to the deficiency which is necessarily involved in the assumption
of any selection, sexual selection included, namely, that the first
beginning of the character which has been intensified by selection
remains obscure. Darwin attached importance to the occurrence of
ordinary individual variation, but it is open to question whether the
insignificant variations thus produced could give an adequate advantage
in the competition for the possession of the females; and, further,
whether we have not grounds for the assumption that larger variations
also occur. This question may also be asked in regard to ordinary
natural selection, although in that case we can imagine the beginnings
to be smaller, since here the advantage of a variation lies only in
the fact that it is useful, not in its being appreciated by others.
As a matter of fact, this very difficulty as to the first beginnings
of variations has been frequently urged against both hypotheses of
selection, and rightly so, inasmuch as this must be above all else
the point of attack for further investigations. But it is a mistake
to deny the whole processes of selection simply because this point is
not yet clear. Later on we shall attempt to gain some insight into
the causes of variation, and then we shall return to this question
of the beginnings of the selective processes. In the meantime let it
suffice to say that Darwin was very well aware that, in addition to the
ordinary individual variations, there were also larger deviations which
occurred discontinuously in single forms. He believed, however, that
such occurrences were very rare, and, on the whole, he was not inclined
to ascribe to them any particular importance in the transformation of
species. He rather referred the organic transformations which have
taken place in the course of the earth's history, in the main, to the
intensification of the ordinary individual variations, and I believe
that he was right in so doing, since adaptations from their very nature
cannot have been brought about by sudden chance leaps in organization,
but can only have become exactly suited to chance conditions of life
through a gradual accumulation of minute variations in the direction
of utility. Whether, however, purely sexual distinctions may not have
had their primary roots in discontinuous variations must be inquired
into later. Theoretically, there is nothing against this assumption,
when such characters are not adaptations like the lasso antennæ of
the Copepods, or the turban eyes of the Ephemerids; mere distinctive
markings, decorative coloration, peculiar outgrowths, and the like,
may, if they arose discontinuously, very well have formed the basis for
further sexual selection, as long as they were not prejudicial to the
existence of the species.
LECTURE XII
INTRA-SELECTION OR SELECTION AMONG TISSUES
Does the Lamarckian principle really play a part in the
transformations of species?--Darwin's position in regard to this
question--Doubts expressed by Galton and others--Neo-Lamarckians
and Neo-Darwinians--Results of exercise and practice: functional
adaptation--Wilhelm Roux, _Kampf der Theile_.
WE have devoted a whole series of lectures to studying the
Darwin-Wallace principle of Natural Selection and the range of its
operation. It seemed to us to make innumerable adaptations intelligible
up to a certain point. We now understand how the purposefulness, which
we meet with everywhere among organisms, can have arisen without
the direct interference of a Power working intentionally towards an
end--simply as the outcome and result of the survival of the fittest.
The two forms of the processes of selection, 'natural selection' in
the narrower sense, and 'sexual selection,' dominate, so to speak, all
parts and all functions of the organism, and are striving to adapt
these as well as possible to the conditions of their life. And although
the range of operation of Natural Selection is incomparably greater,
because it actually affects every part, yet we must attribute to sexual
selection also, at least among animals, a range of influence by no
means unimportant, since through it, as far as we can see at present,
not only do the secondary sexual characters in all their diversity
arise, but by the transference of these to the other sex that too is
modified, and thus the whole species may be influenced, and may indeed
be started afresh on an unlimited series of further transformations.
But although the processes of selection play such an important part in
the transformations of the forms of life, we have to inquire whether
they are the _sole_ factors in these transformations, whether the
accumulation of chance variations in the direction of utility has been
the sole factor in bringing about the evolution of the animate world,
or whether other factors have not also co-operated with it.
