soldiers nor workers reproduce? Zehnder answers this by pointing out
that the sexual animals eat _all_ the kinds of nourishment which are
accumulated in the stores, that is to say, all the different kinds of
food exactly in the proportion in which they have been imported--the
proportion in which the different kinds of persons are represented in
the colony. Thus the kinds of nourishment which caused the appearance
of the newly acquired characters in the non-sexual animals also
reached the sexual animals and their sex-cells, and there gave rise to
substances which evoke the relevant qualities in their descendants, for
instance, in the soldiers, or in the still more modified workers, and
so on; and thus we have an 'inheritance of acquired characters.'
This is certainly ingeniously and cleverly thought out, and it reads
even better and more smoothly in the original than in my brief summary,
but it will hardly be regarded as a refutation of my position; the
hypotheses are all too daring for that. We have no knowledge that
particular modifications in form can be produced and conditioned by
particular kinds of food, and, indeed, the contrary has been proved,
namely, that the two or three different castes of polymorphic species
have precisely similar diet. I need only recall the six forms of female
in _Papilio merope_, of which at least three have been obtained from
the same set of eggs, and by feeding with the same plant.
It is true that there are ants which lay in stores of nourishment,
but these consist, for the most part, of one kind of seeds, or of
honey, not of different substances, and we have no knowledge that
the different persons use different food, or even that there is any
diversity in the mode of feeding the helpless larvæ. The feeding in
some species takes place from mouth to mouth, and therefore cannot be
precisely investigated, and we can only suppose from analogy with bees
that the larvæ of the males and females frequently receive not only
more abundant, but qualitatively different food. They are fed from the
crop unless the food consists of the pith of a tree in which the larvæ
are imbedded, as Dahl informs us is the case with some tropical ants of
the Bismarck Archipelago.
But even if we assume that the soldiers take different food from the
ordinary workers, and different again from that of the sexual animals,
is it by virtue of the quality of their food that they have become
what they are? Have our breeds of pigeons or hens been produced by
different diet, or do we know anything in the whole range of animal
life of such a parallelism between food and bodily structure as Zehnder
here assumes? And if, in reality, let us say, the breeds of pigeon had
arisen through specific dieting, and we were to feed one pair with the
specific food-stuffs of three different breeds, would the descendants
of this pair exhibit the form of these three breeds? Or would they
exhibit them in precisely the proportion in which the food-stuffs had
been mixed? It seems to me that Zehnder's assumptions diverge so far
from what we are accustomed to regard as solid ground in biology that
they hardly require consideration, and yet he not only uses them for
the explanation of the case of the ants, but bases upon them the whole
of his theory of the inheritance of acquired characters.
He considers that the results of use (that is, increased function)
are generally transmitted, because the increase in the organ which
is functioning more strongly changes the composition of the blood,
by withdrawing from it in a greater degree the specific substances
which the organ in question--a muscle, for instance--requires for its
activity. All parts of the animal are thereby affected and modified,
but especially those smallest vital units or 'fistellæ' (corresponding
to biophors) which preside over digestion, and of which there are
several sorts. Among them those work most arduously which have to
produce the specific substances which serve for the nutrition of the
muscles with increased function, because these are needed in larger
quantities. This kind of digestive 'fistella' therefore multiplies,
while other kinds, whose products are not required and therefore not
used up, cease to be so active, diminish in number, and in course of
time disappear. In this way the composition of the blood is altered,
and with it to a greater or less degree all the characters of the whole
organism. Of course the reproductive cells are also under the influence
of this change in the composition of the blood, because the different
nutritive substances are accumulated within them in an altered
proportion corresponding to the changed composition of the blood, the
nutritive substances for the muscles with increased function being
contained in it in a larger quantity, and thus the greater development
of the muscle will repeat itself in the progeny, that is to say, _the
acquired character is transmitted_.
It is obvious that this is precisely the same line of argument as that
used in reference to the origin of the worker and soldier ants. The
different kinds of 'digestive fistellæ' correspond to the different
food-carrying workers, and the blood to the assumed storehouse
from which soldiers and workers select the food suitable for their
respective needs, while the sex-cells in the one case, the sexual
animals in the other, partake of all kinds exactly in the proportion
in which they are stored, and thus the organ which functions most
vigorously must be stronger in the offspring.
