(a) Saltatory evolution.
The Darwinian doctrine of evolution depends essentially on THE
CUMULATIVE AUGMENTATION of minute variations in the direction of
utility. But can such minute variations, which are undoubtedly
continually appearing among the individuals of the same species,
possess any selection-value; can they determine which individuals are
to survive, and which are to succumb; can they be increased by natural
selection till they attain to the highest development of a purposive
variation?
To many this seems so improbable that they have urged a theory of
evolution by leaps from species to species. Kolliker, in 1872, compared
the evolution of species with the processes which we can observe in the
individual life in cases of alternation of generations. But a polyp only
gives rise to a medusa because it has itself arisen from one, and there
can be no question of a medusa ever having arisen suddenly and de
novo from a polyp-bud, if only because both forms are adapted in their
structure as a whole, and in every detail to the conditions of their
life. A sudden origin, in a natural way, of numerous adaptations is
inconceivable. Even the degeneration of a medusoid from a free-swimming
animal to a mere brood-sac (gonophore) is not sudden and saltatory, but
occurs by imperceptible modifications throughout hundreds of years, as
we can learn from the numerous stages of the process of degeneration
persisting at the same time in different species.
If, then, the degeneration to a simple brood-sac takes place only by
very slow transitions, each stage of which may last for centuries, how
could the much more complex ASCENDING evolution possibly have taken
place by sudden leaps? I regard this argument as capable of further
extension, for wherever in nature we come upon degeneration, it is
taking place by minute steps and with a slowness that makes it not
directly perceptible, and I believe that this in itself justifies us in
concluding that THE SAME MUST BE TRUE OF ASCENDING evolution. But in the
latter case the goal can seldom be distinctly recognised while in cases
of degeneration the starting-point of the process can often be inferred,
because several nearly related species may represent different stages.
In recent years Bateson in particular has championed the idea of
saltatory, or so-called discontinuous evolution, and has collected a
number of cases in which more or less marked variations have suddenly
appeared. These are taken for the most part from among domesticated
animals which have been bred and crossed for a long time, and it is
hardly to be wondered at that their much mixed and much influenced
germ-plasm should, under certain conditions, give rise to remarkable
phenomena, often indeed producing forms which are strongly suggestive of
monstrosities, and which would undoubtedly not survive in free nature,
unprotected by man. I should regard such cases as due to an intensified
germinal selection--though this is to anticipate a little--and from this
point of view it cannot be denied that they have a special interest. But
they seem to me to have no significance as far as the transformation
of species is concerned, if only because of the extreme rarity of their
occurrence.
There are, however, many variations which have appeared in a sudden and
saltatory manner, and some of these Darwin pointed out and discussed
in detail: the copper beech, the weeping trees, the oak with "fern-like
leaves," certain garden-flowers, etc. But none of them have persisted in
free nature, or evolved into permanent types.
On the other hand, wherever enduring types have arisen, we find traces
of a gradual origin by successive stages, even if, at first sight, their
origin may appear to have been sudden. This is the case with SEASONAL
DIMORPHISM, the first known cases of which exhibited marked differences
between the two generations, the winter and the summer brood. Take
for instance the much discussed and studied form Vanessa (Araschnia)
levana-prorsa. Here the differences between the two forms are so great
and so apparently disconnected, that one might almost believe it to be
a sudden mutation, were it not that old transition-stages can be called
forth by particular temperatures, and we know other butterflies, as for
instance our Garden Whites, in which the differences between the
two generations are not nearly so marked; indeed, they are so little
apparent that they are scarcely likely to be noticed except by experts.
Thus here again there are small initial steps, some of which, indeed,
must be regarded as adaptations, such as the green-sprinkled or lightly
tinted under-surface which gives them a deceptive resemblance to parsley
or to Cardamine leaves.
Even if saltatory variations do occur, we cannot assume that these HAVE
EVER LED TO FORMS WHICH ARE CAPABLE OF SURVIVAL UNDER THE CONDITIONS
OF WILD LIFE. Experience has shown that in plants which have suddenly
varied the power of persistence is diminished. Korschinksky attributes
to them weaknesses of organisation in general; "they bloom late, ripen
few of their seeds, and show great sensitiveness to cold." These are not
the characters which make for success in the struggle for existence.
We must briefly refer here to the views--much discussed in the last
decade--of H. de Vries, who believes that the roots of transformation
must be sought for in SALTATORY VARIATIONS ARISING FROM INTERNAL CAUSES,
and distinguishes such MUTATIONS, as he has called them, from ordinary
individual variations, in that they breed true, that is, with strict
inbreeding they are handed on pure to the next generation. I have
elsewhere endeavoured to point out the weaknesses of this theory
("Vortrage uber Descendenztheorie", Jena, 1904, II. 269. English
Translation London, 1904, II. page 317.), and I am the less inclined
to return to it here that it now appears (See Poulton, "Essays on
Evolution", Oxford, 1908, pages xix-xxii.) that the far-reaching
conclusions drawn by de Vries from his observations on the Evening
Primrose, Oenothera lamarckiana, rest upon a very insecure
foundation. The plant from which de Vries saw numerous "species"--his
"mutations"--arise was not, as he assumed, a WILD SPECIES that had been
introduced to Europe from America, but was probably a hybrid form which
was first discovered in the Jardin des Plantes in Paris, and which does
not appear to exist anywhere in America as a wild species.
This gives a severe shock to the "Mutation theory," for the other
ACTUALLY WILD species with which de Vries experimented showed no
"mutations" but yielded only negative results.
