.
Here, then, we seem to have good evidence of all the same facts which
were presented in the case of the scratching reflex. In the first
place, co-adaptation is present in a very high degree, because this
shaking reflex in the dog, unlike the skin-twitching reflex in the
horse, does not involve only a single muscle, or even a single group of
muscles; it involves more or less the co-ordinated activity of many
voluntary muscles all over the body. Such, at any rate, is the case when
the action is performed by the intelligent volition of an adult dog; and
if a brainless dog, or a young puppy, does not perform it so extensively
or so vigorously, this only goes to prove that the reflex has not yet
been sufficiently developed to serve as a substitute for intelligent
volition--i.e. that it is _useless_, or a mere organic shadow of the
really adaptive substance. Again, even if this nascent reflex had been
so far developed as to have been capable of superseding voluntary
action, still we may fairly doubt whether it could have proved of
selective value. For it is questionable whether the immediate riddance
of water after a wetting is a matter of life and death to dogs in a
state of nature. Moreover, even if it were, every individual dog would
always have got rid of the irritation, and so of the danger, by means of
a _voluntary_ shake--with the double result that natural selection has
never had any opportunity of gradually building up a special reflex
mechanism for the purpose of securing a shake, and that the canine race
have not had to wait for any such unnecessary process. Lastly, such a
process, besides being unnecessary, must surely have been, under any
circumstances, impossible. For even if we were to suppose--again for the
sake of saving an hypothesis at any cost--that the presence of a
fully-formed shaking reflex is of selective value in the struggle for
existence, it is perfectly certain that all the stages through which the
construction of so elaborate a mechanism must have passed could not have
been, under any circumstances, of any such value.
But, it is needless to repeat, according to the hypothesis of
use-inheritance, there is no necessity to suppose that these incipient
reflex mechanisms _are_ of any value. If function produces structure in
the race as it does in the individual, the voluntary and frequently
repeated actions of scratching and shaking may very well have led to an
organic integration of the neuro-muscular mechanisms concerned. Their
various parts having been always co-ordinated for the performance of
these actions by the intelligence of innumerable dogs in the past, their
co-adapted activity in their now automatic responses to appropriate
stimuli presents no difficulty. And the consideration that neither in
their prospectively more fully developed condition, nor, _a fortiori_,
in their present and all previous stages of evolution, can these reflex
mechanisms be regarded as presenting any selective--or even so much as
any adaptive--value, is neither more nor less than the theory of
use-inheritance would expect.
Thus, with regard to the phenomena of reflex action in general, all the
facts are such as this theory requires, while many of the facts are such
as the theory of natural selection alone cannot conceivably explain.
Indeed, it is scarcely too much to say, that most of the facts are such
as directly contradict the latter theory in its application to them.
But, be this as it may, at present there are only two hypotheses in the
field whereby to account for the facts of adaptive evolution. One of
these hypotheses is universally accepted, and the only question is
whether we are to regard it as _alone_ sufficient to explain _all_ the
facts. The other hypothesis having been questioned, we can test its
validity only by finding cases which it is fully capable of explaining,
and which do not admit of being explained by its companion hypothesis. I
have endeavoured to show that we have a large class of such cases in the
domain of reflex action, and shall next endeavour to show that there is
another large class in the domain of instinct.
* * * * *
If instinct be, as Professor Hering, Mr. Samuel Butler, and others have
argued, "hereditary habit"--i. e. if it comprises an element of
transmitted experience--we at once find a complete explanation of many
cases of the display of instinct which otherwise remain inexplicable.
For although a large number--or even, as I believe, a large majority--of
instincts are explicable by the theory of natural selection alone, or by
supposing that they were gradually developed by the survival of
fortuitous variations in the way of advantageous psychological
peculiarities, this only applies to comparatively simple instincts, such
as that of a protectively coloured animal exhibiting a preference for
the surroundings which it resembles, or even adopting attitudes in
imitation of objects which occur in such surroundings. But in all cases
where instincts become complex and refined, we seem almost compelled to
accept Darwin's view that their origin is to be sought in consciously
intelligent adjustments on the part of ancestors.
Thus, to give only one example, a species of Sphex preys upon
caterpillars, which it stings in their nerve-centres for the purpose of
paralyzing, without killing them. The victims, when thus rendered
motionless, are then buried with the eggs of the Sphex, in order to
serve as food for her larvae which subsequently develop from these eggs.
Now, in order thus to paralyze a caterpillar, the Sphex has to sting it
successively in nine minute and particular points along the ventral
surface of the animal--and this the Sphex unerringly does, to the
exclusion of all other points of the caterpillar's anatomy. Well, such
being the facts--according to M. Fabre, who appears to have observed
them carefully--it is conceivable enough, as Darwin supposed[48], that
the ancestors of the Sphex, being like many other hymenopterous insects
highly intelligent, should have observed that on stinging caterpillars
in these particular spots a greater amount of effect was produced than
could be produced by stinging them anywhere else; and, therefore, that
they habitually stung the caterpillars in these places only, till, in
course of time, this originally intelligent habit became by heredity
instinctive. But now, on the other hand, if we exclude the possibility
of this explanation, it appears to me incredible that such an instinct
should ever have been evolved at all; for it appears to me incredible
that natural selection, unaided by originally intelligent action, could
ever have developed such an instinct out of merely fortuitous
variations--there being, by hypothesis, nothing to _determine_
variations of an insect's mind in the direction of stinging caterpillars
only in these nine intensely localized spots[49].
[48] For details of his explanation of this particular case, for
which I particularly inquired, see _Mental Evolution in
Animals_, pp. 301-2.
[49] Note B.
Again, there are not a few instincts which appear to be wholly useless
to their possessors, and others again which appear to be even
deleterious. The dusting over of their excrement by certain
freely-roaming carnivora; the choice by certain herbivora of particular
places on which to void their urine, or in which to die; the howling of
wolves at the moon; purring of cats, &c., under pleasurable emotion; and
sundry other hereditary actions of the same apparently unmeaning kind,
all admit of being readily accounted for as useless habits originally
acquired in various ways, and afterwards perpetuated by heredity,
because not sufficiently deleterious to have been stamped out by natural
selection[50]. But it does not seem possible to explain them by survival
of the fittest in the struggle for existence.
[50] For fuller treatment see _Mental Evolution in Animals_, pp.
274-285, 378-379, 381-383.
Finally, in the case of our own species, it is self-evident that the
aesthetic, moral, and religious instincts admit of a natural and easy
explanation on the hypothesis of use-inheritance, while such is by no
means the case if that hypothesis is rejected. Our emotions of the
ludicrous, of the beautiful, and of the sublime, appear to be of the
nature of hereditary instincts; and be this as it may, it would further
appear that, whatever else they may be, they are certainly not of a
life-preserving character. And although this cannot be said of the
moral sense when the theory of natural selection is extended from the
individual to the tribe, still, when we remember the extraordinary
complexity and refinement to which they have attained in civilized man,
we may well doubt whether they can have been due to natural selection
alone. But space forbids discussion of this large and important question
on the present occasion. Suffice it therefore to say, that I doubt not
Weismann himself would be the first to allow that his theory of heredity
encounters greater difficulties in the domain of ethics than in any
other--unless, indeed, it be that of religion[51].
[51] For an excellent essay on the deleterious character of early
forms of religion from a biological point of view, see the Hon.
Lady Welby, _An Apparent Paradox in Mental Evolution_ (Journ.
Anthrop. Inst. May 1891).
* * * * *
I have now given a brief sketch of the indirect evidence in favour of
the so-called Lamarckian factors, in so far as this appears fairly
deducible from the facts of reflex action and of instinct. It will now
be my endeavour to present as briefly what has to be said against this
evidence.
As previously observed, the facts of reflex action have not been
hitherto adduced in the present connexion. This has led me to occupy
considerably more space in the treatment of them than those of instinct.
On this account, also, there is here nothing to quote, or to consider,
_per contra_. On the other hand, however, Weismann has himself dealt
with the phenomena of instinct in animals, though not, I think, in
man--if we except his brilliant essay on music. Therefore let us now
begin this division of our subject by briefly stating, and considering,
what he has said upon the subject.
The answer of Weismann to difficulties which arise against the
ultra-Darwinian theory in the domain of instinct, is as follows:--
"The necessity for extreme caution in appealing to the supposed
hereditary effects of use, is well shown in the case of those
numerous instincts which only come into play once in a life-time,
and which do not therefore admit of improvement by practice. The
queen-bee takes her nuptial flight only once, and yet how many and
complex are the instincts and the reflex mechanisms which come into
play on that occasion. Again, in many insects the deposition of
eggs occurs but once in a life-time, and yet such insects always
fulfil the necessary conditions with unfailing accuracy[52]."
[52] _Essays_, i. p. 93.
But in this rejoinder the possibility is forgotten, that although such
actions are _now_ performed only once in the individual life-time,
_originally_--i.e. when the instincts were being developed in a remote
ancestry--they may have been performed on many frequent and successive
occasions during the individual life-time. In all the cases quoted by
Weismann, instincts of the kind in question bear independent evidence of
high antiquity, by occurring in whole genera (or even families), by
being associated with peculiar and often highly evolved structures
required for their performance, and so on. Consequently, in these cases
ample time has been allowed for subsequent changes of habit, and of
seasonal alterations with respect to propagation--both these things
being of frequent and facile occurrence among animals of all kinds, even
within periods which fall under actual observation. Nevertheless, I do
not question that there are instinctive activities which, as far as we
are able to see, can never have been performed more than once in each
individual life-time[53]. The fact, however, only goes to show what is
fully admitted--that some instincts (and even highly complex instincts)
have apparently been developed by natural selection alone. Which, of
course, is not equivalent to showing that all instincts must have been
developed by natural selection alone. The issue is not to be debated on
general grounds like this, but on those of particular cases. Even if it
were satisfactorily proved that the instincts of a queen-bee have been
developed by natural selection, it would not thereby be proved that such
has been the case with the instincts of a Sphex wasp. One can very well
understand how the nuptial flight of the former, with all its associated
actions, may have been brought about by natural selection alone; but
this does not help us to understand how the peculiar instincts of the
latter can have been thus caused.
[53] See _Mental Evolution in Animals_, pp. 377-8.
Strong evidence in favour of Weismann's views does, however, at first
sight seem to be furnished by social hymenoptera in other respects. For
not only does the queen present highly specialized and altogether
remarkable instincts; but the neuters present totally different and even
still more remarkable instincts--which, moreover, are often divided into
two or more classes, corresponding with the different "castes." Yet the
neuters, being barren females, never have an opportunity of bequeathing
their instincts to progeny. Thus it appears necessary to suppose that
the instincts of all the different castes of neuters are latent in the
queen and drones, together with the other instincts which are patent in
both. Lastly, it seems necessary to suppose that all this wonderful
organization of complex and segregated instincts must have been built up
by natural selection acting exclusively on the queens and drones--seeing
that these exercise their own instincts only once in a life-time, while,
as just observed, the neuters cannot possibly bequeath their individual
experience to progeny. Obviously, however, natural selection must here
be supposed to be operating at an immense disadvantage; for it must have
built up the often diverse and always complex instincts of neuters, not
directly, but indirectly through the queens and drones, which never
manifest any of these instincts themselves.