We are all aware that Lamarck regarded the direct influence of use
and disuse as the most essential factor in transformation, and that
Darwin, though hesitatingly and cautiously, recognized and accepted
this factor, which he believed to be indispensable. Indeed, it seems
at first sight to be so. There is a whole range of facts which seem to
be intelligible only in terms of the Lamarckian theory; in particular,
the existence of numberless vestigial or rudimentary organs which
have degenerated through disuse, the remains of eyes in animals which
live in darkness, of wings in running birds, of hind legs in swimming
mammals (whales), and of ear muscles in Man, who no longer points his
ears, and so forth through a long list.
According to Wiedersheim, there are in Man alone about two hundred of
these vestigial or rudimentary organs, and there is no higher animal
which does not possess some. In all, therefore, a piece of the past
history of the species is embodied in the actually existing organism,
and bears witness to the fact that much of what the ancestors possessed
is now superfluous, and is either transformed, or is gradually set
aside, or is still in process of being set aside. It seems obvious that
this gradual dwindling and degeneration of an organ no longer needed
cannot be explained through natural selection in the Darwin-Wallace
sense, for the process goes on so exceedingly slowly that the minute
differences in the size of an organ, which may occur among individuals
of the species at any given time during the retrogressive process,
cannot possibly have a selection value. Whether the degenerate and
now functionless hind leg of the whale is a little larger or a little
smaller can have no importance in the struggle for existence; the
smaller organ cannot be considered either as a lesser hindrance in
swimming or as a greater economy of material, and the case is the same
in regard to most other instances of degeneration through disuse. We
therefore require another interpretation, and at first sight this seems
to be supplied by the Lamarckian principle.
But the reverse process, the strengthening, the enlarging, and the more
perfect development of a part, very often goes on proportionately to
its more frequent use, and here again the Lamarckian principle seems to
afford a simple explanation. For we know that exercise strengthens a
part, as disuse weakens it, and if we could assume that these results
of use and disuse were transmitted from the individual who brought them
about or 'acquired' them in the course of his life to his offspring,
then there would be nothing to object to in the Lamarckian principle.
But it is precisely here that the difficulty lies. Can we assume such
a transmission of 'acquired' characters? Does it exist? Can it be
demonstrated?
That Lamarck did not even put this question to himself, but assumed
such transmission as a matter of course, is readily intelligible
when we consider the time at which he lived. He was himself one of
the first to grasp the idea of the transmutation-hypothesis, and he
was only too glad to have any sort of principle of interpretation
ready to work with. But Charles Darwin, too, attributed a not
inconsiderable influence to this principle, although the transmission
of 'acquired' characters which it took for granted was not accepted
without reflective hesitation. He even directed his own particular
theory of heredity, as we shall see, especially to the explanation of
this supposed form of inheritance, and we can very well understand
this, after what I have said as to the impossibility of explaining
the disappearance of organs which have become superfluous by the
Darwin-Wallace theory of Natural Selection. Darwin needed the
Lamarckian principle for the explanation of these phenomena, and it
was this that decided him to assume the transmission of 'acquired'
characters, although the proofs of it can hardly have satisfied him.
For when we are confronted with facts which we see no possibility
of understanding save on a single hypothesis, even though it be an
undemonstrable one, we are naturally led to accept the hypothesis, at
least until a better one can be found. It is in this way, obviously,
that we are to understand Darwin's attitude to the Lamarckian
principle; he did not reject it, because it seemed to him to offer the
only possible explanation of the disappearance of characters which have
become useless; he adhered to it, although the transmission of acquired
characters which it assumed must have seemed, and, in point of fact,
did seem to him doubtful, or at least not definitely proved. Doubts,
some faint, some stronger, as to this assumed form of inheritance were
hardly expressed till somewhat late in the day--almost twenty years
after the appearance of the _Origin of Species_--first by Francis
Galton (1875), then by His, who definitely declared himself at least
against any inheritance of mutilations, and by Du Bois-Reymond, who,
in his address _Ueber die Uebung_ in 1881, said: 'If we are to be
honest, we must admit that the inheritance of acquired characters is a
hypothesis inferred solely from the facts which have to be explained,
and that it is in itself quite obscure.'