Thus we come to the conclusion that Darwin ("Origin of Species" (6th
edition), pages 176 et seq.) was right in regarding transformations as
taking place by minute steps, which, if useful, are augmented in
the course of innumerable generations, because their possessors more
frequently survive in the struggle for existence.
(b) SELECTION-VALUE OF THE INITIAL STEPS.
Is it possible that the significant deviations which we know as
"individual variations" can form the beginning of a process of
selection? Can they decide which is to perish and which to survive? To
use a phrase of Romanes, can they have SELECTION-VALUE?
Darwin himself answered this question, and brought together many
excellent examples to show that differences, apparently insignificant
because very small, might be of decisive importance for the life of the
possessor. But it is by no means enough to bring forward cases of this
kind, for the question is not merely whether finished adaptations have
selection-value, but whether the first beginnings of these, and whether
the small, I might almost say minimal increments, which have led up
from these beginnings to the perfect adaptation, have also had
selection-value. To this question even one who, like myself, has been
for many years a convinced adherent of the theory of selection, can only
reply: WE MUST ASSUME SO, BUT WE CANNOT PROVE IT IN ANY CASE. It is not
upon demonstrative evidence that we rely when we champion the doctrine
of selection as a scientific truth; we base our argument on quite other
grounds. Undoubtedly there are many apparently insignificant features,
which can nevertheless be shown to be adaptations--for instance, the
thickness of the basin-shaped shell of the limpets that live among the
breakers on the shore. There can be no doubt that the thickness of these
shells, combined with their flat form, protects the animals from the
force of the waves breaking upon them,--but how have they become so
thick? What proportion of thickness was sufficient to decide that of two
variants of a limpet one should survive, the other be eliminated? We can
say nothing more than that we infer from the present state of the shell,
that it must have varied in regard to differences in shell-thickness,
and that these differences must have had selection-value,--no proof
therefore, but an assumption which we must show to be convincing.
For a long time the marvellously complex RADIATE and LATTICE-WORK
skeletons of Radiolarians were regarded as a mere outflow of "Nature's
infinite wealth of form," as an instance of a purely morphological
character with no biological significance. But recent investigations
have shown that these, too, have an adaptive significance (Hacker). The
same thing has been shown by Schutt in regard to the lowly unicellular
plants, the Peridineae, which abound alike on the surface of the ocean
and in its depths. It has been shown that the long skeletal processes
which grow out from these organisms have significance not merely as a
supporting skeleton, but also as an extension of the superficial area,
which increases the contact with the water-particles, and prevents
the floating organisms from sinking. It has been established that the
processes are considerably shorter in the colder layers of the ocean,
and that they may be twelve times as long (Chun, "Reise der Valdivia",
Leipzig, 1904.) in the warmer layers, thus corresponding to the greater
or smaller amount of friction which takes place in the denser and less
dense layers of the water.
The Peridineae of the warmer ocean layers have thus become long-rayed,
those of the colder layers short-rayed, not through the direct effect
of friction on the protoplasm, but through processes of selection, which
favoured the longer rays in warm water, since they kept the organism
afloat, while those with short rays sank and were eliminated. If we put
the question as to selection-value in this case, and ask how great
the variations in the length of processes must be in order to possess
selection-value; what can we answer except that these variations must
have been minimal, and yet sufficient to prevent too rapid sinking
and consequent elimination? Yet this very case would give the
ideal opportunity for a mathematical calculation of the minimal
selection-value, although of course it is not feasible from lack of data
to carry out the actual calculation.
But even in organisms of more than microscopic size there must
frequently be minute, even microscopic differences which set going the
process of selection, and regulate its progress to the highest possible
perfection.
Many tropical trees possess thick, leathery leaves, as a protection
against the force of the tropical rain drops. The DIRECT influence
of the rain cannot be the cause of this power of resistance, for the
leaves, while they were still thin, would simply have been torn to
pieces. Their toughness must therefore be referred to selection, which
would favour the trees with slightly thicker leaves, though we cannot
calculate with any exactness how great the first stages of increase in
thickness must have been. Our hypothesis receives further support from
the fact that, in many such trees, the leaves are drawn out into a
beak-like prolongation (Stahl and Haberlandt) which facilitates the
rapid falling off of the rain water, and also from the fact that the
leaves, while they are still young, hang limply down in bunches which
offer the least possible resistance to the rain. Thus there are here
three adaptations which can only be interpreted as due to selection.
The initial stages of these adaptations must undoubtedly have had
selection-value.
But even in regard to this case we are reasoning in a circle, not giving
"proofs," and no one who does not wish to believe in the selection-value
of the initial stages can be forced to do so. Among the many pieces of
presumptive evidence a particularly weighty one seems to me to be THE
SMALLNESS OF THE STEPS OF PROGRESS which we can observe in certain
cases, as for instance in leaf-imitation among butterflies, and
in mimicry generally. The resemblance to a leaf, for instance of a
particular Kallima, seems to us so close as to be deceptive, and yet we
find in another individual, or it may be in many others, a spot added
which increases the resemblance, and which could not have become fixed
unless the increased deceptiveness so produced had frequently led to
the overlooking of its much persecuted possessor. But if we take the
selection-value of the initial stages for granted, we are confronted
with the further question which I myself formulated many years ago: How
does it happen THAT THE NECESSARY BEGINNINGS OF A USEFUL VARIATION ARE
ALWAYS PRESENT? How could insects which live upon or among green leaves
become all green, while those that live on bark become brown? How have
the desert animals become yellow and the Arctic animals white? Why were
the necessary variations always present? How could the green locust lay
brown eggs, or the privet caterpillar develop white and lilac-coloured
lines on its green skin?