Now Darwin fully acknowledged the difficulty of attributing these
results to the unaided influence of natural selection; but the fact of
neuter insects being unable to propagate seemed to him to leave no
alternative. And so it seems to Weismann, who accordingly quotes these
instincts in support of his views. And so it seemed to me, until my work
on _Animal Intelligence_ was translated into French, and an able Preface
was supplied to that translation by M. Perrier. In this Preface it is
argued that we are not necessarily obliged to exclude the possibility of
Lamarckian principles having operated in the original formation of these
instincts. On the contrary, if such principles ever operate at all,
Perrier shows that here we have a case where it is virtually certain
that they must have operated. For although neuter insects are now unable
to propagate, their organization indicates--if it does not actually
prove--that they are descended from working insects which were able to
propagate. Thus, in all probability, what we now call a "hive" was
originally a society of sexually mature insects, all presenting the same
instincts, both as to propagation and to co-operation. When these
instincts, thus common to all individuals composing the hive, had been
highly perfected, it became of advantage in the struggle for existence
(between different hives or communities) that the functions of
reproduction should devolve more upon some individuals, while those of
co-operation should devolve more upon others. Consequently, this
division of labour began, and gradually became complete, as we now find
it in bees and ants. Perrier sustains the hypothesis thus briefly
sketched by pointing to certain species of social hymenoptera where we
may actually observe different stages of the process--from cases where
all the females of the hive are at the same time workers and breeders,
up to the cases where the severance between these functions has become
complete. Therefore, it seems to me, it is no longer necessary to
suppose that in these latter cases all the instincts of the (now) barren
females can only have been due to the unaided influence of natural
selection.
Nevertheless, although I think that Perrier has made good his position
thus far, that his hypothesis fails to account for some of the instincts
which are manifested by neuter insects, such as those which, so far as I
can see, must necessarily be supposed to have originated after the
breeding and working functions had become separated--seeing that they
appear to have exclusive reference to this peculiar state of matters.
Possibly, however, Perrier might be able to meet each of these
particular instincts, by showing how they could have arisen out of
simpler beginnings, prior to the separation of the two functions in
question. There is no space to consider such possibilities in detail;
but, until this shall have been done, I do not think we are entitled to
conclude that the phenomena of instinct as presented by neuter insects
are demonstrably incompatible with the doctrines of Lamarck--or, that
these phenomena are available as a logical proof of the unassisted
agency of natural selection in the case of instincts in general[54].
[54] [See H. Spencer, _The Inadequacy of Natural Selection, A
Rejoinder to Professor Weismann_, Contemp. Rev. 1893; and
_Weismannism once more_, Ibid. Oct. 1894; Weismann, _The
All-sufficiency of Natural Selection_, Ibid. 1893; and _The
Effect of External Influences upon Development_, "Romanes
Lecture" 1894: also _Neuter Insects and Lamarckism_, W. Platt
Ball, Natural Science, Feb. 1894, and _Neuter Insects and
Darwinism_, J. T. Cunningham, Ibid. April 1894. C. Ll. M.]
(B.)
_Inherited Effects of Use and of Disuse._
There is no doubt that Darwin everywhere attaches great weight to this
line of evidence. Nevertheless, in my opinion, there is equally little
doubt that, taken by itself, it is of immeasurably less weight than
Darwin supposed. Indeed, I quite agree with Weismann that the whole of
this line of evidence is practically worthless; and for the following
reasons.
The evidence on which Darwin relied to prove the inherited effects of
use and disuse was derived from his careful measurements of the increase
or decrease which certain bones of our domesticated animals have
undergone, as compared with the corresponding bones of ancestral stocks
in a state of nature. He chose domesticated animals for these
investigations, because, while yielding unquestionable cases of
increased or diminished use of certain organs over a large number of
sequent generations, the results were not complicated by the possible
interference of natural selection on the one hand, or by that of the
economy of nutrition on the other. For "with highly-fed domesticated
animals there seems to be no economy of growth, or any tendency to the
elimination of superfluous details[55];" seeing that, among other
considerations pointing in the same direction, "structures which are
rudimentary in the parent species, sometimes become partially
re-developed in our domesticated productions[56]."
[55] _Variation of Plants and Animals_, vol. ii. p. 289.
[56] _Ibid._ p. 346.
The method of Darwin's researches in this connexion was as follows.
Taking, for example, the case of ducks, he carefully weighed and
measured the wing-bones and leg-bones of wild and tame ducks; and he
found that the wing-bones were smaller, while the leg-bones were larger,
in the tame than in the wild specimens. These facts he attributed to
many generations of tame ducks using their wings less, and their legs
more, than was the case with their wild ancestry. Similarly he compared
the leg-bones of wild rabbits with those of tame ones, and so forth--in
all cases finding that where domestication had led to increased use of a
part, that part was larger than in the wild parent stock; while the
reverse was the case with parts less used. Now, although at first sight
these facts certainly do seem to yield good evidence of the inherited
effects of use and disuse, they are really open to the following very
weighty objections.
First of all, there is no means of knowing how far the observed effects
may have been due to increased or diminished use during only the
individual life-time of each domesticated animal. Again, and this is a
more important point, in all Darwin's investigations the increase or
decrease of a part was estimated, not by directly comparing, say the
wing-bones of a domesticated duck with the wing-bones of a wild duck,
but by comparing the _ratio_ between the wing and leg bones of a tame
duck with the _ratio_ between the wing and leg bones of a wild duck.
Consequently, if there be any reason to doubt the supposition that a
really inherited decrease in the size of a part thus estimated is due to
the inherited effects of disuse, such a doubt will also extend to the
evidence of increased size being due to the inherited effects of use.
Now there is the gravest possible doubt lying against the supposition
that any really inherited decrease in the size of a part is due to the
inherited effects of disuse. For it may be--and, at any rate to some
extent, must be--due to another principle, which it is strange that
Darwin should have overlooked. This is the principle which Weismann has
called Panmixia, and which cannot be better expressed than in his own
words:--
"A goose or a duck must possess strong powers of flight in the
natural state, but such powers are no longer necessary for
obtaining food when it is brought into the poultry-yard; so that a
rigid selection of individuals with well-developed wings at once
ceases among its descendants. Hence, in the course of generations,
a deterioration of the organs of flight must necessarily
ensue[57]."
[57] _Essays_, i. p. 90.
Or, to state the case in another way: if any structure which was
originally built up by natural selection on account of its use, ceases
any longer to be of so much use, in whatever degree it ceases to be of
use, in that degree will the premium before set upon it by natural
selection be withdrawn. And the consequence of this withdrawal of
selection as regards that particular part will be to allow the part to
degenerate in successive generations. Such is the principle which
Weismann calls Panmixia, because, by the withdrawal of selection from
any particular part, promiscuous breeding ensues with regard to that
part. And it is easy to see that this principle must be one of very
great importance in nature; because it must necessarily come into
operation in all cases where any structure or any instinct has, through
any change in the environment or in the habits of a species, ceased to
be useful. It is likewise easy to see that its effect must be the same
as that which was attributed by Darwin to the inherited effect of
disuse; and, therefore, that the evidence on which he relied in proof of
the inherited effects both of use and of disuse is vitiated by the fact
that the idea of Panmixia did not occur to him.
Here, however, it may be said that the idea first occurred to me[58]
just after the publication of the last edition of the _Origin of
Species_. I called the principle the Cessation of Selection--which I
still think a better, because a more descriptive, term than Panmixia;
and at that time it appeared to me, as it now appears to Weismann,
entirely to supersede the necessity of supposing that the effect of
disuse is ever inherited in any degree at all. Thus it raised the whole
question as to the admissibility of Lamarckian principles in general; or
the question on which we are now engaged touching the possible
inheritance of acquired, as distinguished from congenital, characters.
But on discussing the matter with Mr. Darwin, he satisfied me that the
larger question was not to be so easily closed. That is to say, although
he fully accepted the principle of the Cessation of Selection, and as
fully acknowledged its obvious importance, he convinced me that there
was independent evidence for the transmission of acquired characters,
sufficient in amount to leave the general structure of his previous
theory unaffected by what he nevertheless recognized as a factor which
must necessarily be added. All this I now mention in order to show that
the issue which Weismann has raised since Darwin's death was expressly
contemplated during the later years of Darwin's life. For if the idea of
Panmixia--in the absence of which Weismann's entire system would be
impossible--had never been present to Darwin's mind, we should have been
left in uncertainty how he would have regarded this subsequent revolt
against what are generally called the Lamarckian principles[59].
[58] _Nature_, vol. ix. pp. 361-2, 440-1; and vol. x. p. 164.
[59] Appendix I.
Moreover, in this connexion we must take particular notice that the
year after I had published these articles on the Cessation of Selection,
and discussed with Mr. Darwin the bearing of this principle on the
question of the transmission of acquired characters, Mr. Galton followed
with his highly important essay on Heredity. For in this essay Mr.
Galton fully adopted the principle of the Cessation of Selection, and
was in consequence the first publicly to challenge the Lamarckian
principles--pointing out that, if it were thus possible to deny the
transmission of acquired characters _in toto_, "we should be relieved
from all further trouble"; but that, if such characters are transmitted
"in however faint a degree, a complete theory of heredity must account
for them." Thus the question which, in its revived condition, is now
attracting so much attention, was propounded in all its parts some
fifteen or sixteen years ago; and no additional facts or new
considerations of any great importance bearing upon the subject have
been adduced since that time. In other words, about a year after my own
conversations with Mr. Darwin, the whole matter was still more
effectively brought before his notice by his own cousin. And the result
was that he still retained his belief in the Lamarckian factors of
organic evolution, even more strongly than it was retained either by Mr.
Galton or myself[60].
[60] For a fuller statement of Mr. Galton's theory of Heredity, and
its relation to Weismann's, see _An Examination of
Weismannism_.
We have now considered the line of evidence on which Darwin chiefly
relied in proof of the transmissibility of acquired characters; and it
must be allowed that this line of evidence is practically worthless.
What he regarded as the inherited effects of use and of disuse may be
entirely due to the cessation of selection in the case of our
domesticated animals, combined with an active _reversal_ of selection in
the case of natural species. And in accordance with this view is the
fact that the degeneration of disused parts proceeds much further in the
case of wild species than it does in that of domesticated varieties. For
although it may be said that in the case of wild species more time has
been allowed for a greater accumulation of the inherited effects of
disuse than can have been the case with domesticated varieties, the
alternative explanation is at least as probable--that in the case of
wild species the merely negative, or passive, influence of the
_cessation_ of selection has been continuously and powerfully assisted
by the positive, or active, influence of the _reversal_ of selection,
through economy of growth and the general advantage to be derived from
the abolition of useless parts[61].