How the minute quantity of nutritive material contained in the ovum,
still less in the sperm, is to effect the strengthening of the
particular muscles in the descendants is not stated; moreover, such
minimal quantities of food must soon be exhausted, and cannot possibly
increase. It would seem as if the muscles could not even begin by
being stronger, much less that they should remain so, if they were
not exercised equally vigorously by the descendants. If the specific
nutritive stuffs were 'fistellæ,' that is to say, were living units
capable of multiplication, one could understand it. But there can,
of course, be no possibility of a production of living units through
digestion; that can only give rise to digested substances. Or if the
alteration in the composition of the blood produced in the determinant
system of the germ-plasm just those variations requisite to bring
about a strengthening of the muscular system, it would remain to be
shown how this could happen, for the gist of the problem lies in this.
For muscles do not lie in the germ-plasm as miniature models of the
subsequent muscular system, and even if they did, would not all the
muscles, and not merely those which were no longer exercised, decrease
hereditarily when a particular group, like the muscles of the ear in
man, degenerates? Zehnder replies to this with the hypothesis that
the muscles are not all chemically alike, but that each possesses a
particular chemical formula, though they may all be very similar, and
that, therefore, the nutritive materials required by each must be
slightly different. In that case there would require to be, in the
ovum and sperm of man, in order that functional modifications might be
transmitted, as many special nutritive substances as there are muscles,
and, in addition to these, innumerable hosts of other kinds of specific
nutritive substances for all the other parts of the body, since all of
them can be strengthened by exercise and weakened by disuse. And even
if we suppose that all these millions of specific nutritive substances
are accommodated within the germ-cells, as Zehnder's theory requires,
they could not perform what Zehnder ascribes to them, for, as we have
already said, they cannot multiply in the manner of living units, and
so control the growing organism. The different specific nutritive
materials contained in the blood are just as powerless to perform the
task ascribed to them by Zehnder as the specific kinds of food in the
hypothetical storehouse of the ants are to give rise to the different
persons of the ant-colony.
Zehnder also attempts to overthrow the arguments against the Lamarckian
principle which I based on the skeleton of Arthropods.
It does not seem to him probable that the chitinous coat of mail can
be an absolutely dead structure, and he supposes that very delicate
nerve-fibrils penetrate into all its most minute parts, and so are
stimulated by 'every pressure and every strain' exerted on the
chitinous skeleton. They 'work' when they are stimulated, and in
doing so they use up 'their specific food-stuffs.' At places which
are frequently stimulated the corresponding nerves develop more than
elsewhere. The necessary specific food-stuffs for these particular
nerves therefore increase proportionately within the body, and also
in the reproductive cells. Accordingly, in the germ-cells there is an
increase of the aforesaid nervous substances, which in the offspring
become associated with the relevant part of the chitinous covering, and
induce in development the secretion of chitin at this part. At this
particular spot, then, the chitin will be specially thick.
This clearly implies that each particular part of the skin has its
specific nutritive substances, necessitated by the nerves which
traverse it! Thus there must be as many nerve-nutritive substances
as there are skin-nerves, specific chemical combinations for every
part of the body which is capable of heritable variation. This is so
extraordinarily improbable that I need say nothing more about it. If
the Lamarckian principle requires this kind of hypothesis to bolster it
up, it is undoubtedly doomed.
If we disregard altogether the positive aspect of Zehnder's hypothesis,
and assume that the skin-nerves are really stimulated through the
chitin by every strain and pressure to which a spot of skin is exposed,
and that they cause a correspondingly greater secretion of chitin,
which would then, according to the Lamarckian principle, be hereditary,
does this harmonize with what actually occurs in the development of
the skeleton as we know it in the case of Insects and Crustaceans? Not
at all! Can we suppose that the carapace of a crab or the enormously
hard wing-covers of a water-beetle are exposed to a continual pounding
and pressing and pushing? Exactly the contrary is the case. Every
assailant takes care not to grasp the animal where it is so well
protected, and seeks out the most vulnerable parts for its attacks. It
may be answered that, while this is certainly the case now, the animals
were badly protected when the ancestral forms were evolving. But that
they could not have become hard by dint of being frequently bitten or
otherwise wounded should be obvious from the fact that the whole of the
wing-covers and the whole of the carapace is uniformly covered with
thick chitin, while each wound would only stimulate particular spots;
and we should also have to admit that, since these parts of the skin
which are now so well protected are no longer seized and stimulated,
they would long ago have become thin again, according to the principle
of the degeneration of parts no longer used, or, in this case, no
longer stimulated. But there is no need for wasting time over such
quibbles, since there is a fact which absolutely contradicts Zehnder's
hypothesis. I mean the degeneration of the chitinous skeleton in those
Crustaceans and Insects which protect the abdomen within a shelter
like the hermit-crabs, the caddis-flies (Phryganidæ), (Fig. 107) and
the sack-carrying caterpillars of the Psychidæ among Lepidoptera. The
hermit-crabs, as is well known, squeeze their abdomen into a usually
spirally-coiled Gasteropod shell, and they always choose houses which
are wide enough to conceal the whole body up to the hard claws when
necessity arises. In this case there is surely a continual pressure on
the abdomen, which, being soft, must be squeezed very tightly every
time the animal retreats into its shell. One of my opponents has
described the disappearance of the tough integumentary skeleton from
the abdomen of these animals as an inherited result of this pressure,
and another regards it as the inherited result of the degeneration
of the muscles in this part of the body. But, according to Zehnder,
this continuous pressure, and the frequent rubbing up and down of
the abdomen on the inner surface of the Gasteropod's shell, would
undoubtedly have a stimulating effect on the skin-nerves, and would
therefore bring about a thickening of the chitinous cuticle. In regard
to the larval Phryganidæ and Psychidæ, the case would be the same,
though perhaps hardly to the same degree, for while these larvæ make
their own houses, and will therefore at least make them big enough to
begin with, the pressure and friction must increase with the growth of
the animal.