It is of no use answering to this that the question is wrongly
formulated (Plate, "Selektionsprinzip u. Probleme der Artbildung" (3rd
edition), Leipzig, 1908.) and that it is the converse that is true; that
the process of selection takes place in accordance with the variations
that present themselves. This proposition is undeniably true, but so
also is another, which apparently negatives it: the variation required
has in the majority of cases actually presented itself. Selection cannot
solve this contradiction; it does not call forth the useful variation,
but simply works upon it. The ultimate reason why one and the same
insect should occur in green and in brown, as often happens in
caterpillars and locusts, lies in the fact that variations towards brown
presented themselves, and so also did variations towards green: THE
KERNEL OF THE RIDDLE LIES IN THE VARYING, and for the present we
can only say, that small variations in different directions present
themselves in every species. Otherwise so many different kinds of
variations could not have arisen. I have endeavoured to explain this
remarkable fact by means of the intimate processes that must take place
within the germ-plasm, and I shall return to the problem when dealing
with "germinal selection."
We have, however, to make still greater demands on variation, for it
is not enough that the necessary variation should occur in isolated
individuals, because in that case there would be small prospect of its
being preserved, notwithstanding its utility. Darwin at first believed,
that even single variations might lead to transformation of the species,
but later he became convinced that this was impossible, at least
without the cooperation of other factors, such as isolation and sexual
selection.
In the case of the GREEN CATERPILLARS WITH BRIGHT LONGITUDINAL STRIPES,
numerous individuals exhibiting this useful variation must have been
produced to start with. In all higher, that is, multicellular organisms,
the germ-substance is the source of all transmissible variations, and
this germ-plasm is not a simple substance but is made up of many primary
constituents. The question can therefore be more precisely stated thus:
How does it come about that in so many cases the useful variations
present themselves in numbers just where they are required, the white
oblique lines in the leaf-caterpillar on the under surface of the body,
the accompanying coloured stripes just above them? And, further, how has
it come about that in grass caterpillars, not oblique but longitudinal
stripes, which are more effective for concealment among grass and
plants, have been evolved? And finally, how is it that the same
Hawk-moth caterpillars, which to-day show oblique stripes, possessed
longitudinal stripes in Tertiary times? We can read this fact from the
history of their development, and I have before attempted to show
the biological significance of this change of colour. ("Studien
zur Descendenz-Theorie" II., "Die Enstehung der Zeichnung bei den
Schmetterlings-raupen," Leipzig, 1876.)
For the present I need only draw the conclusion that one and the same
caterpillar may exhibit the initial stages of both, and that it depends
on the manner in which these marking elements are INTENSIFIED and
COMBINED by natural selection whether whitish longitudinal or oblique
stripes should result. In this case then the "useful variations"
were actually "always there," and we see that in the same group of
Lepidoptera, e.g. species of Sphingidae, evolution has occurred in both
directions according to whether the form lived among grass or on broad
leaves with oblique lateral veins, and we can observe even now that the
species with oblique stripes have longitudinal stripes when young, that
is to say, while the stripes have no biological significance. The white
places in the skin which gave rise, probably first as small spots,
to this protective marking could be combined in one way or another
according to the requirements of the species. They must therefore either
have possessed selection-value from the first, or, if this was not
the case at their earliest occurrence, there must have been SOME OTHER
FACTORS which raised them to the point of selection-value. I shall
return to this in discussing germinal selection. But the case may be
followed still farther, and leads us to the same alternative on a still
more secure basis.
Many years ago I observed in caterpillars of Smerinthus populi (the
poplar hawk-moth), which also possess white oblique stripes, that
certain individuals showed RED SPOTS above these stripes; these spots
occurred only on certain segments, and never flowed together to form
continuous stripes. In another species (Smerinthus tiliae) similar
blood-red spots unite to form a line-like coloured seam in the last
stage of larval life, while in S. ocellata rust-red spots appear in
individual caterpillars, but more rarely than in S. Populi, and they
show no tendency to flow together.
Thus we have here the origin of a new character, arising from small
beginnings, at least in S. tiliae, in which species the coloured stripes
are a normal specific character. In the other species, S. populi and
S. ocellata, we find the beginnings of the same variation, in one more
rarely than in the other, and we can imagine that, in the course of
time, in these two species, coloured lines over the oblique stripes will
arise. In any case these spots are the elements of variation, out
of which coloured lines MAY be evolved, if they are combined in this
direction through the agency of natural selection. In S. populi the
spots are often small, but sometimes it seems as though several had
united to form large spots. Whether a process of selection in this
direction will arise in S. populi and S. ocellata, or whether it is
now going on cannot be determined, since we cannot tell in advance what
biological value the marking might have for these two species. It is
conceivable that the spots may have no selection-value as far as these
species are concerned, and may therefore disappear again in the course
of phylogeny, or, on the other hand, that they may be changed in another
direction, for instance towards imitation of the rust-red fungoid
patches on poplar and willow leaves. In any case we may regard the
smallest spots as the initial stages of variation, the larger as a
cumulative summation of these. Therefore either these initial stages
must already possess selection-value, or, as I said before: THERE MUST
BE SOME OTHER REASON FOR THEIR CUMULATIVE SUMMATION. I should like to
give one more example, in which we can infer, though we cannot directly
observe, the initial stages.