[61] For a fuller explanation of the important difference between
the mere cessation and the actual reversal of selection, see
Appendix I.
The absence of any good evidence of this direct kind in favour of
use-inheritance will be rendered strikingly apparent to any one who
reads a learned and interesting work by Professor Semper[62]. His object
was to show the large part which he believed to have been played by
external conditions of life in directly modifying organic types--or, in
other words, of proving that side of Lamarckianism which refers to the
immediate action of the environment, whether with or without the
co-operation of use-inheritance and natural selection. Although Semper
gathered together a great array of facts, the more carefully one reads
his book the more apparent does it become that no single one of the
facts is in itself conclusive evidence of the transmission to progeny of
characters which are acquired through use-inheritance or through direct
action of the environment. Every one of the facts is susceptible of
explanation on the hypothesis that the principle of natural selection
has been the only principle concerned. This, however, it must be
observed, is by no means equivalent to proving that characters thus
acquired are not transmitted. As already pointed out, it is
impracticable with species in a state of nature to dissociate the
distinctively Darwinian from the possibly Lamarckian factors; so that
even if the latter are largely operative, we can only hope for direct
evidence of the fact from direct experiments on varieties in a state of
domestication. To this branch of our subject, therefore, we will now
proceed.
[62] _Animal Life_, International Scientific Series, vol. xxxi.
CHAPTER IV.
CHARACTERS AS HEREDITARY AND ACQUIRED
(_continued_).
(C.)
_Experimental Evidence in favour of the Inheritance of Acquired
Characters._
Notwithstanding the fact already noticed, that no experiments have
hitherto been published with reference to the question of the
transmission of acquired characters[63], there are several researches
which, with other objects in view, have incidentally yielded seemingly
good evidence of such transmission. The best-known of these
researches--and therefore the one with which I shall begin--is that of
Brown-Sequard touching the effects of certain injuries of the nervous
system in guinea-pigs.
[63] The experiments of Galton and Weismann upon this subject are
nugatory, as will be shown later on. But since the above was
written an important research has been published by Mr.
Cunningham, of the Marine Biological Association. For a full
account I must refer the reader to his forthcoming paper in the
_Philosophical Transactions_. The following is his own
statement of the principal results:--
"A case which I have myself recently investigated
experimentally seems to me to support very strongly the theory
of the inheritance of acquired characters, I have shown that in
normal flat-fishes, if the lower side be artificially exposed
to light for a long time, pigmentation is developed on that
side; but when the exposure is commenced while the specimens
are still in process of metamorphosis, when pigment-cells are
still present on the lower side, the action of light does not
prevent the disappearance of these pigment-cells. They
disappear as in individuals living under normal conditions, but
after prolonged exposure pigment-cells reappear. The first fact
proves that the disappearance of the pigment-cells from the
lower side in the metamorphosis is an hereditary character, and
not a change produced in each individual by the withdrawal of
the lower side from the action of light. On the other hand, the
experiments show that the absence of pigment-cells from the
lower side throughout life is due to the fact that light does
not act upon that side, for, when it is allowed to act,
pigment-cells appear. It seems to me the only reasonable
conclusion from these facts is, that the disappearance of
pigment-cells was originally due to the absence of light, and
that this change has now become hereditary. The pigment-cells
produced by the action of light on the lower side are in all
respects similar to those normally present on the upper side of
the fish. If the disappearance of the pigment-cells were due
entirely to a variation of the germ-plasm, no external
influence could cause them to reappear, and, on the other hand,
if there were no hereditary tendency, the colouration of the
lower side of the flat-fish when exposed would be rapid and
complete."--_Natural Science_, Oct. 1893.
During a period of thirty years Brown-Sequard bred many thousands of
guinea-pigs as material for his various researches; and in those whose
parents had not been operated upon in the ways to be immediately
mentioned, he never saw any of the peculiarities which are about to be
described. Therefore the hypothesis of coincidence, at all events, must
be excluded. The following is his own summary of the results with which
we are concerned:--
1st. Appearance of epilepsy in animals born of parents which had
been rendered epileptic by an injury to the spinal cord.
2nd. Appearance of epilepsy also in animals born of parents which
had been rendered epileptic by 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 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 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 were 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.
These results[64] have been independently vouched for by two of
Brown-Sequard's former assistants--Dr. Dupuy, and the late Professor
Westphal. Moreover, his results with regard to epilepsy have been
corroborated also by Obersteiner[65]. I may observe, in passing, that
this labour of testing Brown-Sequard's statements is one which, in my
opinion, ought rather to have been undertaken, if not by Weismann
himself, at all events by some of his followers. Both he and they are
incessant in their demand for evidence of the transmission of acquired
characters; yet they have virtually ignored the foregoing very
remarkable statements. However, be this as it may, all that we have now
to do is to consider what the school of Weismann has had to say with
regard to these experiments on the grounds of general reasoning which
they have thus far been satisfied to occupy.
[64] For Professor Weismann's statement of and discussion of these
results see _Essays_, vol. i. p. 313.
[65] _Oesterreichische medicinische Jahrbuecher_, 1875, 179.
In view of Obersteiner's corroboration of Brown-Sequard's results
touching the artificial production and subsequent transmission of
epilepsy, Weismann accepts the facts, but, in order to save his theory
of heredity, he argues that the transmission may be due to a traumatic
introduction of "some unknown microbe" which causes the epilepsy in the
parent, and, by invading the ova or spermatozoa as the case may be, also
produces epilepsy in the offspring. Here, of course, there would be
transmission of epilepsy, but it would not be, technically speaking, an
hereditary transmission. The case would resemble that of syphilis, where
the sexual elements remain unaffected as to their congenital endowments,
although they have been made the vehicles for conveying an organic
poison to the next generation.
Now it would seem that this suggestion is not, on the face of it, a
probable one. For "some unknown microbe" it indeed must be, which is
always on hand to enter a guinea-pig when certain operations are being
performed on certain parts of the nervous system, but yet will never
enter when operations of any kind are being effected elsewhere.
Moreover, Westphal has produced the epilepsy _without any incision_, by
striking the heads of the animals with a hammer[66]. This latter fact,
it appears to me, entirely abolishes the intrinsically improbable
suggestion touching an unknown--and strangely eclectic--microbe.
However, it is but fair to state what Weismann himself has made of this
fact. The following is what he says:--
[66] _Loc. cit._
"It is obvious that the presence of microbes can have nothing to do
with such an attack, but the shock alone must have caused
morphological and functional changes in the centre of the pons and
medulla oblongata, identical with those produced by microbes in the
other cases.... Various stimuli might cause the nervous centres
concerned to develop the convulsive attack which, together with its
after-effects, we call epilepsy. In Westphal's case, such a
stimulus would be given by a powerful mechanical shock (viz. blows
on the head with a hammer); in Brown-Sequard's experiments, by the
penetration of microbes[67]."
[67] _Essays_, vol. i. p. 315.
But from this passage it would seem that Weismann has failed to notice
that in "Westphal's case," as in "Brown-Sequard's experiments," the
epilepsy was _transmitted to progeny_. That epilepsy may be produced in
guinea-pigs by a method which does not involve any cutting (i.e.
possibility of inoculation) would no doubt tend to corroborate the
suggestion of microbes being concerned in its transmission when it is
produced by cutting, _if in the former case there were no such
transmission_. But as there _is_ transmission in _both_ cases, the
facts, so far as I can see, entirely abolish the suggestion. For they
prove that even when epilepsy is produced in the parents under
circumstances which render "it obvious that the presence of microbes can
have nothing to do with such an attack," the epileptiform condition is
notwithstanding transmitted to the progeny. What, then, is gained by
retaining the intrinsically improbable hypothesis of microbes to explain
the fact of transmission "in Brown-Sequard's experiments," when this
very same fact is proved to occur without the possibility of microbes
"in Westphal's case"?
The only other objection with regard to the seeming transmission of
traumatic epilepsy which Weismann has advanced is, that such epilepsy
may be produced by two or three very different operations--viz. division
of the sciatic nerves (one or both), an injury to the spinal cord, and a
stroke on the head. Does not this show, it is asked, that the epileptic
condition of guinea-pigs is due to a generally unstable condition of the
whole nervous system and is not associated with any particular part
thereof? Well, supposing that such is the case, what would it amount to?
I cannot see that it would in any way affect the only question in
debate--viz. What is the significance of the fact that epilepsy is
_transmitted_? Even if it be but "a tendency," "a disposition," or "a
diathesis" that is transmitted, it is none the less a case of
transmission, in fact quite as much so as if the pathological state were
dependent on the impaired condition of any particular nerve-centre. For,
it must be observed, there can be no question that it is always produced
by an operation of _some_ kind. If it were ever to originate in
guinea-pigs spontaneously, there might be some room for supposing that
its transmission is due to a congenital tendency running through the
whole species--although even then it would remain unaccountable, on the
ultra-Darwinian view, why this tendency should be congenitally
_increased_ by means of an operation. But epilepsy does not originate
spontaneously in guinea-pigs; and therefore the criticism in question
appears to me irrelevant.
Again, it may be worth while to remark that Brown-Sequard's experiments
do not disprove the possibility of its being some one nerve-centre which
is concerned in all cases of traumatic epilepsy. And this possibility
becomes, I think, a probability in view of Luciani's recent experiments
on the dog. These show that the epileptic condition can be produced in
this animal by injury to the cortical substance of the hemispheres, and
is then transmitted to progeny[68]. These experiments, therefore, are of
great interest--first, as showing that traumatic and transmissible
epilepsy is not confined to guinea-pigs; and next, as indicating that
the pathological state in question is associated with the highest
nerve-centres, which may therefore well be affected by injury to the
lower centres, or even by section of a large nerve trunk.
[68] _Les fonctions du Cerveau_, p. 102.
So much, then, with regard to the case of transmitted epilepsy. But now
it must be noted that, even if Weismann's suggestion touching microbes
were fully adequate to meet this case, it would still leave unaffected
those of transmitted protrusion of the eye, drooping of the eyelid,
gangrene of the ear, absence of toes, &c. In all these cases the facts,
as stated by Brown-Sequard, are plainly unamenable to any explanation
which would suppose them due to microbes, or even to any general
neurotic condition induced by the operation. They are much too definite,
peculiar, and localized. Doubtless it is on this account that the school
of Weismann has not seriously attempted to deal with them, but merely
recommends their repetition by other physiologists[69]. Certain
criticisms, however, have been urged by Weismann against the
_interpretation_ of Brown-Sequard's facts as evidence in favour of the
transmission of acquired characters. It does not appear to me that these
criticisms present much weight; but it is only fair that we should here
briefly consider them[70].
[69] _Essays_, vol. i. p. 82.
[70] As Weismann gives an excellent abstract of all the alleged
facts up to date (_Essays_, vol. i. pp. 319-324), it is
needless for me to supply another, further than that which I
have already made from Brown-Sequard.
