ascribed to it by Nägeli; if, as a result of its own growth, it were
compelled to undergo gradual changes, and thus to produce new species,
we should expect that the duration of species, genera, orders, &c.
would be of approximately equal length respectively, at least in forms
of equal structural complexity. The time required by the idioplasm to
undergo such changes as would constitute transformation into a new
species ought to be always the same at equal heights in the scale of
organization, that is, with equal complexity in the molecular structure
of the idioplasm. It appears to me to be a necessary consequence of
Nägeli’s theory that the causes of transformation lie solely in this
molecular structure of the idioplasm. If nothing more than a certain
amount of growth, and consequently a certain period of time during
which the organism reproduces itself with a certain intensity, is
required to produce a change in the idioplasm, then we must conclude
that the alteration in the latter must take place when this certain
amount of growth has been reached, or after this certain period has
elapsed. In other words, the time during which a species exists—from
its origin as a modification of some older species, until its own
transformation into a new one—must be the same in species with the same
degree of organization. But the facts are very far from supporting this
consequence of Nägeli’s theory. The duration of species is excessively
variable: many arise and perish within the limits of a single
geological formation, while others may be restricted to a very small
part of a formation; others again may last through several formations.
It must be admitted that we cannot estimate the exact position of
extinct species in the scale of organization, and the differences may
therefore depend upon differences of organization: or they may be
explained by the supposition that certain species may have become
incapable of transformation, and might, under favourable conditions,
continue to exist for an indefinite period. But this reply would
introduce a new hypothesis in direct antagonism to Nägeli’s theory,
which assumes that the variability of idioplasm takes place as the
consequence of mere growth, and necessarily depends upon molecular
structure. Nägeli himself asserts that the essential substance
(idioplasm) of the descendants of the earliest forms of life is in a
state of perpetual change, which would continue even if the series of
successive generations were indefinitely prolonged[201]. Hence there
can be no rest in the process of change which the idioplasm must
undergo; and this is as true of each single species as it is of the
organic world taken as a whole. We could, perhaps, find shelter in the
insufficiency of our geological knowledge, but the number of
ascertained facts is too great for this to be possible. Thus it is well
known that the genus _Nautilus_ has lasted from Silurian times, through
all the three geological periods, up to the present day: while all its
Silurian allies (_Orthoceras_, _Gomphoceras_, _Goniatites_, &c.) became
extinct at a comparatively early period.
A keen and clever controversialist might still bring forward many
objections against such an argument. I do not therefore place too much
dependence upon the geological facts by themselves, as a disproof of
the self-variability of Nägeli’s idioplasm; for it must be admitted
that the facts are not sufficiently complete for this purpose. For
instance, in the case of _Nautilus_ it might be argued that we do not
know anything about the fossil Cephalopods of pre-Silurian times, and
that it is therefore possible that the above-mentioned allies of
_Nautilus_ may have existed previously for as long a period as that
through which _Nautilus_ has lived in post-Silurian time. However this
may be, it will be at least conceded that the geological facts do not
lend any support to Nägeli’s theory, for we can see no trace of even an
approximately regular succession of forms.
Appendix II. Nägeli’s explanation of adaptation[202].
In order to explain adaptation Nägeli assumes that, under certain
circumstances, external influences may cause slight permanent changes
in the idioplasm. If then such influences act continually in the same
direction during long periods of time, the changes in the idioplasm may
increase to a perceptible amount, i. e. to a degree which makes itself
felt in visible external characters[203]. But such changes alone could
not be considered as adaptations, for the essential character of an
adaptation is that it must be a purposeful change. Nägeli, however,
brings forward the fact that external stimuli often produce their chief
effects at that very part of the organism to which the stimuli
themselves were applied. ‘If the results are detrimental, the organism
attempts to defend itself against the stimulus: a confluence of
nutrient fluid takes place towards the part upon which the stimulus has
acted, and new tissues are formed which restore the integrity of the
organism by replacing the lost structures as far as possible. Thus in
plants the healthy tissues begin to grow actively around the seat of an
injury, tending to close it up, and to afford protection by
impenetrable layers of cork.’ Purposeful reactions of this kind are
certainly common in the organic world, occurring in animals as well as
in plants. Thus in the human body an injury causes a rapid growth of
the surrounding tissues, which leads to the closing-up of the wound;
while in the Salamander even the amputated leg or tail is replaced by
growth. An extreme example of these purposeful reactions is afforded by
the tree-frog (_Hyla_), which is of a light-green colour when seated
upon a light-green leaf, but becomes dark brown when transferred to
dark surroundings. Hence this animal adapts itself to the colour of its
environment, and thus gains protection from its enemies.