All the Holothurians or sea-cucumbers have in the skin calcareous bodies
of different forms, usually thick and irregular, which make the
skin tough and resistant. In a small group of them--the species of
Synapta--the calcareous bodies occur in the form of delicate anchors of
microscopic size. Up till 1897 these anchors, like many other delicate
microscopic structures, were regarded as curiosities, as natural
marvels. But a Swedish observer, Oestergren, has recently shown that
they have a biological significance: they serve the footless Synapta as
auxiliary organs of locomotion, since, when the body swells up in the
act of creeping, they press firmly with their tips, which are embedded
in the skin, against the substratum on which the animal creeps, and thus
prevent slipping backwards. In other Holothurians this slipping is
made impossible by the fixing of the tube-feet. The anchors act
automatically, sinking their tips towards the ground when the
corresponding part of the body thickens, and returning to the original
position at an angle of 45 degrees to the upper surface when the part
becomes thin again. The arms of the anchor do not lie in the same plane
as the shaft, and thus the curve of the arms forms the outermost part
of the anchor, and offers no further resistance to the gliding of the
animal. Every detail of the anchor, the curved portion, the little teeth
at the head, the arms, etc., can be interpreted in the most beautiful
way, above all the form of the anchor itself, for the two arms prevent
it from swaying round to the side. The position of the anchors, too, is
definite and significant; they lie obliquely to the longitudinal axis of
the animal, and therefore they act alike whether the animal is creeping
backwards or forwards. Moreover, the tips would pierce through the skin
if the anchors lay in the longitudinal direction. Synapta burrows in the
sand; it first pushes in the thin anterior end, and thickens this again,
thus enlarging the hole, then the anterior tentacles displace more sand,
the body is worked in a little farther, and the process begins anew. In
the first act the anchors are passive, but they begin to take an
active share in the forward movement when the body is contracted again.
Frequently the animal retains only the posterior end buried in the sand,
and then the anchors keep it in position, and make rapid withdrawal
possible.
Thus we have in these apparently random forms of the calcareous bodies,
complex adaptations in which every little detail as to direction, curve,
and pointing is exactly determined. That they have selection-value in
their present perfected form is beyond all doubt, since the animals
are enabled by means of them to bore rapidly into the ground and so to
escape from enemies. We do not know what the initial stages were, but we
cannot doubt that the little improvements, which occurred as variations
of the originally simple slimy bodies of the Holothurians, were
preserved because they already possessed selection-value for the
Synaptidae. For such minute microscopic structures whose form is so
delicately adapted to the role they have to play in the life of the
animal, cannot have arisen suddenly and as a whole, and every new
variation of the anchor, that is, in the direction of the development
of the two arms, and every curving of the shaft which prevented the tips
from projecting at the wrong time, in short, every little adaptation
in the modelling of the anchor must have possessed selection-value. And
that such minute changes of form fall within the sphere of fluctuating
variations, that is to say, THAT THEY OCCUR is beyond all doubt.
In many of the Synaptidae the anchors are replaced by calcareous rods
bent in the form of an S, which are said to act in the same way. Others,
such as those of the genus Ankyroderma, have anchors which project
considerably beyond the skin, and, according to Oestergren, serve "to
catch plant-particles and other substances" and so mask the animal. Thus
we see that in the Synaptidae the thick and irregular calcareous bodies
of the Holothurians have been modified and transformed in various ways
in adaptation to the footlessness of these animals, and to the peculiar
conditions of their life, and we must conclude that the earlier stages
of these changes presented themselves to the processes of selection in
the form of microscopic variations. For it is as impossible to think of
any origin other than through selection in this case as in the case of
the toughness, and the "drip-tips" of tropical leaves. And as these
last could not have been produced directly by the beating of the heavy
rain-drops upon them, so the calcareous anchors of Synapta cannot have
been produced directly by the friction of the sand and mud at the bottom
of the sea, and, since they are parts whose function is PASSIVE the
Lamarckian factor of use and disuse does not come into question. The
conclusion is unavoidable, that the microscopically small variations of
the calcareous bodies in the ancestral forms have been intensified
and accumulated in a particular direction, till they have led to the
formation of the anchor. Whether this has taken place by the action
of natural selection alone, or whether the laws of variation and the
intimate processes within the germ-plasm have cooperated will become
clear in the discussion of germinal selection. This whole process of
adaptation has obviously taken place within the time that has
elapsed since this group of sea-cucumbers lost their tube-feet, those
characteristic organs of locomotion which occur in no group except the
Echinoderms, and yet have totally disappeared in the Synaptidae.
And after all what would animals that live in sand and mud do with
tube-feet?
(c) COADAPTATION.
Darwin pointed out that one of the essential differences between
artificial and natural selection lies in the fact that the former can
modify only a few characters, usually only one at a time, while Nature
preserves in the struggle for existence all the variations of a species,
at the same time and in a purely mechanical way, if they possess
selection-value.
Herbert Spencer, though himself an adherent of the theory of selection,
declared in the beginning of the nineties that in his opinion the range
of this principle was greatly over-estimated, if the great changes which
have taken place in so many organisms in the course of ages are to
be interpreted as due to this process of selection alone, since no
transformation of any importance can be evolved by itself; it is always
accompanied by a host of secondary changes. He gives the familiar
example of the Giant Stag of the Irish peat, the enormous antlers of
which required not only a much stronger skull cap, but also greater
strength of the sinews, muscles, nerves and bones of the whole anterior
half of the animal, if their mass was not to weigh down the animal
altogether. It is inconceivable, he says, that so many processes of
selection should take place SIMULTANEOUSLY, and we are therefore
forced to fall back on the Lamarckian factor of the use and disuse of
functional parts. And how, he asks, could natural selection follow two
opposite directions of evolution in different parts of the body at the
same time, as for instance in the case of the kangaroo, in which the
forelegs must have become shorter, while the hind legs and the tail were
becoming longer and stronger?