First, with regard to Brown-Sequard's results other than the production
of transmitted epilepsy, Weismann allows that the hypothesis of microbes
can scarcely apply. In order to meet these results, therefore, he
furnishes another suggestion--viz. that where the nervous system has
sustained "a great shock," the animals are very likely to bear "weak
descendants, and such as are readily affected by disease." Then, in
answer to the obvious consideration, "that this does not explain why the
offspring should suffer from the same disease" as that which has been
produced in the parents, he adds--"But this does not appear to have been
by any means invariably the case. For '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.'"
Now, this does not appear to me a good commentary. In the first place,
it does not apply to the other cases (such as the ears and the toes),
where the changes in the offspring, when they occurred at all, _were_
exactly similar to those observed in the parents, save that some of them
occasionally occurred on the _opposite_ side, and frequently also on
_both_ sides of the offspring. These subordinate facts, however, will
not be regarded by any physiologist as making against the more ready
interpretation of the results as due to heredity. For a physiologist
well knows that homologous parts are apt to exhibit correlated
variability--and this especially where variations of a congenital kind
are concerned, and also where there is any reason to suppose that the
nervous system is involved. Moreover, even in the case of the eye, it
was always protrusion that was caused in the parent and transmitted to
the offspring as a result of injuring the restiform bodies of the
former; while it was always partial closure of the eyelids that was
caused and transmitted by section of the sympathetic nerve, or removal
of the cervical ganglia. Therefore, if we call such effects "diseases,"
surely it _was_ "the same disease" which in each case appeared in the
parents and reappeared in their offspring. Again, the "diseases" were so
peculiar, definite, and localized, that I cannot see how they can be
reasonably ascribed to a general nervous "shock." Why, for instance, if
this were the case, should a protruding eye never result from removal
of the cervical ganglia, a drooping eyelid from a puncture of the
restiform body, a toeless foot from either or both of these operations,
and so on? In view of such considerations I cannot deem these
suggestions touching "microbes" and "diseases" as worthy of the
distinguished biologist from whom they emanate.
Secondly, Weismann asks--How can we suppose these results to be
instances of the transmission of acquired characters, when from
Brown-Sequard's own statement of them it appears that the mutilation
itself was not inherited, but only its effects? Neither in the case of
the sciatic nerve, the sympathetic nerve, the cervical ganglion, nor the
restiform bodies, was there ever any trace of transmitted injury in the
corresponding parts of the offspring; so that, if the "diseases" from
which they suffered be regarded as hereditary, we have to suppose that a
consequence was in each case transmitted without the transmission of its
cause, which is absurd. But I do not think that this criticism can be
deemed of much weight by a physiologist as distinguished from a
naturalist. For nothing is more certain to a student of physiology, in
any of its branches, than that negative evidence, if yielded by the
microscope alone, is most precarious. Therefore it does not need a
_visible_ change in the nervous system to be present, in order that the
part affected should be functionally weak or incapable: pathology can
show numberless cases of nerve-disorder the "structural" causes of which
neither the scalpel nor the microscope can detect. So that, if any
peculiar form of nerve-disorder is transmitted to progeny, and if it be
certain that it has been caused by injury to some particular part of
the nervous system, I cannot see that there is any reason to doubt the
transmission of a nervous lesion merely on the ground that it is not
visibly discernible. Of course there may be other grounds for doubting
it; but I am satisfied that this ground is untenable. Besides, it must
be remembered, as regards the particular cases in question, that no one
has thus far investigated the histology of the matter by the greatly
improved methods which are now at our disposal.
* * * * *
I have now considered all the criticisms which have been advanced
against what may be called the Lamarckian interpretation of
Brown-Sequard's results; and I think it will be seen that they present
very little force--even if it can be seen that they present any force at
all. But it must be remembered that this is a different thing from
saying that the Lamarckian interpretation is the true one. The facts
alleged are, without question, highly peculiar; and, on this account
alone, Brown-Sequard's interpretation of them ought to be deemed
provisional. Hence, although as yet they have not encountered any valid
criticism from the side of ultra-Darwinian theory, I do not agree with
Darwin that, on the supposition of their truth as facts, they furnish
positive proof of the transmission of acquired characters. Rather do I
agree with Weismann that further investigation is needed in order to
establish such an important conclusion on the basis of so unusual a
class of facts. This further investigation, therefore, I have
undertaken, and will now state the results.
Although this work was begun over twenty years ago, and then yielded
negative results, it was only within the last decade that I resumed it
more systematically, and under the tutelage of Brown-Sequard himself.
During the last two years, however, the experiments have been so much
interrupted by illness that even now the research is far from complete.
Therefore I will here confine myself to a tabular statement of the
results as far as they have hitherto gone, on the understanding that, in
so far as they are negative or doubtful, I am not yet prepared to
announce them as final.
We may take Brown-Sequard's propositions in his own order, as already
given on page 104.
1st. Appearance of epilepsy in animals born of parents which had
been rendered epileptic by an injury to the spinal cord.
2nd. Appearance of epilepsy also in animals born of parents which
had been rendered epileptic by section of the sciatic nerve.
I did not repeat these experiments with a view to producing epilepsy,
because, as above stated, they had been already and sufficiently
corroborated in this respect. But I repeated many times the experiments
of dividing the sciatic nerve for the purpose of testing the statements
made later on in paragraphs 7 and 8, and observed that it almost always
had the effect of producing epilepsy in the animal thus operated
upon--and this of a peculiar kind, the chief characteristics of which
may here be summarized. The epileptiform habit does not supervene until
some considerable time after the operation; it is then transitory,
lasting only for some weeks or months. While the habit endures the fits
never occur spontaneously, but only as a result of irritating a small
area of skin behind the ear on the same side of the body as that on
which the sciatic nerve had been divided. Effectual irritation may be
either mechanical (such as gentle pinching), electrical, or, though less
certainly, thermal. The area of skin in question, soon after the
epileptiform habit supervenes, and during all the time that it lasts,
swarms with lice of the kind which infest guinea-pigs--i.e. the lice
congregate in this area, on account, I think, of the animal being there
insensitive, and therefore not disturbing its parasites in that
particular spot; otherwise it would presumably throw itself into fits by
scratching that spot. On removing the skin from the area in question, no
kind or degree of irritation supplied to the subjacent tissue has any
effect in producing a fit. A fit never lasts for more than a very few
minutes, during which the animal is unconscious and convulsed, though
not with any great violence. The epileptiform habit is but rarely
transmitted to progeny. Most of these observations are in accordance
with those previously made by Brown-Sequard, and also by others who have
repeated his experiments under this heading. I can have no doubt that
the injury of the sciatic nerve or spinal cord produces a change in some
of the cerebral centres, and that it is this change--whatever it is and
in whatever part of the brain it takes place--which causes the
remarkable phenomena in question.
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 section
of the cervical sympathetic nerve, or the removal of the superior
cervical ganglion.
I have not succeeded in corroborating these results. It must be added,
however, that up to the time of going to press my experiments on this,
the easiest branch of the research, have been too few fairly to prove a
negative.
5th. Exophthalmia in animals born of parents in which an injury to
the restiform body had produced that protrusion of the eyeball....
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.
I have fully corroborated the statement that injury to a particular spot
of the restiform body is quickly followed by a marked protrusion of the
eyeball on the same side. I have also had many cases in which some of
the progeny of parents thus affected have shown considerable protrusion
of the eyeballs on both sides, and this seemingly abnormal protrusion
has been occasionally transmitted to the next generation. Nevertheless,
I am far from satisfied that this latter fact is anything more than an
accidental coincidence. For I have never seen the so-called exophthalmia
of progeny exhibited in so high a degree as it occurs in the parents as
an immediate result of the operation, while, on examining any large
stock of normal guinea-pigs, there is found a considerable amount of
individual variation in regard to prominence of eyeballs. Therefore,
while not denying that the obviously abnormal amount of protrusion due
to the operation may be inherited in lesser degrees, and thus may be the
cause of the unusual degree of prominence which is sometimes seen in the
eyeballs of progeny born of exophthalmic parents, I am unable to affirm
so important a conclusion on the basis supplied by these experiments.
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.
As regards the animals operated upon (i. e. the parents), I find that
the haematoma and dry gangrene may supervene either several weeks after
the operation, or at any subsequent time up to many months. When it does
supervene it usually affects the upper parts of both ears, and may then
eat its way down until, in extreme cases, it has entirely consumed
two-thirds of the tissue of both ears. As regards the progeny of animals
thus affected, in some cases, but by no means in all, a similarly morbid
state of the ears may arise apparently at any time in the life-history
of the individual. But I have observed that in cases where two or more
individuals _of the same litter_ develop this diseased condition, they
usually do so at about the same time--even though this be many months
after birth, and therefore after the animals are fully grown. But in
progeny the morbid process never goes so far as in the parents which
have been operated upon, and it almost always affects the _middle_
thirds of the ears. In order to illustrate these points, reproductions
of two of my photographs are appended. They represent the consequences
of the operation on a male and a female guinea-pig. Among the progeny of
both these animals there were several in which a portion of each ear was
consumed by apparently the same process, where, of course, there had
been no operation.
[Illustration: FIG. 1.--Reproduction of photographs from life of a male
and female guinea-pig, whose left restiform bodies had been injured by a
scalpel six months previously. The loss of tissue in both ears was due
to haematoma and dry gangrene, which, however, had ceased when the
photograph was taken.]
It should be observed that not only is a different _part_ of the ear
affected in the progeny, but also a very much less _quantity_ thereof.
Naturally, therefore, the hypothesis of heredity seems less probable
than that of mere coincidence on the one hand, or of transmitted
microbes on the other. But I hope to have fairly excluded both these
alternative explanations. For, as regards merely accidental coincidence,
I have never seen this very peculiar morbid process in the ears, or in
any other parts, of guinea-pigs which have neither themselves had their
restiform bodies injured, nor been born of parents thus mutilated. As
regards the hypothesis of microbes, I have tried to inoculate the
corresponding parts of the ears of normal guinea-pigs, by first
scarifying those parts and then rubbing them with the diseased surfaces
of the ears of mutilated guinea-pigs; but have not been able in this way
to communicate the disease.
It will be seen that the above results in large measure corroborate the
statements of Brown-Sequard; and it is only fair to add that he told me
they are the results which he had himself obtained most frequently, but
that he had also met with many cases where the diseased condition of the
ears in parents affected the same parts in their progeny, and also
occurred in more equal degrees. Lastly, I should like to remark, with
regard to these experiments on restiform bodies, and for the benefit of
any one else who may hereafter repeat them, that it will be necessary
for him to obtain precise information touching the _modus operandi_. For
it is only one very localized spot in each restiform body which has to
be injured in order to produce any of the results in question. I myself
lost two years of work on account of not knowing this exact spot before
going to Paris for the purpose of seeing Brown-Sequard himself perform
the operation. I had in the preceding year seen one of his assistants do
so, but this gentleman had a much more careless method, and one which in
my hands yielded uniformly negative results. The exact spot in question
in the restiform body is as far forwards as it is possible to reach, and
as far down in depth as is compatible with not producing rotatory
movements.
