coadaptation, and it is certain that functional adaptation is operative
during the individual life, and that it compensates in a certain measure
the inequalities of the inherited constitutions. I shall not repeat what I
have said before on this subject, nor maintain, in refutation of Spencer's
contention, that functional adaptation is itself nothing more than the
efflux of _intra-biontic_ selective processes, as Spencer himself once
suggested in a prophetic moment, but which it was left for Wilhelm Roux to
introduce into science as "the struggle of the parts" of organisms.[10] I
shall only remark that if functional adaptations were themselves
inheritable, this would still be insufficient {30} for the explanation of
coadaptation, for the reason that precisely similar coadaptive
modifications occur in _purely passively_ functioning parts, in which,
consequently, modification _by_ function is excluded. This is the case with
the skeletal parts of Articulata; e. g., it is true of their articular
surfaces with their complex adaptations to the most varied forms of
locomotion. In all these cases the ready-made, hard, unalterable, chitinous
part is _first_ set into activity; consequently its adaptation to the
function must have been _previously_ effected, independently of that
function. These joints, and divers other parts, accordingly, have been
developed in the precisest manner for the function, and the latter could
have had no direct share in their formation. When we consider, now, that it
is impossible that every one of the numerous surfaces, ridges, furrows, and
corners found in a single such articulation, let alone in all the
articulations of the body, should hold in its hands the power of life and
death over individuals for untold successions of generations, the fact is
again unmistakably impressed upon our attention that the conception of the
selective processes which has hitherto obtained is insufficient, that the
root of the process in fact lies deeper, that it is to be found in the
place where it is determined what variations of the parts of the organism
shall appear--namely _in the germ_.
The phenomena observed in the _stunting_, or _degeneration_, _of parts
rendered useless_, point to the same conclusion. They show distinctly that
ordinary selection which operates by the removal of entire persons,
_personal selection_, as I prefer to call it, cannot be the only cause of
degeneration; for in most cases of degeneration it cannot be assumed that
slight individual {31} vacillations in the size of the organ in question
have possessed selective value. On the contrary, we see such retrogressions
affected apparently _in the shape of a continuous evolutionary process
determined by internal causes_, in the case of which there can be no
question whatever of selection of persons or of a survival of the fittest,
that is, of individuals with the smallest rudiments.
It is this consideration principally that has won so many adherents for the
Lamarckian principle in recent times, particularly among the
paleontologists. They see the outer toes of hoofed animals constantly and
steadily degenerating through long successions of generations and species,
concurrently with the re-enforcement of one or two middle toes, which are
preferred or are afterwards used exclusively for stepping, and they believe
correctly enough that these results should not be ascribed to the effects
of personal selection alone. They demand a principle which shall effect the
degeneration by internal forces, and believe that they have found it in
functional adaptation.[11] {32} On this last point, now, I believe, they
are mistaken, be they ever so strongly convinced of the correctness of
their view and ever so aggressive and embittered in their defence of it.
Recently, an inquirer of great caution and calmness of judgment, Prof. C.
Lloyd Morgan, has expressed the opinion that the Lamarckian principle must
at least be admitted as a working hypothesis. But with this I cannot agree,
at least as things stand at present. A working hypothesis may be false, and
yet lead to further progress; that is, it may constitute an advance to the
extent of being useful in formulating the problem and in illuminating paths
that are likely to lead to results. But it seems to me that a hypothesis of
this kind has performed its services and must be discarded the moment it is
found to be at hopeless variance with the facts. If it can be proved that
precisely the same degenerative processes also take place in such
superfluous parts as have only _passive_ and not active functions, as is
the case with the _chitinous parts of the skeleton of Arthropoda_, then it
is a demonstrated fact, that the cessation of functional action is not the
efficient cause of the process of degeneration. At once your legitimate
working hypothesis is transformed into an illegitimate dogma--illegitimate
because it no longer serves as a guide on the path to knowledge but {33}
blocks that path. For the person who is convinced he has found the right
explanation is not going to seek for it.