Spencer's main object was to substantiate the validity of the Lamarckian
principle, the cooperation of which with selection had been doubted
by many. And it does seem as though this principle, if it operates
in nature at all, offers a ready and simple explanation of all such
secondary variations. Not only muscles, but nerves, bones, sinews,
in short all tissues which function actively, increase in strength
in proportion as they are used, and conversely they decrease when the
claims on them diminish. All the parts, therefore, which depend on the
part that varied first, as for instance the enlarged antlers of the
Irish Elk, must have been increased or decreased in strength, in exact
proportion to the claims made upon them,--just as is actually the case.
But beautiful as this explanation would be, I regard it as untenable,
because it assumes the TRANSMISSIBILITY OF FUNCTIONAL MODIFICATIONS
(so-called "acquired" characters), and this is not only undemonstrable,
but is scarcely theoretically conceivable, for the secondary variations
which accompany or follow the first as correlative variations, occur
also in cases in which the animals concerned are sterile and THEREFORE
CANNOT TRANSMIT ANYTHING TO THEIR DESCENDANTS. This is true of WORKER
BEES, and particularly of ANTS, and I shall here give a brief survey of
the present state of the problem as it appears to me.
Much has been written on both sides of this question since the published
controversy on the subject in the nineties between Herbert Spencer and
myself. I should like to return to the matter in detail, if the space
at my disposal permitted, because it seems to me that the arguments I
advanced at that time are equally cogent to-day, notwithstanding all the
objections that have since been urged against them. Moreover, the matter
is by no means one of subordinate interest; it is the very kernel of the
whole question of the reality and value of the principle of selection.
For if selection alone does not suffice to explain "HARMONIOUS
ADAPTATION" as I have called Spencer's COADAPTATION, and if we require
to call in the aid of the Lamarckian factor it would be questionable
whether selection could explain any adaptations whatever. In this
particular case--of worker bees--the Lamarckian factor may be excluded
altogether, for it can be demonstrated that here at any rate the effects
of use and disuse cannot be transmitted.
But if it be asked why we are unwilling to admit the cooperation of
the Darwinian factor of selection and the Lamarckian factor, since this
would afford us an easy and satisfactory explanation of the phenomena,
I answer: BECAUSE THE LAMARCKIAN PRINCIPLE IS FALLACIOUS, AND BECAUSE BY
ACCEPTING IT WE CLOSE THE WAY TOWARDS DEEPER INSIGHT. It is not a spirit
of combativeness or a desire for self-vindication that induces me to
take the field once more against the Lamarckian principle, it is the
conviction that the progress of our knowledge is being obstructed by the
acceptance of this fallacious principle, since the facile explanation it
apparently affords prevents our seeking after a truer explanation and a
deeper analysis.
The workers in the various species of ants are sterile, that is to say,
they take no regular part in the reproduction of the species, although
individuals among them may occasionally lay eggs. In addition to this
they have lost the wings, and the receptaculum seminis, and their
compound eyes have degenerated to a few facets. How could this last
change have come about through disuse, since the eyes of workers are
exposed to light in the same way as are those of the sexual insects and
thus in this particular case are not liable to "disuse" at all? The same
is true of the receptaculum seminis, which can only have been disused
as far as its glandular portion and its stalk are concerned, and also
of the wings, the nerves tracheae and epidermal cells of which could
not cease to function until the whole wing had degenerated, for the
chitinous skeleton of the wing does not function at all in the active
sense.
But, on the other hand, the workers in all species have undergone
modifications in a positive direction, as, for instance, the greater
development of brain. In many species large workers have evolved,--the
so-called SOLDIERS, with enormous jaws and teeth, which defend the
colony,--and in others there are SMALL workers which have taken over
other special functions, such as the rearing of the young Aphides. This
kind of division of the workers into two castes occurs among several
tropical species of ants, but it is also present in the Italian species,
Colobopsis truncata. Beautifully as the size of the jaws could be
explained as due to the increased use made of them by the "soldiers," or
the enlarged brain as due to the mental activities of the workers, the
fact of the infertility of these forms is an insurmountable obstacle
to accepting such an explanation. Neither jaws nor brain can have been
evolved on the Lamarckian principle.
The problem of coadaptation is no easier in the case of the ant than in
the case of the Giant Stag. Darwin himself gave a pretty illustration to
show how imposing the difference between the two kinds of workers in one
species would seem if we translated it into human terms. In regard to
the Driver ants (Anomma) we must picture to ourselves a piece of work,
"for instance the building of a house, being carried on by two kinds of
workers, of which one group was five feet four inches high, the other
sixteen feet high." ("Origin of Species" (6th edition), page 232.)
Although the ant is a small animal as compared with man or with the
Irish Elk, the "soldier" with its relatively enormous jaws is hardly
less heavily burdened than the Elk with its antlers, and in the ant's
case, too, a strengthening of the skeleton, of the muscles, the nerves
of the head, and of the legs must have taken place parallel with the
enlargement of the jaws. HARMONIOUS ADAPTATION (coadaptation) has here
been active in a high degree, and yet these "soldiers" are sterile!