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 were absent.
As I found that the results here described were usually given by
division of the sciatic nerve alone--or, more correctly, by excision of
a considerable portion of the nerve, in order to prevent regeneration--I
did not also divide the crural. But, although I have bred numerous
litters from parents thus injured, there has been no case of any
inherited deficiency of toes. My experiments in this connexion were
carried on through a series of six successive generations, so as to
produce, if possible, a cumulative effect. Nevertheless, no effect of
any kind was produced. On the other hand, Brown-Sequard informed me that
he had observed this inherited absence of toes only in about one or two
per cent. of cases. Hence it is possible enough, that my experiments
have not been sufficiently numerous to furnish a case. It may be added
that there is here no measurable possibility of accidental coincidence
(seeing that normal guinea-pigs do not seem ever to produce young with
any deficiency of toes), while the only possibility of mal-observation
consists in some error with regard to the isolation (or the tabulation)
of parents and progeny. Such an error, however, may easily arise. For
gangrene of the toes does not set in till some considerable time after
division of the sciatic nerve. Hence, if the wound be healed before the
gangrene begins, and if any mistake has been made with regard to the
isolation (or tabulation) of the animal, it becomes possible that the
latter should be recorded as an uninjured, instead of an injured,
individual. On this account one would like to be assured that
Brown-Sequard took the precaution of examining the state of the sciatic
nerve in those comparatively few specimens which he alleges to have
displayed such exceedingly definite proof of the inheritance of a
mutilation. For it is needless to remark, after what has been said in
the preceding chapter on the analogous case of epilepsy, that the proof
would not be regarded by any physiologist as displaced by the fact that
there is no observable deficiency in the sciatic nerve of the toeless
young.
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.
I have not paid any attention to this paragraph, because the facts which
it alleges did not seem of a sufficiently definite character to serve as
a guide to further experiment.
On the whole, then, as regards Brown-Sequard's experiments, it will be
seen that I have not been able to furnish any approach to a full
corroboration. But I must repeat that my own experiments have not as yet
been sufficiently numerous to justify me in repudiating those of his
statements which I have not been able to verify.
The only other experimental results, where animals are concerned, which
seemed to tell on the side of Lamarckianism, are those of Mr.
Cunningham, already alluded to. But, as the research is still in
progress, the school of Weismann may fairly say that it would be
premature to discuss its theoretical bearings.
* * * * *
Passing now from experiments on animals to experiments on plants, I must
again ask it to be borne in mind, that here also no researches have been
published, which have had for their object the testing of the question
on which we are engaged. As in the case of animals, therefore, so in
that of plants, we are dependent for any experimental results bearing
upon the subject to such as have been gained incidentally during the
course of investigations in quite other directions.
Allusion has already been made, in my previous essay, to De Vries'
observations on the chromatophores of algae passing from the ovum of the
mother to the daughter organism; and we have seen that even Weismann
admits, "It appears possible that a transmission of somatogenetic
variation has here occurred[71]." It will now be my object to show that
such variations appear to be sometimes transmitted in the case of
higher plants, and this under circumstances which carry much less
equivocal evidence of the inheritance of acquired characters, than can
be rendered by the much more simple organization of an alga.
[71] _Examination of Weismannism_, p. 83.
I have previously mentioned Hoffmann's experiments on transplantation,
the result of which was to show that variations, directly induced by
changed conditions of life, were reproduced by seed[72]. Weismann,
however, as we have seen, questions the _somatogenetic_ origin of these
variations--attributing the facts to a _blastogenetic_ change produced
in the plants by a direct action of the changed conditions upon the
germ-plasm itself[73]. And he points out that whether he is right or
wrong in this interpretation can only be settled by ascertaining whether
the observable somatic changes occur in the generation which is first
exposed to the changed conditions of life. If they do occur in the first
generation, they are somatogenetic changes, which afterwards react on
the substance of heredity, so as to transmit the acquired peculiarities
to progeny. But if they do not occur till the second (or any later)
generation, they are presumably blastogenetic. Unfortunately Hoffmann
does not appear to have attended to this point with sufficient care, but
there are other experiments of the same kind where the point has been
specially observed.
[72] _Examination of Wiesmannism_, p. 93.
[73] _Ibid._ p. 153.
For instance, M. L. A. Carriere[74] gathered seed from the wild radish
(_Raphanus Raphanistrum_) in France, and sowed one lot in the light dry
soil near the Museum of Natural History in Paris, while another lot was
sown by him at the same time in heavy soil elsewhere. His object was to
ascertain whether he could produce a good cultivated radish by
methodical selection; and this he did; in a wonderfully rapid manner,
during the course of a very few generations. But the point for us is,
that _from the first_ the plants grown in the light soil of Paris
presented sundry marked differences from those grown in the heavy soil
of the country; and that these points of difference had nothing to do
with the variations on which his artificial selection was brought to
bear. For while his artificial selection was directed to increasing the
_size_ of the "root," the differences in question had reference to its
_form_ and _colour_. In Paris an elongated form prevailed, which
presented either a white or a rose colour: in the country the form was
more rounded, and the colour violet, dark brown, or "almost black." Now,
as these differences were strongly apparent in the first generation, and
were not afterwards made the subject of selection, both in origin and
development they must have been due to "climatic" influences acting on
the somatic tissues. And although the author does not appear to have
tested their hereditary characters by afterwards sowing the seed from
the Paris variety in the country, or _vice versa_, we may fairly
conclude that these changes must have been hereditary--1st, from the
fact of their intensification in the course of the five sequent
generations over which the experiment extended, and, 2nd, from the very
analogous results which were similarly obtained in the following case
with another genus, where both the somatogenetic and the hereditary
characters of the change were carefully and specially observed. This
case is as follows.
[74] _Origine des Plantes Domestiques, demontree par la culture du
Radis Sauvage_ (Paris, 1869).
The late Professor James Buckman, F.R.S., saved some seed from wild
parsnips (_P. sativa_) in the summer of 1847, and sowed under changed
conditions of life in the spring of 1848. The plants grown from these
wild seeds were for the most part like wild plants; but some of them had
"already (i.e. in the autumn of 1848) the light green and smooth aspect
devoid of hairs which is peculiar to the cultivated plant; and among the
latter there were a few with longer leaves and broader divisions of
leaf-lobes than the rest--the leaves, too, all growing systematically
round one central bud. The roots of the plant when taken up were
observed to be for the most part more fleshy than those of wild
examples[75]."
[75] _Journl. Agric. Soc._ 1848.
Professor Buckman then proceeds to describe how he selected the best
samples for cultivation in succeeding generations, till eventually the
variety which he called "The Student" was produced, and which Messrs.
Sutton still regard as the best variety in their catalogue. That is to
say, it has come true to seed for the last forty years; and although
such great excellence and stability are doubtless in chief part due to
the subsequent process of selection by Professor Buckman in the years
1848-1850, this does not affect the point with which we are here
concerned--namely, that the somatogenetic changes of the plants in the
first generation were transmitted by seed to the second generation, and
thus furnished Professor Buckman with the material for his subsequent
process of selection. And the changes in question were not merely of a
very definite character, but also of what may be termed a very _local_
character--affecting only particular tissues of the soma, and therefore
expressive of a high degree of _representation_ on the part of the
subsequently developed seed, by which they were faithfully reproduced in
the next generation.
Here is another case. M. Lesage examined the tissues of a large number
of plants growing both near to, and remote from, the sea. He suspected
that the characteristic fleshiness, &c. of seaside plants was due to the
influence of sea-salt; and proved that such was the case by causing the
characters to occur in inland plants as a result of watering them with
salt-water. Then he adds:--
"J'ai reussi surtout pour le _Lepidium sativum_ cultive en 1888;
j'ai obtenu pour la meme plante des resultats plus nets encore dans
la culture de 1889, entreprise en semant les graines recoltees avec
soin des pots de l'annee precedente et traitees exactement de la
meme facon[76]."
[76] _Rev. Gen. de Bot._ tom. ii. p. 64.
Here, it will be observed, there was no selection; and therefore the
increased hereditary effect in the second generation must apparently be
ascribed to a continuance of influence exercised by somatic tissues on
germinal elements; for at the time when the changes were produced no
seed had been formed. In other words, the accumulated change, like the
initial change, would seem to have been exclusively of somatogenetic
origin; and yet it so influenced the qualities of the seed (as this was
afterwards formed), that the augmented changes were transmitted to the
next generation, part for part, as the lesser changes had occurred in
the preceding generation. "This experiment, therefore, like Professor
Buckman's, shows that the alteration of the tissues was carried on in
the second generation from the point gained in the first. In both cases
no germ-plasm (in the germ-cells) existed at the time during which the
alterations arose, as they were confined to the vegetative system; and
in the case of the parsnips and carrots, being biennials no germ-cells
are produced till the second year has arrived[77]."
[77] I am indebted to the Rev. G. Henslow for the references to
these cases. This and the passages which follow are quoted from
his letters to me.
Once more, Professor Bailey remarks:--
"Squashes often show remarkable differences when grown upon
different soils; and these differences can sometimes be perpetuated
for a time by seeds. The writer has produced, from the same parent,
squashes so dissimilar, through the simple agency of a change of
soil in one season, that they might readily be taken for distinct
varieties. Peas are known to vary in the same manner. The seeds of
a row of peas of the same kind, last year gave the writer marked
variations due to differences of soil.... Pea-growers characterize
soils as 'good' and 'viney.' Upon the latter sort the plants run to
vine at the expense of the fruit, and their offspring for two or
three generations have the same tendency[78]."
[78] _Gardener's Chronicle_, May 31, 1890, p. 677.
I think these several cases are enough to show that, while the
Weismannian assumption as to the seeming transmission of somatogenetic
characters being restricted to the lowest kinds of plants is purely
gratuitous, there is no small amount of evidence to the contrary--or
evidence which seems to prove that a similar transmission occurs
likewise in the higher plants. And no doubt many additional cases might
be advanced by any one who is well read in the literature of economic
botany.
It appears to me that the only answer to such cases would be furnished
by supposing that the hereditary changes are due to an alteration of the
residual "germ-plasm" in the wild seed, when this is first exposed to
the changed conditions of life, due to its growth in a strange kind of
soil--e.g. while germinating in an unusual kind of earth for producing
the first generation. But this would be going a long way to save an
hypothesis. In case, however, it should now be suggested, I may remark
that it would be negatived by the following facts.[79]
[79] Since the above was written Professor Weismann has advanced, in
_The Germ-plasm_, a suggestion very similar to this. It is
sufficient here to remark, that nearly all the facts and
considerations which ensue in the present chapter are
applicable to his suggestion, the essence of which is
anticipated in the above paragraph.