Admitting this capability on the part of organisms to react under
certain stimuli in a purposeful manner, the question remains whether
such a power is a primitive original quality belonging to the essential
nature of each organism. The power of changing the colour of the skin
in correspondence with that of the surroundings is not very common in
the animal kingdom. In the frog this power depends upon a highly
complex reflex mechanism. Certain chromatophores in the skin are
connected with nerves[204] which pass to the brain and are there
brought into relation, by means of nerve-cells, with the nervous
centres of the organ of vision. The relation is of such a kind that
strong light falling upon the retina constitutes a stimulus for the
production of an impulse, which is conducted, along the previously
mentioned motor nerves, from the brain to the chromatophores, thus
determining the contraction of these latter and the consequent
appearance of a light-coloured skin. When the strong stimulus (of
light) ceases, the chromatophores expand again, and the skin becomes
dark. That the chromatophores do not themselves react upon the direct
stimulus of light was proved by Lister[205], who showed that blind
frogs do not possess the power of altering their colour in
correspondence with that of their environment. It is quite obvious that
in this case we are not dealing with a primary, but with a secondarily
produced character; and it has yet to be proved that all the purposeful
reactions mentioned by Nägeli are not similarly secondary characters or
adaptations, and thus very far from being primitive qualities of the
organic substance of the forms in which they occur.
I do not by any means doubt that some of the reactions witnessed in
organisms do not depend upon adaptation, but such reactions are not
usually purposeful. Curiously enough, Nägeli mentions the formation of
galls in plants among his instances of purposeful reactions under
external stimuli. I think, however, that it can hardly be maintained
that the galls are of any use to the plant: on the contrary, they may
even be very injurious to it. The gall is only useful to the insect
which it protects and supplies with food. The recent and most excellent
investigations of Adler[206] and of Beyerinck[207] have shown that the
puncture made by the _Cynips_ in depositing its eggs is not the
stimulus which produces the gall, as was formerly believed to be the
case, but that such a stimulus is provided by the larva which developes
from the egg. The presence of this small, actively moving, foreign body
stimulates the tissue of the plant in a definite manner, always
producing a result which is advantageous to the larva and not to the
plant. It would be to the advantage of the latter if it killed the
intruding larva, either enclosing it by woody tissue devoid of
nourishment, or poisoning it by some acrid secretion, or simply
crushing it by the active growth of the surrounding tissues. But
nothing of the kind occurs: in fact an active growth of cells (forming
the so-called ‘Blastem’ of Beyerinck) takes place around the embryo,
while it is still enclosed in the egg-capsule; but the growth is not
such as to crush the embryo, which remains free in the cavity, the
so-called larval chamber, which is formed around it. It would be out of
place to discuss here the question as to how we can conceive that the
plant is thus compelled to produce a growth which is at any rate
indifferent and may be injurious to it; and which, moreover, is exactly
adapted to the needs of its insect-enemy. But it is at all events
obvious that this cannot be an example of a self-protecting reaction
under a stimulus, and that therefore an organism does not always
respond to external stimuli in a manner useful to itself.