I can understand perfectly well the hesitation that has prevailed on this
point in many minds, from their having seen _one_ aspect of the facts more
distinctly than the other. From this sceptical point of view Osborn has
drawn the following perfectly correct conclusion: "If acquired variations
are transmitted, there must be some unknown principle in heredity; if they
are not transmitted, there must be some unknown factor in evolution."[12]
Such in fact is the case and I shall attempt to point out to you what this
factor is. My inference is a very simple one: if we are forced by the facts
on all hands to the assumption that the useful variations which render
selection possible are always present, then _some profound connection must
exist between the utility of a variation and its actual appearance_, or, in
other words, _the direction of the variation of a part must be determined
by utility_, and we shall have to see whether facts exist that confirm our
conjecture.
The facts do indeed exist and lie before our very eyes, despite their not
having been recognised as such before. All _artificial selection_ practised
by man rests on the fact that by means of the selection of individuals
having a given character slightly more pronounced than usual, there is
gradually produced a general augmentation of this character, which
subsequently reaches a point never before attained by any individual {34}
of this species. I shall choose an example which seems to me especially
clear and simple because only one character has been substantially modified
here. The long-tailed variety of domestic cock, now bred in Japan and
Corea, owes its existence to skilful selection and not at all to the
circumstance that at some period of the race's history a cock with
tail-feathers six feet in length suddenly and spasmodically appeared. At
the present day even, as Professor Ishikawa of Tokio writes me, the
breeders still make extraordinary efforts to increase the length of the
tail, and every inch gained adds considerably to the value of the bird. Now
nothing has been done here whatever except always to select for purposes of
breeding the cocks with the longest feathers; and in this way alone were
these feathers, after the lapse of many generations, prolonged to a length
far exceeding every previous variation.
I once asked a famous dove-fancier, Mr. W. B. Tegetmeier of London, whether
it was his opinion that by artificial selection alone a character could be
augmented. He thought a long time and finally said: "It is without our
power to do anything if the variation which we seek is not presented, but
once that variation is given, then I think the augmentation can be
effected." And that in fact is the case. If cocks had never existed whose
tail-feathers were a little longer than usual the Japanese breed could
never have originated; but as the facts are, always the cocks with the
longest feathers were chosen from each generation, and these only were
bred, and thus a hereditary augmentation of the character in question was
effected, which would hardly have been deemed possible.
Now what does this mean? Simply that the {35} hereditary diathesis, the
constitutional predisposition (_Anlage_) of the breed was changed in the
respect in question, and our conclusion from this and numerous similar
facts of artificial selection runs as follows: _by the selection alone of
the plus or minus variations of a character is the constant modification of
that character in the plus or minus direction determined._ Obviously the
hereditary _diminution_ of a part is also effected by the simple selection
of the individuals in each generation possessing the smallest parts, as is
proved, for example, by the tiny bills and feet of numerous breeds of
doves. We may assert, therefore, in general terms: a definitely directed
progressive variation of a given part is produced by continued selection in
that definite direction. This is no hypothesis, but a direct inference from
the facts and may also be expressed as follows: _By a selection of the kind
referred to the germ is progressively modified in a manner corresponding
with the production of a definitely directed progressive variation of the
part._
In this general form the proposition is not likely to encounter opposition,
as certainly no one is prepared to uphold the view that the germ remains
unchanged whilst the products proceeding from it, its descendants, are
modified. On the contrary, all will agree when I say that the germ in this
case must have undergone modifications, and that their character must
correspond with the modifications undergone by its products. Thus far,
then, we find ourselves, not on the ground of the hypothesis that has been
lately so much maligned, but on the ground of facts and of direct
inferences from facts. But if we attempt to pierce deeper into the problem,
we are in need of the hypothesis. {36}
The first and most natural explanation will be this--that through selection
the zero-point, about which, figuratively speaking, the organ may be said
to oscillate in its plus and minus variations, is displaced upwards or
downwards. Darwin himself assumed that the variations oscillated about a
mean point, and the statistical researches of Galton, Weldon, and others
have furnished a proof of the assumption. If selection, now, always picks
out the plus variations for imitation, perforce, then, the mean or
zero-point will be displaced in the upward direction, and the variations of
the following generation will oscillate about a higher mean than before.