In the first place, an endless number of cases might be quoted where
somatogenetic changes thus produced by changed conditions of life are
not hereditary. Therefore, in all these cases it is certainly not the
"germ-plasm" that is affected. In other words, there can be no question
that somatogenetic changes of the kinds above mentioned do very readily
admit of being produced in the first generation by changes of soil,
altitude, &c. And that somatogenetic changes thus produced should not
always--or even generally--prove themselves to be hereditary from the
first moment of their occurrence, is no more than any theory of
heredity would expect. Indeed, looking to the known potency of
reversion, the wonder is that in any case such changes should become
hereditary in a single generation. On the other hand, there is no reason
to imagine that the hypothetical germ-plasm--howsoever _unstable_ we may
suppose it to be--can admit of being directly affected by a change of
soil in a single generation. For, on this view, it must presumably be
chiefly affected during the short time that the seed is germinating; and
during that time the changed conditions can scarcely be conceived as
having any points of attack, so to speak, upon the residual germ-plasm.
There are no roots on which the change of _soil_ can make itself
perceptible, nor any stem and leaves on which the change of _atmosphere_
can operate. Yet the changed condition's may produce hereditary
modifications in any parts of the plant, which are not only precisely
analogous to non-hereditary changes similarly produced in the somatic
tissues of innumerable other plants, but are always of precisely the
same kind in the same lot of plants that are affected. When all the
radishes grown from wild seed in Paris, for instance, varied in the
direction of rotundity and dark colour, while those grown in the country
presented the opposite characters, we can well understand the facts as
due to an entire season's action upon the whole of the growing plant,
with the result that all the changes produced in each set of plants were
similar--just as in the cases where similarly "climatic" modifications
are not hereditary, and therefore unquestionably due to changed
conditions acting on roots, stems, leaves, or flowers, as the case may
be. On the other hand, it is not thus intelligible that during the
short time of germination the changed conditions should effect a
re-shuffling (or any other modification) of the "germ-plasm" in the
seeds--and this in such a manner that the effect on the residual
germ-plasm reserved for future generations is precisely similar to that
produced on the somatic tissues of the developing embryo.
In the second place, as we have seen, in some of the foregoing cases the
changes were produced months--and even years--before the seeds of the
first germination were formed. Therefore the hereditary effect, if
subsequent to the period of embryonic germination, must have been
produced on germ-plasm as this occurs diffused through the somatic
tissues. But, if so, we shall have to suppose that such germ-plasm is
afterwards gathered in the seeds when these are subsequently formed.
This supposition, however, would be radically opposed to Weismann's
theory of heredity: nor do I know of any other theory with which it
would be reconcilable, save such as entertain the possibility of the
Lamarckian factors.
Lastly, in the third place, I deem the following considerations of the
highest importance:--
"As other instances in which peculiar structures are now hereditary
may be mentioned aquatic plants and those producing subterraneous
stems. Whether they be dicotyledons or monocotyledons, there is a
fundamental agreement in the anatomy of the roots and stem of
aquatic plants, and, in many cases, of the leaves as well. Such has
hitherto been attributed to the aquatic habit. The inference or
deduction was, of course, based upon innumerable coincidences; the
water being supposed to be the direct cause of the degenerate
structures, which are hereditary and characteristic of such plants
in the wild state. M. Costantin has, however, verified this
deduction, by making terrestrial and aerial stems to grow
underground and in water: the structures _at once_ began to assume
the subterranean or aquatic type, as the case might be; and,
conversely, aquatic plants made to grow upon land _at once_ began
to assume the terrestrial type of structure, while analogous
results followed changes from a subterranean to an aerial position,
and _vice versa_."
This is also quoted from the Rev. Prof. Henslow's letters to me, and the
important point in it is, that the great changes in question are proved
to be of a purely "somatogenetic" kind; for they occurred "at once" _in
the ready-grown plant_, when the organs concerned were exposed to the
change from aquatic to terrestrial life, or _vice versa_--and also from
a subterranean to an aerial position, or _vice versa_. Consequently,
even the abstract possibility of the changed conditions of life having
operated on the _seed_ is here excluded. Yet the changes are of
precisely the same kind as are now _hereditary_ in the wild species. It
thus appears undeniable that all these remarkable and uniform changes
must originally have been somatogenetic changes; yet they have now
become blastogenetic. This much, I say, seems undeniable; and therefore
it goes a long way to prove that the non-blastogenetic character of the
changes has been due to their originally somatogenetic character. For,
if not, how did natural selection ever get an opportunity of making any
of them blastogenetic, when every individual plant has always presented
them as already given somatogenetically? This last consideration appears
in no small measure to justify the opinion of Mr. Henslow, who
concludes--"These experiments prove, not only that the influence of the
environment is _at once_ felt by the organ; but that it is indubitably
the _cause_ of the now specific and hereditary traits peculiar to
normally aquatic, subterranean, and aerial stems, or roots[80]."
[80] It also serves to show that Weismann's newer doctrine of
similar "determinants" occurring both in the germ and in the
somatic tissues is a doctrine which cannot be applied to rebut
this evidence of the transmission of acquired characters in
plants. Therefore even its hypothetical validity as applied by
him to explain the seasonal variation of butterflies is
rendered in a high degree dubious.
He continues to furnish other instances in the same line of proof--such
as the distinctive "habits" of insectivorous, parasitic, and climbing
plants; the difference in structure between the upper and under sides of
horizontal leaves, &c. "For here, as in all organs, we discover by
experiment how easily the anatomy of plants can be affected by their
environment; and that, as long as the latter is constant, so are the
characters of the plants constant and hereditary."
[The following letter, contributed by Dr. Hill to _Nature_, vol. I.
p. 617, may here be quoted. C. Ll. M.
"It may be of interest to your readers to know that two guinea-pigs
were born at Oxford a day or two before the death Dr. Romanes, both
of which exhibited a well-marked droop of the left upper eyelid.
These guinea-pigs were the offspring of a male and a female
guinea-pig in both of which I had produced for Dr. Romanes, some
months earlier, a droop of the left upper eyelid by division of the
left cervical sympathetic nerve. This result is a corroboration of
the series of Brown-Sequard's experiments on the inheritance of
acquired characteristics. A very large series of such experiments
are of course needed to eliminate all sources of error, but this I
unfortunately cannot carry out at present, owing to the need of a
special farm in the country, for the proper care and breeding of
the animals.--LEONARD HILL.
"Physiological Laboratory, Univ. Coll. London, Oct. 18, 1894."]
CHAPTER V.
CHARACTERS AS HEREDITARY AND ACQUIRED
(_continued_).
(A. and B.)
_Direct and Indirect Evidence in favour of the Non-inheritance of
Acquired Characters_[81].
[81] [_See_ note appended to Preface. C. LI. M.]
The strongest argument in favour of "continuity" is that based upon the
immense difference between congenital and acquired characters in respect
of heritability. For that there is a great difference in this respect is
a matter of undeniable fact. And it is obvious that this difference, the
importance of which must be allowed its full weight, is just what we
should expect on the theory of the continuity of the germ-plasm, as
opposed to that of pangenesis. Indeed it may be said that the difference
in question, while it constitutes important _evidence_ in favour of the
former theory, is a _difficulty_ in the way of the latter. But here two
or three considerations must be borne in mind.
In the first place, this fact has long been one which has met with wide
recognition and now constitutes the main ground on which the theory of
continuity stands. That is to say, it was the previous knowledge of
this contrast between congenital and acquired characters which led to
the formulation of a theory of continuity by Mr. Galton, and to its
subsequent development by Prof Weismann.
But, in the second place, there is a wide difference between the
certainty of this fact and that of the theory based upon it. The certain
fact is, that a great distinction in respect of heritability is
observable between congenital and acquired characters. The theory, as
formulated by Weismann, is that the distinction is not only great but
absolute, or, in other words, that in no case and in no degree can any
acquired character be ever inherited. This hypothesis, it will be
observed, goes far beyond the observed fact, for it is obviously
possible that, notwithstanding this great difference in regard to
heritability between congenital and acquired characters, the latter may
nevertheless, sometimes and in some degree, be inherited, however much
difficulty we may experience in observing these lesser phenomena in
presence of the greater. The Weismannian hypothesis of _absolute_
continuity is one thing, while the observed fact of at least a _high
relative degree_ of continuity is quite another thing. And it is
necessary to be emphatic on this point, since some of the reviewers of
my _Examination of Weismannism_ confound these two things. Being
apparently under the impression that it was reserved for Weismann to
perceive the fact of there being a great difference between the
heritability of congenital and acquired characters, they deem it
inconsistent in me to acknowledge this fact while at the same time
questioning the hypothetical basis of his fundamental postulate touching
the absolute continuity of germ-plasm. It is one merit of Galton's
theory, as against Weismann's, that it does not dogmatically exclude the
possible interruption of continuity on some occasions and in some
degree. Herein, indeed, would seem to lie the central core of the whole
question in dispute. For it is certain and has long been known that
individually acquired characters are at all events much less heritable
than are long-inherited or congenital ones. But Lamarckian theory
supposes that congenital characters were in some cases originally
acquired, and that what are now blastogenetic characters were in some
cases at first somatogenetic and have become blastogenetic only in
virtue of sufficiently long inheritance. Since Darwin's time, however,
evolutionists (even of the so-called Lamarckian type) have supposed that
natural selection greatly assists this process of determining which
somatogenetic characters shall become congenital or blastogenetic. Hence
all schools of evolutionists are, and have long been, agreed in
regarding the continuity principle as true in the main. No evolutionist
would at any time have propounded the view that one generation depends
for _all_ its characters on those acquired by its _immediate_ ancestors,
for this would merely be to unsay the theory of Evolution itself, as
well as to deny the patent facts of heredity as shown, for example, in
atavism. At most only some fraction of a _per cent._ could be supposed
to do so. But Weismann's contention is that this principle is not only
true in the main, but _absolutely_ true; so that natural selection
becomes all in all or not at all. Unless Weismannism be regarded as
this doctrine of absolutism it permits no basis for his attempted theory
of evolution.
And, whatever may be said to the contrary by the more enthusiastic
followers of Prof. Weismann, I must insist that there is the widest
possible difference between the truly scientific question of fact which
is assumed by Weismann as answered (the base-line of the diagram on p.
43), and the elaborate structure of deductive reasoning which he has
reared on this assumption (the Y-like structure). Even if the assumption
should ever admit of inductive proof, the almost bewildering edifice of
deductive reasoning which he has built upon it would still appear to me
to present extremely little value of a scientific kind. Interesting
though it may be as a monument of ingenious speculation hitherto unique
in the history of science, the mere flimsiness of its material must
always prevent its far-reaching conclusions from being worthy of serious
attention from a biological point of view. But having already attempted
to show fully in my _Examination_ this great distinction between the
scientific importance of the question which lies at the base of
"Weismannism," and that of the system which he has constructed on his
assumed answer thereto, I need not now say anything further with regard
to it.
Again, on the present occasion and in this connexion I should like to
dissipate a misunderstanding into which some of the reviewers of the
work just mentioned have fallen. They appear to have concluded that
because I have criticized unfavourably a considerable number of
Weismann's theories, I have shown myself hostile to his entire system.