There thus remains nothing for it but to refer all their adaptations,
positive and negative alike, to processes of selection which have taken
place in the rudiments of the workers within the egg and sperm-cells
of their parents. There is no way out of the difficulty except the one
Darwin pointed out. He himself did not find the solution of the riddle
at once. At first he believed that the case of the workers among social
insects presented "the most serious special difficulty" in the way of
his theory of natural selection; and it was only after it had become
clear to him, that it was not the sterile insects themselves but their
parents that were selected, according as they produced more or less
well adapted workers, that he was able to refer to this very case of the
conditions among ants "IN ORDER TO SHOW THE POWER OF NATURAL SELECTION"
("Origin of Species", page 233; see also edition 1, page 242.). He
explains his view by a simple but interesting illustration. Gardeners
have produced, by means of long continued artificial selection, a
variety of Stock, which bears entirely double, and therefore infertile
flowers (Ibid. page 230.). Nevertheless the variety continues to be
reproduced from seed, because in addition to the double and infertile
flowers, the seeds always produce a certain number of single, fertile
blossoms, and these are used to reproduce the double variety. These
single and fertile plants correspond "to the males and females of an
ant-colony, the infertile plants, which are regularly produced in large
numbers, to the neuter workers of the colony."
This illustration is entirely apt, the only difference between the two
cases consisting in the fact that the variation in the flower is not
a useful, but a disadvantageous one, which can only be preserved
by artificial selection on the part of the gardener, while the
transformations that have taken place parallel with the sterility of the
ants are useful, since they procure for the colony an advantage in the
struggle for existence, and they are therefore preserved by
natural selection. Even the sterility itself in this case is not
disadvantageous, since the fertility of the true females has at the same
time considerably increased. We may therefore regard the sterile forms
of ants, which have gradually been adapted in several directions to
varying functions, AS A CERTAIN PROOF that selection really takes place
in the germ-cells of the fathers and mothers of the workers, and that
SPECIAL COMPLEXES OF PRIMORDIA (IDS) are present in the workers and in
the males and females, and these complexes contain the primordia of the
individual parts (DETERMINANTS). But since all living entities vary, the
determinants must also vary, now in a favourable, now in an unfavourable
direction. If a female produces eggs, which contain favourably varying
determinants in the worker-ids, then these eggs will give rise to
workers modified in the favourable direction, and if this happens with
many females, the colony concerned will contain a better kind of worker
than other colonies.
I digress here in order to give an account of the intimate processes,
which, according to my view, take place within the germ-plasm, and which
I have called "GERMINAL SELECTION." These processes are of importance
since they form the roots of variation, which in its turn is the root
of natural selection. I cannot here do more than give a brief outline of
the theory in order to show how the Darwin-Wallace theory of selection
has gained support from it.
With others, I regard the minimal amount of substance which is contained
within the nucleus of the germ-cells, in the form of rods, bands, or
granules, as the GERM-SUBSTANCE or GERM-PLASM, and I call the individual
granules IDS. There is always a multiplicity of such ids present in the
nucleus, either occurring individually, or united in the form of rods
or bands (chromosomes). Each id contains the primary constituents of a
WHOLE individual, so that several ids are concerned in the development
of a new individual.
In every being of complex structure thousands of primary constituents
must go to make up a single id; these I call DETERMINANTS, and I mean
by this name very small individual particles, far below the limits of
microscopic visibility, vital units which feed, grow, and multiply
by division. These determinants control the parts of the developing
embryo,--in what manner need not here concern us. The determinants
differ among themselves, those of a muscle are differently constituted
from those of a nerve-cell or a glandular cell, etc., and every
determinant is in its turn made up of minute vital units, which I
call BIOPHORS, or the bearers of life. According to my view, these
determinants not only assimilate, like every other living unit, but they
VARY in the course of their growth, as every living unit does; they may
vary qualitatively if the elements of which they are composed vary, they
may grow and divide more or less rapidly, and their variations give rise
to CORRESPONDING variations of the organ, cell, or cell-group which they
determine. That they are undergoing ceaseless fluctuations in regard to
size and quality seems to me the inevitable consequence of their unequal
nutrition; for although the germ-cell as a whole usually receives
sufficient nutriment, minute fluctuations in the amount carried to
different parts within the germ-plasm cannot fail to occur.
Now, if a determinant, for instance of a sensory cell, receives for a
considerable time more abundant nutriment than before, it will grow more
rapidly--become bigger, and divide more quickly, and, later, when the
id concerned develops into an embryo, this sensory cell will become
stronger than in the parents, possibly even twice as strong. This is an
instance of a HEREDITARY INDIVIDUAL VARIATION, arising from the germ.
The nutritive stream which, according to our hypothesis, favours the
determinant N by chance, that is, for reasons unknown to us, may remain
strong for a considerable time, or may decrease again; but even in
the latter case it is conceivable that the ascending movement of the
determinant may continue, because the strengthened determinant now
ACTIVELY nourishes itself more abundantly,--that is to say, it attracts
the nutriment to itself, and to a certain extent withdraws it from its
fellow-determinants. In this way, it may--as it seems to me--get into
PERMANENT UPWARD MOVEMENT, AND ATTAIN A DEGREE OF STRENGTH FROM WHICH
THERE IS NO FALLING BACK. Then positive or negative selection sets in,
favouring the variations which are advantageous, setting aside those
which are disadvantageous.