This elevation of the zero-point of a variation would be continued in this
manner until the total equilibrium of the organism was in danger of being
disturbed.
There is involved here, however, an assumption which is by no means
self-evident, that every advancement gained by the variation in question
constitutes a new centre for the variations occurring in the following
generation. _That this is a fact_, is proved by such actual results of
selection as are obtained in the case of the Japanese cock. But the
question remains, Why is this the fact?
Now here, I think, my theory of determinants gives a satisfactory answer.
According to that theory every independently and hereditarily variable part
is represented in the germ by a _determinant_, that is by a determinative
group of vital units, whose size and power of assimilation correspond to
the size and vigor of the part. These determinants multiply, as do all
vital units, by growth and division, and necessarily they increase rapidly
in every individual, and the more rapidly the greater the quantity of the
germinal cells {37} the individual produces. And since there is no more
reason for excluding irregularities of passive nutrition, and of the supply
of nutriment in these minute, microscopically invisible parts, than there
is in the larger visible parts of the cells, tissues, and organs,
consequently the descendants of a determinant can never all be exactly
alike in size and capacity of assimilation, but they will oscillate in this
respect to and fro about the maternal determinant as about their
zero-point, and will be partly greater, partly smaller, and partly of the
same size as that. In these oscillations, now, the material for further
selection is presented, and in the inevitable fluctuations of the nutrient
supply I see the reason why every stage attained becomes immediately the
zero-point of new fluctuations, and consequently why the size of a part can
be augmented or diminished by selection without limit, solely by the
displacement of the zero-point of variation as the result of selection.
We should err, however, if we believed that we had penetrated to the root
of the phenomenon by this insight. There is certainly some other and
mightier factor involved here than the simple selection of persons and the
consequent displacement of the zero-point of variation. It would seem,
indeed, as if in one case, _videlicet_, in that of the Japanese cock, the
augmentation of the character in question were completely explained by this
factor _alone_. In fact, in this and similar cases we cannot penetrate
deeper into the processes of variation, and therefore cannot say _a priori_
whether other factors have or have not been involved in the augmentation of
the character in question--other characters, that is, than the simple
displacement of the zero-point. There is, however, another class of
phyletic modifications, which point {38} unmistakably to the conclusion
that the displacement of the zero-point of variation by personal selection
is not and cannot be the only factor in the determination and
accomplishment of the direction of variation. I refer to _retrogressive
development_, the gradual degeneration of parts or characters that have
grown useless, the gradual disappearance of the eye in cave-animals, of the
legs in snakes and whales, of the wings in certain female butterflies, in
short, to that entire enormous mass of facts comprehended under the
designation of "rudimentary organs."
I have endeavored on a previous occasion to point out the significance of
the part played in the great process of animate evolution by these
retrogressive growths, and I made at the time the statement that "the
phenomena of retrogressive growth enabled us in a greater measure almost
than those of progressive growth to penetrate to the causes which produce
the transformations of animate nature." Although at that time[13] I had no
inkling of certain processes which today I shall seek to prove the
existence of, yet my statement receives a fresh confirmation from these
facts.