Such, however, is by no means the case; and the misunderstanding can
only be accounted for by supposing that the strongly partisan spirit
which these critics display on the side of neo-Darwinism has rendered
them incapable of appreciating any attempt at impartial--or even so much
as independent--criticism. At all events, it is a matter of fact that
throughout the work in question I have been particularly careful to
avoid this misunderstanding as to my own position. Over and over again
it is there stated that, far from having any objection to the principle
of "Continuity" as represented in the base-line of the above diagram, I
have been convinced of its truth ever since reading Mr. Galton's _Theory
of Heredity_ in 1875. All the "hard words" which I have written against
Weismann's system of theories have reference to those parts of it which
go to constitute the Y-like structure of the diagram.
It is, however, desirable to recur to another point, and one which I
hope will be borne in mind throughout the following discussion. It has
already been stated, a few pages back, that the doctrine of continuity
admits of being held in two very different significations. It may be
held as absolute, or as relative. In the former case we have the
Weismannian doctrine of germ-plasm: the substance of heredity is taken
to be a substance _per se_, which has always occupied a separate
"sphere" of its own, without any contact with that of somatoplasm
further than is required for its lodgement and nutrition; hence it can
never have been in any degree modified as to its hereditary qualities by
use-inheritance or any other kind of somatogenetic change; it has been
_absolutely_ continuous "since the first origin of life." On the other
hand, the doctrine of continuity may be held in the widely different
sense in which it has been presented by Galton's theory of Stirp. Here
the doctrine is, that while for the most part the phenomena of heredity
are due to the continuity of the substance of heredity through
numberless generations, this substance ("Stirp") is nevertheless not
absolutely continuous, but may admit, in small though cumulative
degrees, of modification by use-inheritance and other factors of the
Lamarckian kind. Now this all-important distinction between these two
theories of continuity has been fully explained and thoroughly discussed
in my _Examination_; therefore I will not here repeat myself further
than to make the following remarks.
The Weismannian doctrine of continuity as absolute (base-line of the
diagram) is necessary for the vast edifice of theories which he has
raised upon it (the Y), first as to the minute nature and exact
composition of the substance of heredity itself ("Germ-plasm"), next as
to the precise mechanism of its action in producing the visible
phenomena of heredity, variation, and all allied phenomena, and, lastly,
the elaborate and ever-changing theory of organic evolution which is
either founded on or interwoven with this vast system of hypothetic
speculation. Galton's doctrine of continuity, on the other hand, is a
"Theory of Heredity," and a theory of heredity alone. It does not meddle
with any other matters whatsoever, and rigidly avoids all speculation
further than is necessary for the bare statement and inductive support
of the doctrine in question. Hence, it would appear that this, the only
important respect wherein the doctrine of continuity as held by Galton
differs from the doctrine as held by Weismann, arisen from the necessity
under which the latter finds himself of postulating _absolute_
continuity as a logical basis for his deductive theory of the precise
mechanism of heredity on the one hand, and of his similarly deductive
theory of evolution on the other. So far as the doctrine of continuity
is itself concerned (i.e. the question of the inheritance of acquired
characters), there is certainly no more inductive reason for supposing
the continuity absolute "since the first origin of life," than there is
for supposing it to be more or less susceptible of interruption by the
Lamarckian factors. In other words, but for the sake of constructing a
speculative foundation for the support of his further theories as to
"the architecture of germ-plasm" and the factors of organic evolution,
there is no reason why Weismann should maintain the absolute separation
of the "sphere" of germ-plasm from that of somatoplasm. On the contrary,
he has no reason for concluding against even a considerable and a
frequent amount of cutting, or overlapping, on the part of these two
spheres.
But although this seems to me sufficiently obvious, as I have shown at
greater length in the _Examination of Weismannism_, it must not be
understood that I hold that there is room for any large amount of such
overlapping. On the contrary, it appears to me as certain as anything
can well be that the amount of such overlapping from one generation to
another, if it ever occur at all, must be exceedingly small, so that,
if we have regard to only a few sequent generations, the effects of
use-inheritance, and Lamarckian factors are, at all events as a rule,
demonstrably imperceptible. But this fact does not constitute any
evidence--as Weismann and his followers seem to suppose--against a
possibly important influence being exercised by the Lamarckian factors,
in the way of gradual increments through a long series of generations.
It has long been well known that acquired characters are at best far
less fully and far less certainly inherited than are congenital ones.
And this fact is of itself sufficient to prove the doctrine of
continuity to the extent that even the Lamarckian is rationally bound to
concede. But the fact yields no proof--scarcely indeed so much as a
presumption--in favour of the doctrine of continuity as absolute. For it
is sufficiently obvious that the adaptive work of heredity could not be
carried on at all if there had to be a discontinuity in the substance of
heredity at every generation, or even after any very large number of
generations.
Little more need be said concerning the arguments which fall under the
headings A and B. The Indirect evidence is considered in Appendix I of
the _Examination of Weismannism_; while the Direct evidence is
considered in the text of that work in treating of Professor Weismann's
researches on the _Hydromedusae_ (pp. 71-76).
The facts of karyokinesis are generally claimed by the school of
Weismann as making exclusively in favour of continuity as absolute. But
this is a partisan view to take. In any impartial survey it should be
seen that while the facts are fairly interpretable on Weismann's theory,
they are by no means proof thereof. For any other theory of Heredity
must suppose the material of heredity to be of a kind more or less
specialized, and the mechanism of heredity extremely precise and well
ordered. And this is all that the facts of karyokinesis prove. Granting
that they prove continuity, they cannot be held to prove that continuity
to be absolute. In other words, the facts are by no means incompatible
with even a large amount of commerce between germ-plasm and
somato-plasm, or a frequent transmission of acquired characters.
Again, Weismann's theory, that the somatic and the germ-plasm
determinants may be similarly and simultaneously modified by external
conditions may be extended much further than he has used it himself, so
as to exclude, or at any rate invalidate, _all_ evidence in favour of
Lamarckianism, other than the inheritance of the effects of use and
disuse. All evidence from apparently inherited effects produced by
change of external conditions is thus virtually put out of court,
leaving only evidence from the apparently inherited effects of
functionally produced modifications. And this line of evidence is
invalidated by Panmixia. Hence there remain only the arguments from
selective value and co-adaptation. Weismann meets these by adducing the
case of neuter insects, which have been already considered at sufficient
length.
(C.)
_Experimental Evidence as to the Non-inheritance of Acquired
Characters._
Let us now proceed to the experimental evidence which has been adduced
on the side of Weismannism.
Taking this evidence in order of date, we have first to mention that on
which the school of Weismann has hitherto been satisfied almost
exclusively to rely. This is the line of negative evidence, or the
seeming absence of any experimental demonstration of the inheritance of
acquired characters. This kind of evidence, however, presents much less
cogency than is usually supposed. And it has been shown in the last
chapter that the amount of experimental evidence in favour of the
transmission of acquired characters is more considerable than the school
of Weismann seems to be aware--especially in the vegetable kingdom. I do
not think that this negative line of evidence presents much weight; and,
to show that I am not biassed in forming this judgement, I may here
state that few have more reason than myself for appreciating the weight
of such evidence. For, as already stated, when first led to doubt the
Lamarckian factors, now more than twenty years ago, I undertook a
research upon the whole question--only a part of which was devoted to
testing the particular case of Brown-Sequard's statements, with the
result recorded in the preceding chapter. As this research yielded
negative results in all its divisions--and, not only in the matter of
Brown-Sequard's statements--I have not hitherto published a word upon
the subject. But it now seems worth while to do so, and for the
following reasons.
First, as just observed, a brief account of my old experiences in this
field will serve to show what good reason I have for feeling the weight
of such negative evidence in favour of Continuity as arises from failure
to produce any good experimental evidence to the contrary. In the second
place, now that the question has become one of world-wide interest, it
would seem that even negative results deserve to be published for
whatever they may be worth on the side of Neo-Darwinism. Lastly, in the
third place, although the research yielded negative results in my hands,
it is perhaps not undesirable to state the nature of it, if only to
furnish suggestions to other physiologists, in whose hands the
experiments--especially in these days of antiseptics--may lead to a
different termination. Altogether I made thousands of experiments in
graft-hydridization (comprising bines, bulbs of various kinds, buds, and
tubers); but with uniformly negative results. With animals I tried a
number of experiments in grafting characteristic congenital tissues from
one variety on another--such as the combs of Spanish cocks upon the
heads of Hamburgs; also, in mice and rats, the grafting together of
different varieties; and, in rabbits and bitches, the transplantation of
ovaries of newly-born individuals belonging to different well-marked
breeds. This latter experiment seems to be one which, if successfully
performed (so that the transplanted ovaries would form their attachment
in a young bitch puppy and subsequently yield progeny to a dog of the
same breed as herself) would furnish a crucial test as to the
inheritance or non-inheritance of acquired characters. Therefore I
devoted to it a large share of my attention, and tried the experiment in
several different ways. But I was never able to get the foreign
ovary--or even any portion thereof--to graft. Eventually the passing of
the Vivisection Act caused me to abandon the whole research as far as
animals were concerned--a research, indeed, of which I had become
heartily tired, since in no one instance did I obtain any adhesion.
During the last few years, however, I have returned to these experiments
under a licence, and with antiseptic precautions, but with a similar
want of success. Perhaps this prolonged and uniformly fruitless
experience may now have the effect of saving the time of other
physiologists, by warning them off the roads where there seems to be no
thoroughfare. On the other hand, it may possibly lead some one else to
try some variation in the method, or in the material, which has not
occurred to me. In particular, I am not without hope that the
transplantation of ovaries in very young animals may eventually prove to
be physiologically possible; and, if so, that the whole issue as between
the rival theories of heredity will be settled by the result of a single
experiment. Possibly some of the invertebrata will be found to furnish
the suitable material, although I have been unable to think of any of
these which present sufficiently well-marked varieties for the purpose.
But, pending the successful accomplishment of this particular experiment
in the grafting of any animal tissue, I think it would be clearly
unjustifiable to conclude against the Lamarckian factors on the ground
of any other experiments yielding negative results in but one generation
or even in a large number of sequent generations.
For instance, the latter consideration applies to the negative results
of Mr. Francis Galton's celebrated _Experiments in Pangenesis_.[82].
These consisted in transfusing the blood of one variety of rabbit into
the veins of both sexes of another, and then allowing the latter to
breed together: in no case was there any appearance in the progeny of
characters distinctive of the variety from which the transfused blood
was derived. But, as Mr. Galton himself subsequently allowed, this
negative result constitutes no disproof of pangenesis, seeing that only
a portion of the parents' blood was replaced; that this portion, even if
charged with "gemmules," would contain but a very small number of these
hypothetical bodies, compared with those contained in all the tissues of
the parents; and that even this small proportional number would
presumably be soon overwhelmed by those contained in blood newly-made by
the parents. Nevertheless the experiment was unquestionably worth
trying, on the chance of its yielding a positive result; for, in this
event, the question at issue would have been closed. Accordingly I
repeated these experiments (with the kind help of Professor Schaefer),
but with slight differences in the method, designed to give pangenesis a
better chance, so to speak.