[Illustration: FIG. 107. Larva of a Caddis-fly, after Rösel. _A_,
removed from its case, showing the hooks (_h_) which attach it thereto,
and the whitish abdomen, covered only by a thin cuticle. _B_, the same
larva, moving about with its case.]
If the regulation of the strength of the integumentary skeleton be
referred to selection, we see at once why carapace and wing-covers
should be of equal thickness throughout their whole extent, and why
they do not disappear, although they do not function actively, and
are less stimulated than any other parts of the skeleton; and we also
understand why the abdomen of hermit-crabs and of larval Phryganidæ
and Psychidæ has become soft, whether it be exposed to pressure or
friction in a greater or a less degree. It no longer requires to be
hard, because it is protected by the house, and in the case of the
Pagurids it must not be hard because it could not then be readily
squeezed into the hard-walled and narrow recesses of the Gasteropod
shell; in this case there has therefore been positive selection. I have
not yet referred to the fact that the chitinous covering is certainly
not living, though it is not exactly dead; it is a secretion of the
epidermic cells, not a tissue, and we cannot suppose that there are
any nerve-endings in it. It is almost superfluous to say that the fact
that the skin is cast is in itself enough to make such an assumption
untenable, for the whole of the assumed delicate nervous network would
be shed at every moult and torn away from the nerves which lead to it.
As far as my knowledge goes, nothing of this kind occurs anywhere in
the whole range of the animal kingdom.
Even if we assume, for the benefit of Zehnder's hypothesis, that
although there are no nerves in the chitin itself yet irritations
affecting the chitinous coat may be transmitted through it to the
delicate nerve-endings lying beneath it, this should take place in a
greater degree at the thin places of the skeleton than _at the thick
parts_! But this interpretation is again fallacious, for we see that
the tactile organs of Arthropods always break through the chitinous
cuticle and protrude beyond it in the form of setæ.
Of the many other opponents of my views in regard to the
transmissibility of acquired functional modifications, I need only deal
in detail with Oscar Hertwig.
He seeks for direct proofs of an inheritance of acquired characters,
and believes that he has found these in the hereditary transmission of
acquired immunity from certain diseases. He reminds us of Ehrlich's
well-known experiments on mice with ricin and abrin.
Even small doses of these two poisons kill mice, but they are tolerated
in very minute doses, and if their administration be continued for some
time in such minute doses, the animals gradually acquire a high degree
of insensitiveness to these poisons; they become immune to ricin and
abrin.
This immunity is transmitted from mother to young, but it only lasts
for a short time, about six to eight weeks after birth. Yet this is
regarded by Hertwig as an illustration of the transmission of an
acquired character, as an acquired modification of the cells of the
body, for he explains the immunity on the assumption that all the cells
of the body undergo a particular variation due to the influence of the
poison, and are thus, to a certain extent, modified in their nature,
and that the ovum also undergoes this variation and transmits it to the
young animal. The immunization might certainly come under the category
of functional modifications, and it might be thought that we have here
a case of transmission of such an acquired character.
Against this, however, we have to put the fact that _the acquired
immunity is not transmitted from the father to the descendants_.