In a similar manner a DOWNWARD variation of the determinants may take
place, if its progress be started by a diminished flow of nutriment. The
determinants which are weakened by this diminished flow will have less
affinity for attracting nutriment because of their diminished strength,
and they will assimilate more feebly and grow more slowly, unless chance
streams of nutriment help them to recover themselves. But, as will
presently be shown, a change of direction cannot take place at EVERY
stage of the degenerative process. If a certain critical stage of
downward progress be passed, even favourable conditions of food-supply
will no longer suffice permanently to change the direction of
the variation. Only two cases are conceivable; if the determinant
corresponds to a USEFUL organ, only its removal can bring back the
germ-plasm to its former level; therefore personal selection removes the
id in question, with its determinants, from the germ-plasm, by causing
the elimination of the individual in the struggle for existence. But
there is another conceivable case; the determinants concerned may be
those of an organ which has become USELESS, and they will then continue
unobstructed, but with exceeding slowness, along the downward path,
until the organ becomes vestigial, and finally disappears altogether.
The fluctuations of the determinants hither and thither may thus be
transformed into a lasting ascending or descending movement; and THIS IS
THE CRUCIAL POINT OF THESE GERMINAL PROCESSES.
This is not a fantastic assumption; we can read it in the fact of the
degeneration of disused parts. USELESS ORGANS ARE THE ONLY ONES WHICH
ARE NOT HELPED TO ASCEND AGAIN BY PERSONAL SELECTION, AND THEREFORE IN
THEIR CASE ALONE CAN WE FORM ANY IDEA OF HOW THE PRIMARY CONSTITUENTS
BEHAVE, WHEN THEY ARE SUBJECT SOLELY TO INTRA-GERMINAL FORCES.
The whole determinant system of an id, as I conceive it, is in a state
of continual fluctuation upwards and downwards. In most cases the
fluctuations will counteract one another, because the passive streams of
nutriment soon change, but in many cases the limit from which a return
is possible will be passed, and then the determinants concerned will
continue to vary in the same direction, till they attain positive or
negative selection-value. At this stage personal selection intervenes
and sets aside the variation if it is disadvantageous, or favours--that
is to say, preserves--it if it is advantageous. Only THE DETERMINANT
OF A USELESS ORGAN IS UNINFLUENCED BY PERSONAL SELECTION, and,
as experience shows, it sinks downwards; that is, the organ that
corresponds to it degenerates very slowly but uninterruptedly till,
after what must obviously be an immense stretch of time, it disappears
from the germ-plasm altogether.
Thus we find in the fact of the degeneration of disused parts the proof
that not all the fluctuations of a determinant return to equilibrium
again, but that, when the movement has attained to a certain strength,
it continues IN THE SAME DIRECTION. We have entire certainty in regard
to this as far as the downward progress is concerned, and we must
assume it also in regard to ascending variations, as the phenomena of
artificial selection certainly justify us in doing. If the Japanese
breeders were able to lengthen the tail feathers of the cock to
six feet, it can only have been because the determinants of the
tail-feathers in the germ-plasm had already struck out a path of
ascending variation, and this movement was taken advantage of by the
breeder, who continually selected for reproduction the individuals in
which the ascending variation was most marked. For all breeding depends
upon the unconscious selection of germinal variations.
Of course these germinal processes cannot be proved mathematically,
since we cannot actually see the play of forces of the passive
fluctuations and their causes. We cannot say how great these
fluctuations are, and how quickly or slowly, how regularly or
irregularly they change. Nor do we know how far a determinant must be
strengthened by the passive flow of the nutritive stream if it is to
be beyond the danger of unfavourable variations, or how far it must be
weakened passively before it loses the power of recovering itself by its
own strength. It is no more possible to bring forward actual proofs in
this case than it was in regard to the selection-value of the initial
stages of an adaptation. But if we consider that all heritable
variations must have their roots in the germ-plasm, and further, that
when personal selection does not intervene, that is to say, in the case
of parts which have become useless, a degeneration of the part, and
therefore also of its determinant must inevitably take place; then we
must conclude that processes such as I have assumed are running their
course within the germ-plasm, and we can do this with as much certainty
as we were able to infer, from the phenomena of adaptation, the
selection-value of their initial stages. The fact of the degeneration
of disused parts seems to me to afford irrefutable proof that the
fluctuations within the germ-plasm ARE THE REAL ROOT OF ALL HEREDITARY
VARIATION, and the preliminary condition for the occurrence of the
Darwin-Wallace factor of selection. Germinal selection supplies the
stones out of which personal selection builds her temples and
palaces: ADAPTATIONS. The importance for the theory of the process of
degeneration of disused parts cannot be over-estimated, especially when
it occurs in sterile animal forms, where we are free from the doubt as
to the alleged LAMARCKIAN FACTOR which is apt to confuse our ideas in
regard to other cases.
If we regard the variation of the many determinants concerned in the
transformation of the female into the sterile worker as having come
about through the gradual transformation of the ids into worker-ids,
we shall see that the germ-plasm of the sexual ants must contain three
kinds of ids, male, female, and worker ids, or if the workers have
diverged into soldiers and nest-builders, then four kinds. We understand
that the worker-ids arose because their determinants struck out a useful
path of variation, whether upward or downward, and that they continued
in this path until the highest attainable degree of utility of the parts
determined was reached. But in addition to the organs of positive or
negative selection-value, there were some which were indifferent as far
as the success and especially the functional capacity of the workers was
concerned: wings, ovarian tubes, receptaculum seminis, a number of the
facets of the eye, perhaps even the whole eye. As to the ovarian tubes
it is possible that their degeneration was an advantage for the workers,
in saving energy, and if so selection would favour the degeneration; but
how could the presence of eyes diminish the usefulness of the workers to
the colony? or the minute receptaculum seminis, or even the wings? These
parts have therefore degenerated BECAUSE THEY WERE OF NO FURTHER VALUE
TO THE INSECT. But if selection did not influence the setting aside of
these parts because they were neither of advantage nor of disadvantage
to the species, then the Darwinian factor of selection is here
confronted with a puzzle which it cannot solve alone, but which at once
becomes clear when germinal selection is added. For the determinants
of organs that have no further value for the organism, must, as we have
already explained, embark on a gradual course of retrograde development.