This is how it must appear to every one who examines it simply in
respect of its theoretical possibility, its conceivability. This is how
it appeared to me when I attempted, in 1883, to arrive at clearness on
the subject, and I then expressed my conviction that such a form of
inheritance was not only unproved, but that it was even theoretically
unthinkable, and that we ought to try to explain the fact of the
disappearance of disused parts in some other way, and I attempted to
give an explanation, as will be seen later.
Thus war was declared against the Lamarckian principle of the direct
effect of use and disuse, and there arose a strife which has continued
down to the present day, the strife between the Neo-Lamarckians and the
Neo-Darwinians, as the two disputing parties have been called.
In order to form an independent opinion in regard to this famous
dispute, it is, first of all, necessary to examine what actually takes
place when an organ is exercised or is left inactive, and further,
whether we can assume that the results of this exercise or inaction can
be transmitted to descendants.
That exercise in general has a strengthening, and neglect of it a
weakening influence on the relevant organ has long been known and is
familiar to all; gymnastics make the muscles stronger, the thickness of
the exercised muscle and the number of its fibres increases; the right
arm, which is much more used than the left, is capable of performing
twenty per cent. more work. Similarly, the activity of glands is
increased by exercise, and the glands themselves are increased in
size, as are the milk-glands of the cow through frequent milking; and
that even the nerve-elements can be favourably influenced by exercise
is proved by actors and professors of mnemonics, who have by practice
increased their powers of memory to an almost incredible degree. I have
heard of a singer who had learned by heart 160 operas; and which of us
has not experienced how quickly the capacity for learning by rote can
be again increased by practice, even after it has been neglected or
left unexercised for a long time?
I have always been particularly struck with the practising of a piece
of music, with its long succession of periods of different phrase,
with its changes in melody, rhythm, and harmony, which nevertheless
becomes so firmly stamped on the memory that it can be played, not
only consciously, but quite unconsciously, when the player is thinking
intensely of other things. It is in this case not the memory alone, but
the whole complicated mechanism of successive muscle-impulses, with
all the details of fast and slow, loud and soft, that is engraved on
the brain elements, just like a long series of reflex movements which
set one another a-going. Though in this case we cannot demonstrate
the material changes which have taken place in the nervous elements,
there can be no doubt that changes have taken place, and that these
consist in a strengthening of definite elements and parts of elements.
The strengthening causes certain ganglion-cells to give a stronger
impulse in a particular direction, and this impulse acquires increasing
transmissive power, and so on.
Our first theoretical insight into these relations came through Wilhelm
Roux, who, in 1881, gave expression to what had previously been
an open, if not quite conscious, secret, that 'functional stimulus
strengthens the organ,' that is to say, that an organ increases through
its own specific activity. Up till that time it had been believed that
it was merely the increased flow of blood that caused the increase in
the size of a much-used part. Roux showed that there is a 'quantitative
self-regulation of the organ according to the strength of the stimulus
supplied to it'; that the stimulated organ, that is, the organ which
is performing its normal function, may, in spite of the increased
breaking down or combustion (dissimilation), assimilate all the more
rapidly; that its used-up material is 'over-compensated,' and that
therefore it grows. He called this the 'trophic' or nutritive effect of
the stimulus, and in terms of this he explained the increase and the
heightened functional capacity of the much-used organ. Conversely, he
referred the decrease of a disused organ to 'functional atrophy,' which
sets in when there is a deficient compensation for the substance used
up in the metabolism.
But if we press for deeper analysis, we must ask: 'On what does this
trophic effect of functional stimulus depend?' Roux could not answer
this question when he wrote, nor can we do so now, as Plate has justly
emphasized. We are here face to face with the fundamental phenomenon
of life, metabolism; and, since we do not understand the causes of
this, we are not in a position to say why it varies in this way or in
that according to the 'stimulus.' But the fact itself is certain that
the organs respond up to a certain point to the claims made upon them;
they increase in proportion as they function more frequently or more
vigorously, they are able to respond to increased functional demands,
and this Roux has called 'functional adaptation.' As an animal adapts
itself to the claims of the conditions of its life, for instance, by
taking on a green or a brown protective colour according as it lives
on green or brown parts of plants, so the individual organ adapts
itself to the strength of the stimulus which impels it to function,
and increases or decreases in proportion to it. If _one_ kidney in Man
degenerate, or be surgically removed, the other begins to grow, and
goes on increasing until it has reached nearly twice its former size.