But even if we could accept the suggestion that the purposeful reaction
of an organism under stimulation is a primary and not a secondarily
produced character, such a principle would by no means suffice for the
explanation of existing adaptations. Nägeli attempts to explain certain
selected cases of adaptation as the direct results of external stimuli.
He looks upon the thick hairy coat of mammals in arctic regions, and
the winter covering of animals in temperate regions, as a direct
reaction of the skin under the influence of cold. He considers that the
horns, claws, and tusks of animals have arisen directly as reactions
under stimuli applied to certain parts of the surface of the body in
attack and defence[208]. This interpretation is similar to that offered
by Lamarck at the beginning of this century. At first sight such a
suggestion appears to be plausible, for the acquisition of a thick
hairy covering by the mammals of temperate regions is actually
contemporaneous with the cold season of the year. But the question
arises as to whether the production of a larger number of hairs at the
beginning of winter is not merely another instance of a secondary
character, like the assumption of a green colour by the tree-frog under
the stimulus exerted by strong light.
In the case of the hairy coat it is only necessary to produce a larger
number of structures such as had existed previously; but how can it
have been possible for the petals of flowers, with their peculiar and
complex forms, to have been developed from stamens as a direct result
of the insects which visit them in order to obtain pollen and nectar?
How could the creeping of these insects and the small punctures made by
them constitute stimuli for the production of an increased rate of
growth? And how is it possible in any way to explain, by mere increase
in growth, the origin of a structure in which each part has its own
distinct meaning and plays a peculiar part in attracting insects and in
the process of cross-fertilization effected by them? Even if the
manifold peculiarities of form could be explained in this way, how can
such an explanation possibly hold for the colours of flowers? How could
the white colour of flowers which open at night be explained as the
direct result of the creeping of insects? How can the suggestion of
such a cause offer any interpretation of the fact that flowers which
open by day are tinted with various colours, or of the fact that there
is often a bright or highly coloured spot which shows the way to the
hidden nectary?
There are, moreover, a large number of very striking adaptations in
form and colour, for which no stimulus acting directly upon the
organism can be found. Can we imagine that the green caterpillar[209],
plant-bug, or grasshopper, sitting among green surroundings, is thus
exposed to a stimulus which directly produces the green colour in the
skin? Can the walking-stick insect, which resembles a brown twig, be
subject to a transforming stimulus by sitting on such branches or by
looking at them? Or again, if we consider the phenomena of mimicry, how
can one species of butterfly, by flying about with another species,
exercise upon the latter such an influence as to render it similar to
the first in appearance? In many cases of mimicry, the mimicked and the
mimicking species do not even live in the same place, as we see in the
moths, flies, and beetles which resemble in appearance the much-dreaded
wasps.
The interpretation of adaptation is the weak part of Nägeli’s theory,
and it is somewhat remarkable that so acute a thinker should not have
perceived this himself. One very nearly gains the impression that
Nägeli does not wish to understand the theory of natural selection. He
says, for instance, in speaking of the mutual adaptation observable
between the proboscis, the so-called ‘tongue’ of butterflies, and
flowers with tubular corolla[210]:—‘Among the most remarkable and
commonest adaptations observable in the forms of flowers, are the
corollas with long tubes considered in relation to the long “tongues”
of insects, which suck the nectar from the bottom of the long narrow
tubes, and at the same time effect the cross-fertilization of the
plant. Both these arrangements have been gradually developed to their
present degree of complexity—the long-tubed corollas from those without
tubes, and from those with short ones, the long “tongues” from short
ones. Undoubtedly both have been developed at the same rate so that the
length of both sets of structures has always remained the same.’