Hertwig attempts to explain this by saying that the short duration
of the experiments has only allowed the poison to affect the
cell-substance (cytoplasm) and not the nucleus, that is, the hereditary
substance of the sperm-cells, an assumption which has little
probability considering the intimate nutritive relations between the
cell-nucleus and the cytoplasm. I should be rather inclined to conclude
from the difference in the transmitting power of the sperm and of the
ovum, that this 'inheritance of immunity' does not depend, as Hertwig
supposes, on a modification of the cells to 'ricin immunity,' but, as
Ehrlich and the bacteriologists believe, on the production of so-called
'anti-toxins,' and that these anti-toxins are handed on to the embryo
not by the ovum itself, but by the interchange of blood between mother
and offspring which lasts throughout the whole embryonic period. It is
then self-evident why no transmission of immunity through the father
occurs.
But it would lead me too far were I to attempt to refute all the
attempts that have been made to interpret individual cases as due to
the inheritance of acquired characters. I should, however, like to say
something as to the theoretical possibility of such an assumption.
When we try to conceive how experiences and their consequences can
be entailed, how new acquirements of the 'personal part can have
representative effects on the germinal part,' we find ourselves
confronted with almost, if not entirely, insuperable difficulties.
How could it happen that the constant exercise of memory throughout
a lifetime, as, for instance, in the case of an actor, could
influence the germ-cells in such a way that in the offspring the same
brain-cells which preside over memory will likewise be more highly
developed--that is, capable of greater functional activity? We know
what Zehnder's answer to such a question would be; he would make the
blood the intermediary between the brain-cells and the germ-cells, but
we have seen that specific food-stuffs for each specific cell-group
cannot be assumed, and that, even if they could, they would not meet
the necessities of the case. Yet every one who does not regard the
germ-plasm as composed of determinants is constrained to make some such
assumption. But if we take our stand upon the theory of determinants,
it would be necessary to a transmission of acquired strength of
memory that the states of these brain-cells should be communicated by
the telegraphic path of the nerve-cells to the germ-cells, and should
there modify only the determinants of the brain-cells, and should do
so in such a way that, in the subsequent development of an embryo
from the germ-cell, the corresponding brain-cells should turn out to
be capable of increased functional activity. But as the determinants
are not miniature brain-cells, but only groups of biophors of unknown
constitution, and are assuredly different from those cells; as they
are not 'seed-grains' of the brain-cells, but only living germ-units
which, in co-operation with the rest exercise a decisive influence on
the memory-cells of the brain, I can only compare the assumption of the
transmission of the results of memory-exercise to the telegraphing of a
poem, which is handed in in German, but at the place of arrival appears
on the paper translated into Chinese.
Nevertheless, as I have said before, I do not disagree with those who
say, with Oscar Hertwig, that the impossibility of forming a conception
of the physiological nexus involved in the assumed transmission does
not _ipso facto_ constrain us to conclude that the transmission
does not occur. I cannot, however, agree with Hertwig that the case
is exactly the same as in the 'converse process,' that is, 'in the
development of the given invisible primary constituents in the
inheritance of the cell into the visible characters of the personal
part.' Certainly no one can state with any definiteness how the germ
goes to work, so that from it there arises an eye or a brain with its
millionfold intricacies of nerve-paths, but although the process cannot
be understood in detail, it can in principle, and this is just what is
impossible in regard to the communication of functional modifications
to the germ. Moreover, in addition to this, there is the very important
difference that, in the one case, we know with certainty that the
process actually takes place, although we cannot understand its
mechanical sequence in detail, while in the other we cannot even prove
that the supposed process is a real one at all. From the fact that we
are unable to form clear conceptions of a hypothetical process, we are
not justified, it seems to me, in assuming it to be real, even though
we are aware of many other processes in nature which we are unable to
understand.
Nor does Hertwig take up this position, for he is at pains to show
the mechanical possibility of the process of inheritance which he
assumes, and he bases this upon the suggestions made by Hering in his
famous work _Ueber das Gedächtniss als eine allgemeine Function der
organisirten Materie_ [_On memory as a general function of organized
matter_], 1870. As this essay probably contains the best that can be
said in favour of a transmission of functional modifications, and as
it also includes some indisputable truths, we may consider it in some
detail.
Hering is undoubtedly right in regarding 'the phenomena of
consciousness as functions of the material changes of organic
substance, and conversely.' That is, he believes that every sensation,
every perception, every act of will arises from material changes in
the relevant nerve-substances. But we know that 'whole groups of
impressions, which our brain has received through the sense-organs, are
stored up in it, as if resting, and below the margin of consciousness,
to be reproduced when occasion arises, in correct order of space and
time, and with such vividness, that we may be deceived into regarding
as a present reality what has long ceased to be present.' There
must therefore remain in the nerve-substance a 'material impact,' a
modification of the molecular or atomic structure, which enables it 'to
ring out to-day the note that it gave forth yesterday if only it be
rightly struck.'