The specific stimulus which is brought to bear upon it by the urea
contained in the blood, and which forces it to grow, is twice as great
in the absence of the other kidney, and therefore the remaining kidney
grows in response to the increased stimulus and its 'trophic effect'
until its increase in size has reduced the functional intensity to the
normal proportion.
Adaptation of an organ in the opposite direction takes place when the
function diminishes or ceases. If a nerve supplying a muscle or a
gland be cut through, the organ concerned begins to degenerate and to
lose its normal structure to a greater or less degree. Sensory nerves
also degenerate in their peripheral part when they are cut through. In
such cases there may be no alteration either in the nutritive mechanism
or in the blood-vessels, &c., but the functional stimulus--in the case
of the muscle, the stimulus from the will--no longer affects the organ,
and its metabolism is so much lowered in consequence that it begins to
degenerate.
When we admit that the fit adaptation of the organism, as far as we
understand it, must depend upon processes of selection, we may refer
this 'functional adaptation' also to primitive processes of selection,
which prevailed at the very beginning of life upon our earth, and
represented, so to speak, the first adaptation that was established,
but we can say nothing with certainty in regard to this matter as long
as we do not understand the essence of assimilation. It is conceivable,
however, that a _primary_ adaptiveness may have arisen, so to speak,
abruptly, through a concurrence of favourable circumstances, as we
shall endeavour to show later on when we discuss the beginnings of life.
Even although we cannot lay bare the primary roots of 'functional
adaptation' we can gain from the fact itself very valuable insight
into phenomena which would otherwise be unintelligible and mysterious:
_the perfectly adapted structure of many tissues and their power of
adaptation to changed conditions_. In this lies, in the main, the
advance in our knowledge which is due to Roux's _Kampf der Theile_.
If a number of embryonic cells of different capacity, say _A_, _B_, and
_C_, be affected by different kinds of functional stimuli, _a_, _b_,
and _c_, those cells will grow most rapidly which are most frequently
affected by the stimulus appropriate to them. The proportion in which
the cells _A_, _B_, and _C_ will ultimately be present in the tissues
will depend upon the frequency with which the stimuli _a_, _b_, and
_c_ act upon the tissue. But the tissue will be still more precisely
determined as to its structure if the three kinds of stimuli affect the
cell-mass, not uniformly all over, but only at certain spots, or along
particular paths, one in this, the other in that. Thus the cells _A_
will predominate over the cells _B_ and _C_ at all the places which
are most frequently affected by the stimulus _a_, the cells _B_ in the
sphere of the stimulus _b_, and the cells _C_ in that of the stimulus
_c_; there they will increase most rapidly and so crowd out the other
kinds of cells, and thus a spatial arrangement will be established
within the tissue, a 'structure' which corresponds and is well adapted
to its end. This is what Roux deduced from his _Struggle of the
Parts_, and I subsequently defined the process as histonal or tissue
selection.
Let us first take an example. The anatomist Hermann Meyer showed in
1869 that the so-called 'spongiosa,' that is, the bony tissue of spongy
structure within the terminal portions of the long bones in Man and
Mammals, has a minute structure conspicuously well adapted to its
office. The thin bone lamellæ of this 'spongiosa' lie precisely in the
direction of the strongest strain or pressure which is exerted upon the
bone at the particular area. Arch-like in form, they are kept apart by
means of buttresses, and no architect could have done better if he had
been entrusted with the task of making a complicated system of arches
with the greatest possible carrying and resisting power combined with
the greatest possible economy of material.