[82] _Proc. R. S. 1871._
Thus I chose wild rabbits to supply the blood, and Himalayan to receive
it--the former being the ancestral type (and therefore giving reversion
an opportunity of coming into play), while the latter, although a
product of domestication, is a remarkably constant variety, and one
which differs very much in size and colour from the parent species.
Again, instead of a single transfusion, there were several transfusions
performed at different times. Moreover, we did not merely allow the
blood of one rabbit to flow into the veins of the other (whereby little
more than half the blood could be substituted); but sacrificed three
wild rabbits for refilling the vascular system of each tame one on each
occasion. Even as thus improved, however, the experiment yielded only
negative results, which, therefore, we never published.
Subsequently I found that all this labour, both on Mr. Galton's part and
our own, was simply thrown away--not because it yielded only negative
results, but because it did not serve as a crucial experiment at all.
The material chosen was unserviceable for the purpose, inasmuch as
rabbits, even when crossed in the ordinary way, never throw intermediate
characters. Needless to say, had I been aware of this fact before, I
should never have repeated Mr. Galton's experiments--nor, indeed, would
he have originally performed them had he been aware of it. So all this
work goes for nothing. The research must begin all over again with some
other animals, the varieties of which when crossed do throw intermediate
characters.
Therefore I have this year made arrangements for again repeating the
experiments in question--only, instead of rabbits, using well-marked
varieties of dogs. A renewed attack of illness, however, has
necessitated the surrender of this research to other hands, with a
consequent delay in its commencement.
My ignorance of the unfortunate peculiarity displayed by rabbits in not
throwing intermediate characters has led to a further waste of time in
another line of experiment. On finding that mammalian ovaries did not
admit of being grafted, it seemed to me that the next best thing to try
would be the transplantation of fertilized ova from one variety to
another, for the purpose of ascertaining whether, if a parturition
should take place under such circumstances, gestation by the uterine
mother would affect the characters of the ovum derived from the ovarian
mother--she, of course, having been fertilized by a male of her own
variety. Of course it was necessary that both the mothers should be in
season at about the same time, and therefore I again chose rabbits,
seeing that in the breeding season they are virtually in a chronic state
of "heat." I selected Himalayans and Belgian hares, because they are
well-marked varieties, breed true, and in respect of colour are very
different from one another. It so happened that while I was at work upon
this experiment, it was also being tried, unknown to me, by Messrs.
Heape and Buckley who, curiously enough, employed exactly the same
material. They were the first to obtain a successful result. Two
fertilized ova of the Angora breed having been introduced into the
fallopian tube of a Belgian hare, developed there in due course, and
gave rise to two Angora rabbits in no way modified by their Belgian hare
gestation[83].
[83] _Proc. R. S. 1890_, vol. xlviii. p. 457. It should be stated
that the authors do not here concern themselves with any theory
of heredity.
But, interesting and suggestive as this experiment is in other
connexions, it is clearly without significance in the present one, for
the reason already stated. It will have to be tried on well-marked
varieties of other species of animals, which are known to throw
intermediate characters. Even, however, if it should then yield a
similarly negative result, the fact would not tell against the
inheritance of acquired characters; seeing that an ovum by the time it
is ripe is a finished product, and therefore not to be expected, on any
theory of heredity, to be influenced as to its hereditary potentialities
by the mere process of gestation. On the other hand, if it should prove
that it does admit of being thus affected, so that against all
reasonable expectation the young animal presents any of the hereditary
characters of its uterine mother, the fact would terminate the question
of the transmission of acquired characters--and this quite as
effectually as would a similarly positive result in the case of progeny
from an ingrafted ovary of a different variety. In point of fact, the
only difference between the two cases would be, that in the former it
_might_ prove possible to close the question on the side of
Lamarckianism, in the latter it would _certainly_ close the question,
either on this side or on the opposite as the event would determine.
The only additional fact that has hitherto been published by the school
of Weismann is the result of Weismann's own experiment in cutting off
the tails of mice through successive generations. But this experiment
does not bear upon any question that is in debate; for no one who is
acquainted with the literature of the subject would have expected any
positive result to follow from such a line of inquiry. As shown further
back in the text, Darwin had carefully considered the case of
mutilations, and explained that their non-transmissibility constitutes
no valid objection to his theory of pangenesis. Furthermore, it may now
be added, he expressly alluded in this connexion to the cutting off of
tails, as practised by horse-breeders and dog-fanciers, "through a
number of generations, without any inherited effect." He also alluded to
the still better evidence which is furnished by the practice of
circumcision. Therefore it is difficult to understand the object of
Weismann's experiment. Yet, other than the result of this experiment, no
new fact bearing on the question at issue has been even so much as
alleged.
CHAPTER VI.
CHARACTERS AS HEREDITARY AND ACQUIRED
(_conclusion_[84]).
[84] _See_ note appended to Preface. C. Ll. M.
In the foregoing chapters I have endeavoured to be, before all things,
impartial; and if it seems that I have been arguing chiefly in favour of
the Lamarckian principles, this has been because the only way of
examining the question is to consider what has to be said on the
affirmative side, and then to see what the negative side can say in
reply. Before we are entitled to discard the Lamarckian factors _in
toto_, we must be able to destroy all evidence of their action. This,
indeed, is what the ultra-Darwinians profess to have done. But is not
their profession premature? Is it not evident that they have not
sufficiently considered certain general facts of nature, or certain
particular results of experiment, which at all events appear
inexplicable by the theory of natural selection alone? In any case the
present discussion has been devoted mainly to indicating such general
facts and particular results. If I have fallen into errors, either of
statement or of reasoning, it is for the ultra-Darwinians to correct
them; but it may be well to remark beforehand, that any criticism of a
merely general kind touching the comparative paucity of the facts thus
adduced in favour of Lamarckian doctrine, will not stand as a valid
criticism. For, as we have seen in the opening part of the discussion,
even if use-inheritance and direct action of the environment have been
of high importance as factors of organic evolution, it must be in almost
all cases impossible to dissociate their influence from that of natural
selection--at any rate where plants and animals in a state of nature are
concerned. On the other hand, experiments expressly devised to test the
question have not hitherto been carried out. Besides, the facts and
arguments here adduced are but _comparatively_ few. For, unless it can
be shown that what has been said of reflex action, instinct, so-called
"self-adaptation" in plants, &c., is wrong in principle, the facts which
tell in favour of Lamarckian theory are _absolutely_ very numerous. Only
when considered in relation to cases where we are unable to exclude the
conceivable possibility of natural selection having been at work, can it
be said that the facts in question are not numerous.
Comparatively few, then, though the facts may be of which I have given
some examples, in my opinion they are amply sufficient for the purpose
in hand. This purpose is to show that the question which we are now
considering is very far from being a closed question; and, therefore,
that the school of Weismann is much too precipitate in alleging that
there is neither any necessity for, nor evidence of, the so-called
Lamarckian factors[85]. And this opinion, whatever it may be worth, is
at all events both deliberate and impartial. As one of the first to
doubt the transmission of acquired characters, and as one who has spent
many years in experimental inquiries upon the subject, any bias that I
may have is assuredly against the Lamarckian principles--seeing that
nearly all my experiments have yielded negative results. It was Darwin
himself who checked this bias. But if the ultra-Darwinians of the last
ten years had succeeded in showing that Darwin was mistaken, I should be
extremely glad to fall into line with them. As already shown, however,
they have in no way affected this question as it was left by Galton in
1875. And if it be supposed a matter of but little importance whether we
agree with Galton in largely diminishing the comparative potency of the
Lamarckian principles, or whether we agree with Weismann in abolishing
them together, it cannot be too often repeated that such is an entirely
erroneous view. No matter how faintly or how fitfully acquired
characters may be transmitted, in so far as they are likewise adaptive
characters, their transmission (and therefore their development) must be
cumulative. Hence, the only effect of attenuating our estimate of their
_intensity_, is that of increasing our estimate of their
_duration_--i.e. of the time over which they have to operate in order to
produce important results. And, even so, it is to be remembered that
the importance of such results is not to be estimated by the magnitude
of modification. Far more is it to be estimated by the character of
modification as adaptive. For if functionally produced changes, and
changes produced in adaptive response to the environment, are ever
transmitted in a cumulative manner, a time must sooner or later arrive
when they will reach a selective value in the struggle for
existence--when, of course, they will be rapidly augmented by natural
selection. Thus, if in any degree operative at all, the great function
of these principles must be that of supplying to natural selection those
incipient stages of adaptive modifications in all cases where, but for
their agency, there would have been nothing of the kind to select.
Themselves in no way dependent on adaptive modifications having already
attained a selective value, these Lamarckian principles are (under the
Darwinian theory) direct causes of determinate variation in adaptive
lines; and variation in those lines being cumulative, the result is that
natural selection is in large part presented with the raw material of
its manufacture--special material of the particular kinds required, as
distinguished from promiscuous material of all kinds. And the more
complex the manufacture the more important will be the work of this
subordinate factory. We can well imagine how the shell of a nut, for
instance, or even the protective colouring of an insect, may have been
gradually built up by natural selection alone. But just in proportion as
structures or organs are not merely thus of passive _use_ (where, of
course, the Lamarckian principles cannot obtain), but require to be
actively _used_, in that proportion does it become difficult to
understand the _incipient_ construction of them by natural selection
alone. Therefore, in many such cases, if the incipient construction is
not to be explained by the Lamarckian principles, it is difficult to see
how it is to be explained at all.
[85] E.g. "The supposed transmission of this artificially produced
disease (epilepsy) is the only definite instance which has been
brought forward in support of the transmission of acquired
characters."--_Essays_, p. 328.
Furthermore, since the question as to the transmission of acquired
characters stands now exactly as it did after the publication of Mr.
Galton's _Theory of Heredity_ twenty years ago, it would seem that our
judgement with regard to it should remain exactly what it was then.
Although we must "out-Darwin Darwin" to the extent of holding that he
assigned too large a measure of intensity to the Lamarckian factors, no
sufficient reason has been shown for denying the existence of these
factors _in toto_; while, on the other hand, there are certain general
considerations, and certain particular facts, which appear to render it
probable that they have played a highly important part in the process of
organic evolution as a whole. At the same time, and in the present state
of our information, this judgement must be deemed provisional, or liable
eventually to be overturned by experimental proof of the non-inheritance
of acquired characters. But, even if this should ever be finally
accomplished, the question would still remain whether the principle of
natural selection alone is capable of explaining all the facts of
adaptation; and, for my own part, I should then be disposed to believe
that there must be some other, though hitherto undiscovered, principle
at work, which co-operates with natural selection, by playing the
subordinate role which was assigned by Darwin to the principles of
Lamarck.