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.[38]
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 calcereous
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 calcereous
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 raindrops 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 cooeperated 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 cooeperation 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 would 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 cooeperation 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."[39]
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_."[40] 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.[41] 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 _biophores_, 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.