No objection can be raised against these statements, but Nägeli goes on
to say:—‘But how can such a process of development be explained by the
theory of natural selection, for at each stage in the process the
adaptation was invariably complete. The tube of the corolla and the
“tongue” must have reached, for instance, at a certain time, a length
of 5 or 10 mm. If now the tube of the corolla became longer in some
plants, such an alteration would have been disadvantageous because the
insects would be no longer able to obtain food from them, and would
therefore visit flowers with shorter tubes. Hence, according to the
theory of natural selection, the longer tubes ought to have
disappeared. If on the other hand the “tongue” became longer in some
insects, such a change would be superfluous and should have been given
up, according to the same theory, as unnecessary structural waste. The
simultaneous change in the two structures must, according to the theory
of natural selection, be due to the same principle as that by which
Münchhausen pulled himself out of a bog by means of his own pig-tail.’
But, according to the theory of natural selection, the case appears in
a very different light from that in which it is put by Nägeli. The
flower and the insect do not compete for the greater length of their
respective organs: all through the gradual process, the flower is the
first to lengthen its corolla and the butterfly follows. Their relation
is not like that between a certain species of animal and another which
serves as its prey, where each strives to be the quicker, so that the
speed of both is increased to the greatest possible extent in the
course of generations. Nor do they stand in the same relation as that
obtaining between an insectivorous bird and a certain species of
butterfly which forms its principal food; in such a case two totally
different characters may be continually increased up to their highest
point, e.g. in the butterfly similarity to the dead and fallen leaves
among which it seeks protection when pursued, in the bird keenness of
sight. As long as the latter quality is still capable of increase, so
long will it still be advantageous to any individual butterfly to
resemble the leaf a little more completely than other individuals of
the same species; for it will thus be capable of escaping those birds
which possess a rather keener sight than others. On the other hand, a
bird with rather keener sight will have the greatest chance of catching
the better protected butterflies. It is only in this way that we can
explain the constant production of such extraordinary similarities
between insects and leaves or other parts of plants. At every stage of
growth both the insect and its pursuer are completely adapted to each
other; i.e. they are so far protected and so far successful
respectively, as is necessary to prevent that gradual decrease in the
average number of individuals which would lead to the extermination of
the species[211]. But the fact that there is complete adaptation at
each stage does not prevent the two species from increasing those
qualities of protection and of pursuit upon which they respectively
depend. So far from this being the case, they would be necessarily
compelled to gradually increase these qualities so long as the physical
possibility of improvement remained on both sides. As long as some
birds possessed a rather keener sight than those which previously
existed, so long would those butterflies possess an advantage in which
the resemblance to leaf-veining was more distinct than in others. But
from the moment at which the maximum keenness of eyesight attainable
had been reached, at which therefore all butterflies resembled leaves
so completely that even the birds with the keenest eyesight might fail
to detect them when at rest,—from this very point any further
improvement in the similarity to leaves would cease, because the
advantage to be gained from any such improvement would cease at the
same time.
Such reciprocal intensification of adaptive characters appears to me to
have been one of the most important factors in the transformation of
species: it must have persisted through long series of species during
phylogeny: it must have affected the most diverse parts and characters
in the most diverse groups of organisms.
In certain large butterflies of the Indian and African forests—_Kallima
paralecta_, _K. inachis_, and _K. albofasciata_—it has been frequently
pointed out that the deceptive resemblance to a leaf is so striking
that an observer who has received no hint upon the subject believes
that he sees a leaf, even when he is looking at the butterfly very
closely. The similarity is nevertheless incomplete; for out of sixteen
specimens in the collections at Amsterdam and Leyden, I could not find
a single one which had more than two lateral veins on one side of the
mid-rib of the supposed leaf, or more than three upon the other side;
while about six or seven veins should have been present on each side.
But from two to three lateral veins are amply sufficient to produce a
high degree of resemblance; in fact so much so that it is a matter for
wonder as to how it has been possible for such a relatively perfect
copy to have been produced; or how the sight of birds can have become
so highly developed that while flying rapidly they could perceive the
vein-like markings; or to state the case more accurately, that they
could detect those individuals with a less number of veins than others.
It is possible that the process of increase in resemblance is still
proceeding in the species of the genus _Kallima_; at all events, I was
struck by the rather strong individual differences in the markings of
the supposed leaf.