In ants the degeneration has gone so far that there are no
wing-rudiments present in ANY species, as is the case with so many
butterflies, flies, and locusts, but in the larvae the imaginal discs of
the wings are still laid down. With regard to the ovaries, degeneration
has reached different levels in different species of ants, as has been
shown by the researches of my former pupil, Elizabeth Bickford. In many
species there are twelve ovarian tubes, and they decrease from that
number to one; indeed, in one species no ovarian tube at all is present.
So much at least is certain from what has been said, that in this
case EVERYTHING depends on the fluctuations of the elements of the
germ-plasm. Germinal selection, here as elsewhere, presents the
variations of the determinants, and personal selection favours or
rejects these, or,--if it be a question of organs which have become
useless,--it does not come into play at all, and allows the descending
variation free course.
It is obvious that even the problem of COADAPTATION IN STERILE
ANIMALS can thus be satisfactorily explained. If the determinants are
oscillating upwards and downwards in continual fluctuation, and
varying more pronouncedly now in one direction now in the other, useful
variations of every determinant will continually present themselves
anew, and may, in the course of generations, be combined with one
another in various ways. But there is one character of the determinants
that greatly facilitates this complex process of selection, that,
after a certain limit has been reached, they go on varying in the same
direction. From this it follows that development along a path once
struck out may proceed without the continual intervention of personal
selection. This factor only operates, so to speak, at the beginning,
when it selects the determinants which are varying in the right
direction, and again at the end, when it is necessary to put a check
upon further variation. In addition to this, enormously long periods
have been available for all these adaptations, as the very gradual
transition stages between females and workers in many species plainly
show, and thus this process of transformation loses the marvellous and
mysterious character that seemed at the first glance to invest it,
and takes rank, without any straining, among the other processes of
selection. It seems to me that, from the facts that sterile animal forms
can adapt themselves to new vital functions, their superfluous parts
degenerate, and the parts more used adapt themselves in an ascending
direction, those less used in a descending direction, we must draw
the conclusion that harmonious adaptation here comes about WITHOUT
THE COOPERATION OF THE LAMARCKIAN PRINCIPLE. This conclusion once
established, however, we have no reason to refer the thousands of cases
of harmonious adaptation, which occur in exactly the same way among
other animals or plants, to a principle, the ACTIVE INTERVENTION OF
WHICH IN THE TRANSFORMATION OF SPECIES IS NOWHERE PROVED. WE DO NOT
REQUIRE IT TO EXPLAIN THE FACTS, AND THEREFORE WE MUST NOT ASSUME IT.
The fact of coadaptation, which was supposed to furnish the strongest
argument against the principle of selection, in reality yields the
clearest evidence in favour of it. We MUST assume it, BECAUSE NO OTHER
POSSIBILITY OF EXPLANATION IS OPEN TO US, AND BECAUSE THESE ADAPTATIONS
ACTUALLY EXIST, THAT IS TO SAY, HAVE REALLY TAKEN PLACE. With this
conviction I attempted, as far back as 1894, when the idea of germinal
selection had not yet occurred to me, to make "harmonious adaptation"
(coadaptation) more easily intelligible in some way or other, and so
I was led to the idea, which was subsequently expounded in detail by
Baldwin, and Lloyd Morgan, and also by Osborn, and Gulick as ORGANIC
SELECTION. It seemed to me that it was not necessary that all the
germinal variations required for secondary variations should have
occurred SIMULTANEOUSLY, since, for instance, in the case of the
stag, the bones, muscles, sinews, and nerves would be incited by
the increasing heaviness of the antlers to greater activity in THE
INDIVIDUAL LIFE, and so would be strengthened. The antlers can only have
increased in size by very slow degrees, so that the muscles and bones
may have been able to keep pace with their growth in the individual
life, until the requisite germinal variations presented themselves. In
this way a disharmony between the increasing weight of the antlers and
the parts which support and move them would be avoided, since time would
be given for the appropriate germinal variations to occur, and so to set
agoing the HEREDITARY variation of the muscles, sinews, and bones.
("The Effect of External Influences upon Development", Romanes Lecture,
Oxford, 1894.)
I still regard this idea as correct, but I attribute less importance
to "organic selection" than I did at that time, in so far that I do
not believe that it ALONE could effect complex harmonious adaptations.
Germinal selection now seems to me to play the chief part in bringing
about such adaptations. Something the same is true of the principle
I have called "Panmixia". As I became more and more convinced, in the
course of years, that the LAMARCKIAN PRINCIPLE ought not to be called in
to explain the dwindling of disused parts, I believed that this process
might be simply explained as due to the cessation of the conservative
effect of natural selection. I said to myself that, from the moment in
which a part ceases to be of use, natural selection withdraws its
hand from it, and then it must inevitably fall from the height of its
adaptiveness, because inferior variants would have as good a chance of
persisting as better ones, since all grades of fitness of the part in
question would be mingled with one another indiscriminately. This is
undoubtedly true, as Romanes pointed out ten years before I did, and
this mingling of the bad with the good probably does bring about a
deterioration of the part concerned. But it cannot account for the
steady diminution, which always occurs when a part is in process of
becoming rudimentary, and which goes on until it ultimately disappears
altogether. The process of dwindling cannot therefore be explained as
due to panmixia alone; we can only find a sufficient explanation in
germinal selection.