This well-adapted structure is now interpreted through the _Struggle of
the Parts_ as a self-differentiation, for if there be in the rudiments
or primordia of the bone differently endowed elements[10], that is,
cells which respond in diverse ways to different stimuli, these must
arrange themselves locally, owing to the struggle for space and food,
in a manner corresponding to the distribution of the different stimuli
in the bone. The largest amount of bone substance will be formed in the
directions of the strongest strain and the greatest pressure, because
the bone-forming cells are excited by this, their functional stimulus,
to growth and multiplication. Thus the buttress and arch structure
comes about, and between the delicate bone lamellæ spaces will remain
free, and these, being relieved from the burden of strain and pressure
by the aforesaid bony lamellæ, will offer suitable conditions of life
to cells with other functional properties, such as connective tissue
cells or vascular cells.
[10] I do not here enter into the question whether we have not in
this case to do with similar elements, which have the power of
differentiating into one or another kind of cell according to the
nature of the external stimuli by which they are influenced.
The structure of the bone spongiosa is not everywhere the same,
and it is demonstrably related with precision to the conditions of
strain and pressure at each particular region. Thus, just below the
soft cartilaginous covering of the joints there are no long pillars
with short arches, but only rounded meshes, because the pressure is
here almost equally strong from all sides. The long parallel pillars
only occur further down in the bone, and they lie in two directions
which intersect each other obliquely, corresponding to the two main
directions of pressure. But it is only under the functional stimulus
of pressure that the bone-forming cells have an advantage over the
others, and multiply more quickly, thus crowding out those that are not
attuned to the appropriate functional stimulus.
In a similar manner Roux interprets, in the light of the struggle of
the parts, the striking adaptations in the course, the branching, and
the lumen-formation of the blood-vessels, in the direction of the
intersecting connective tissue strands in the tail-fin of the dolphin,
in the direction of the fibres in the tympanum, and in many other
adaptations in the histological structure of complex tissues.
In this there is manifestly an important step of progress, for it is
obvious that the direction of the bone-lamellæ and such like could not
have been determined by individual selection, and the same is true
in regard to many other histological details. It cannot be disputed,
however, that there is a kind of selection-process here also, similar
to that which we think of, with Darwin and Wallace, as occurring
between individual organisms. Just as in the latter, which we shall
henceforward call _personal selection_, variability and inheritance
lead, in the struggle for existence, to the survival of the fittest,
so, in histonal differentiation, the same three factors lead to the
victory of what is best suited to the parts of the body in question.
The tissues and the parts of the tissues have to distribute and
arrange themselves so that each comes to fill the place in which it
is most effectively and frequently affected by its specific stimulus,
that is, the stimulus in regard to which it is superior to other
parts; but these places are also those the occupation of which by
the best re-acting parts makes the whole tissue capable of more
effective function, and therefore makes its structure the fittest.
Variability--in this case that of embryonic cells, with different
primary constituents--must be assumed; inheritance is implied in
the multiplication of the cells by division; and the 'struggle for
existence' here assumes its frequent form of a competition for food
and space; the cells which assimilate more rapidly because of the
more frequent functional stimulus increase more rapidly, draw away
nourishment from the more slowly-multiplying cells around them, and
thus crowd these out to a greater or less extent.
We might even speak of histonal selection among unicellulars, for it
is conceivable that in primitive living substance, such as that of a
moneron, there may be minute differences among the vital particles,
involving also functional distinctions, which, under the influence of
diverse stimuli, may gradually give rise to an increasingly complex
differentiation. For the variations in the primary living substance
most strongly affected by a particular stimulus would tend to
accumulate at the places most frequently reached by that stimulus, and
would crowd out other variations at that spot, just as the body and its
individual parts may be said to have taken their architectural form
in exact response to the demands made upon them by function. In this
case, of course, personal selection and histonal selection co-operate,
for every improvement in the organization of the fundamental living
substance means at the same time a lasting improvement in the whole
individual.