On the other hand, the cause of the increase in length of the tubular
corolla and of the butterfly’s ‘tongue,’ lies neither in the flower nor
in the butterfly, but it is to be found in those other insects which
visit the flower and steal its honey without being of any assistance in
cross-fertilization. It may be stated shortly, that non-tubular
corollas, with the honey freely exposed—for it must be assumed the
ancestral form was of this kind—gradually developed into corollas with
the honey deeply concealed. The whole process was presumably first
started by the flower, for the gradual withdrawal of the honey to
greater depths conferred the advantage of protection from rain (Hermann
Müller), while larger quantities of honey could be stored up, and this
would also increase the number of insects visiting the flower and
render their visits more certain. As soon as this withdrawal occurred,
the mouth-parts of insects began to be subjected to a selective process
whereby these organs in some of them were lengthened at the same rate
as that at which the honey was withdrawn. When once the process had
begun, its continuance was ensured, for as soon as flower-frequenting
insects were divided into two groups with short and with long
mouth-parts respectively, a further increase in the length of the
corolla-tube necessarily took place in all those flowers which were
especially benefited by the assured visits of a relatively small number
of species of insects, viz., those flowers in which cross-fertilization
was more certainly performed in this way than by the uncertain visits
of a great variety of species. This would imply that a still further
increase in length would take place, for it is obvious that the
cross-fertilization of any flower would be more certainly performed by
an insect when the number of species of plants visited by it became
less; and hence the cross-fertilization would be rendered most certain
when the insect became completely adapted—in size, form, character of
its surface, and the manner in which it obtained the honey—to the
peculiarities of the flower. Those insects which obtain honey from a
great variety of flowers are sure to waste a great part of the pollen
by carrying it to the flowers of many different species, while insects
which can only obtain honey from a few species of plants must
necessarily visit many flowers of the same species one after the other,
and they would therefore more generally distribute the pollen in an
effective manner.
Hence the tube of the corolla, and the ‘tongue’ of the butterfly which
brings about fertilization, would have continued to increase in length
as long as it remained advantageous for the flower to exclude other
less useful visitors, and as long as it was advantageous for the
butterfly to secure the sole possession of the flower. Hence there is
no competition between the flower and the butterfly which fertilizes
it, but between these two on the one side, and the other would-be
visitors of the flower on the other. Further details as to the
advantages which the flower gains by excluding all other visitors, and
the butterfly by being the only visitor of the flower, and also as to
the manifold and elaborate mutual adaptations between insects and
flowers, and as to the advantages and disadvantages which follow from
the concealment of the honey—will be found in Hermann Müller’s[212]
work on the fertilization of flowers, in which all these subjects are
minutely discussed, and are clearly explained in a most admirable
manner.
Appendix III. Adaptations in Plants[213].
It is well known that Christian Conrad Sprengel was the first to
recognise that the forms and colours of flowers are not due to chance,
that they are not the mere sport of nature, and that they are not made
for the enjoyment of man, but that their purpose is to attract insects
for the performance of cross-fertilization. It is also well known that
this discovery—which was made at the end of the last century, and which
caused much excitement at that time—was completely forgotten, and was
brought to light again by Charles Darwin when attacking the same
problem.
In his work entitled ‘The Solution of Nature’s Secret in the Structure
and Fertilization of Flowers’ (‘Das entdeckte Geheimniss der Natur im
Bau und der Befruchtung der Blumen’), published at Berlin, in 1793,
Sprengel showed, in several hundred cases, that the peculiarities in
the structure and colours of flowers were calculated to attract
insects, and to ensure the fertilization of the flowers by their
instrumentality. But it was due to his successor in this line of
investigation that the whole significance of the cross-fertilization
effected by insects was made clear. Darwin[214] showed that in many
cases, although not in all, the intention of nature was to avoid
self-fertilization, and he showed that stronger and more numerous
descendants are produced after cross-fertilization.