For, in most retrogressive processes _active_ selection in Darwin's sense
plays no part, and advocates of the Lamarckian principle, as above
remarked, have rightly denied that active selection, that is, the selection
of individuals possessing the useless organ in its most reduced state, is
sufficient to explain the process of degeneration. I, for my part, have
never assumed this, {39} and I enunciated precisely on this account the
_principle of panmixia_. Now, although this, as I still have no reason for
doubting, is a perfectly correct principle, which really does have an
essential and indispensable share in the process of retrogression, still it
is not _alone_ sufficient for a full explanation of the phenomena. My
opponents, in advancing this objection, were right, to the extent indicated
and as I expressly acknowledge, although they were unable to substitute
anything positive in its stead or to render my explanation complete. The
very fact of the cessation of control over the organ is sufficient to
explain its _degeneration_, that is, its deterioration, the disharmony of
its parts, but not the fact which actually and always occurs where an organ
has become useless--viz., _its gradual and unceasing diminution continuing
for thousands and thousands of years culminating in its final and absolute
effacement._
If, now, neither the selection of persons nor the cessation of personal
selection can explain this phenomenon, assuredly some other principle must
be the efficient cause here, and this cause I believe I have indicated in
an essay written at the close of last year and only recently published.[14]
I call it _germinal selection_.
The principle in question reposes on the application, made some fifteen
years ago by Wilhelm Roux, of the principle of selection to the _parts_ of
organisms--on the _struggle of the parts_, as he called it. If such a
struggle obtains among organs, tissues, and cells, it must also obtain
between the smallest and for us invisible vital particles, not only between
those of the body-cells, strictly so called, but also between those of the
{40} germinal cells. Roux himself spoke of the struggle of the molecules,
by which he presumably understood the smallest ultimate units of vital
phenomena--elements which De Vries designated pangenes, Wiesner plasomes,
and I _biophores_, after Bruecke's ingenious conception[15] of these
invisible entities had been almost totally forgotten, or at least had lain
unnoticed for thirty years. No struggle, as that is understood in the
theory of selection, could take place between real {41} molecules, for
molecules are neither nourished, subject to growth, nor propagated.
The gradual degeneration of organs grown useless may be explained, now, by
the theory of determinants very simply and without any co-operation on the
part of active personal selection, as follows.
Nutrition, it is known, is not merely a passive process. A part is not only
_nourished_ but also actively _nourishes_ itself, and the more vigorously,
the more powerful and capable of assimilation it is. Hence powerful
determinants in the germ will absorb nutriment more rapidly than weaker
determinants. The latter, accordingly, will grow more slowly and will
produce weaker descendants than the former.
Let us assume, now, that a part of the body, say the hinder extremities of
the quadruped ancestors of {42} our common whales, are rendered useless.
Panmixia steps in, _i. e._, selection ceases to influence these organs.
Individuals with large and individuals with small hind legs are equally
favored in the struggle for existence.
From this fact alone would result a degradation of the organ, but of course
it would not be very marked in extent, seeing that the minus variations
which occur are no longer removed. According to our assumption, however,
such minus variations repose on the weaker determinants of the germ, that
is, on such as absorb nutriment less powerfully than the rest. And since
every determinant battles stoutly with its neighbors for food, that is,
takes to itself as much of it as it can, consonantly with its power of
assimilation and proportionately to the nutrient supply, therefore the
unimpoverished neighbors of this minus determinant will deprive it of its
nutriment more rapidly than was the case with its more robust ancestors;
hence, it will be unable to obtain the full quantum of food corresponding
even to its weakened capacity of assimilation, and the result will be that
its ancestors will be weakened still more. Inasmuch, now, as no weeding out
of the weaker determinants of the hind leg by personal selection takes
place on our hypothesis, inevitably the average strength of this
determinant must slowly but constantly diminish, that is, the leg must grow
smaller and smaller until finally it disappears altogether. The
determinants[16] of the useless organ are constantly at {43} a disadvantage
as compared with the determinants of their environment in the germinal
tenement, because no assistance is offered to them by personal selection
after they have once been weakened by a decrease of the passive nutrient
influx. Nor is the degeneration stopped by the uninterrupted crossing of
individuals in sexual propagation, but only slightly retarded. The number
of individuals with weaker determinants must, despite this fact, go on
increasing from generation to generation, so that soon every determinant
that still happens to be endowed with exceptional vigor will be confronted
by a decided overplus of weaker determinants, and by continued crossing
therefore will become more and more impoverished. Panmixia is the
indispensable precondition of the whole process; for owing to the fact that
persons with weak determinants are just as capable of life as those with
strong, owing to the fact that they cannot now, as formerly, when the organ
was still useful, be removed by personal selection, solely by this means is
a further weakening effected in the following generations--in short, only
by this means are the determinants of the useless organ brought upon the
inclined plane, down which they are destined slowly but incessantly to
slide towards their completed extinction.
The foregoing explanation will be probably accepted as satisfactory _in a
purely formal regard_, but it will be objected that, even granting this, it
has not yet been proved to be the correct one. In answer I can of course
adduce nothing except that it is at present the only one that can be given.
It may be that the actual state of things in nature is different, but if it
can be shown that a self-direction of variation merely from the need of it
is at all conceivable by mechanical means, {44} that in itself, it seems to
me, is a decided gain. It must also not be forgotten that some process or
other _must_ take place in the germ-plasm when an organ becomes
rudimentary, and that as the result of it this organ, and only this organ,
must disappear. Now in what shall this process consist, if not in a
modification of the constitution of the germ? And how could the effect of
such a modification be limited only to _one_ organ which was becoming
rudimentary if the modification itself were not a local one? These are
questions which it is incumbent on those to answer who conceive the
germinal substance to be composed of like units.
Applying, now, the explanation derived from the disappearance of organs to
the opposed transformation, namely, to the _enlargement_ of a part, the
presumption lies close at hand that the production of the long
tail-feathers of the Japanese cock does not repose solely on the
displacement directly effected by personal selection, of the zero-point of
variation upwards, but that _it is also fostered and strengthened by
germinal selection_. Were that not so, the phenomena of the transmutation
of species, in so far as fresh growth and the enlargement and complication
of organs already present are concerned, _would not be a whit more
intelligible than they were before_. We should know probably how it comes
to pass that the constitutional predisposition (group of determinants) of a
_single_ organ is intensified by selection, but the flood of objections
against the theory of selection touching its inability to modify _many_
parts at once would not be repressed by such knowledge. The initial impulse
conditioning the independent maintenance of the useful direction of
variation in the germ-plasm must rather be sought {45} in the utility of
the modification itself, and this also seems to me intelligible from the
side of the theory. For as soon as personal selection favors the more
powerful variations of a determinant, the moment that these come to
predominate in the germ-plasm of the species, at once the tendency must
arise for them to vary _still more strongly_ in the plus direction, not
solely because the zero-point has been pushed farther upwards, but because
they themselves now oppose a relatively more powerful front to their
neighbors, that is, actively absorb more nutriment, and upon the whole
increase in vigor and produce more robust descendants. From the relative
vigor or dynamic status of the particles of the germ-plasm, thus, will
issue spontaneously an ascending line of variation, precisely as the facts
of evolution require. For, as I have already said, it is not sufficient
that the augmentation of a character should be brought about by
uninterrupted personal selection, even supposing that the displacement of
the zero-point were possible without germinal selection.
Thus, I think, may be explained how personal selection imparts the initial
impulse to processes in the germ-plasm, which, when they are once set
agoing, persist of themselves in the same direction, and are, therefore, in
no need of the continued supplementary help of personal selection, _as
directed exclusively to a definite part_. If but from time to time, that
is, if upon the average the poorest individuals, the bearers of the weakest
determinants, are eliminated, the variational direction of the part in
question, now reposing on germinal selection, must persist, and it will
very slowly but very surely increase until further development is impeded
by its inutility and personal selection {46} arrests the process, that is,
ceases to eliminate the weaker individuals.