find the greatest number of such cases. Several of these have been
minutely studied and every detail has been investigated so that it is
difficult to understand how there can still be disbelief in regard to
them. If the many and exact observations which have been carefully
collected and critically discussed for instance by Poulton[47] were
thoroughly studied the arguments which are still frequently urged
against mimicry would be found untenable; we can hardly hope to find
more convincing proof of the actuality of the processes of selection
than these cases put into our hands. The preliminary postulates of the
theory of mimicry have been disputed, for instance, that diurnal
butterflies are persecuted and eaten by birds, but observations
specially directed towards this point in India, Africa, America and
Europe have placed it beyond all doubt. If it were necessary I could
myself furnish an account of my own observations on this point.
In the same way it has been established by experiment and observation
in the field that in all the great regions of distribution there are
butterflies which are rejected by birds and lizards, their chief
enemies, on account of their unpleasant smell or taste. These
butterflies are usually gaily and conspicuously coloured and thus--as
Wallace first interpreted it--are furnished with an easily
recognisable sign: a sign of unpalatableness or _warning colours_. If
they were not thus recognisable easily and from a distance, they would
frequently be pecked at by birds, and then rejected because of their
unpleasant taste; but as it is, the insect-eaters recognise them at
once as unpalatable booty and ignore them. Such _immune_[48] species,
wherever they occur, are imitated by other palatable species, which
thus acquire a certain degree of protection.
It is true that this explanation of the bright, conspicuous colours
is only a hypothesis, but its foundations--unpalatableness, and the
liability of other butterflies to be eaten,--are certain, and its
consequences--the existence of mimetic palatable forms--conform it in
the most convincing manner. Of the many cases now known I select one,
which is especially remarkable, and which has been thoroughly
investigated, _Papilla dardanus_ (_merope_), a large, beautiful,
diurnal butterfly which ranges from Abyssinia throughout the whole of
Africa to the south coast of Cape Colony.
The males of this form are everywhere _almost_ the same in colour and
in form of wings, save for a few variations in the sparse black
markings on the pale yellow ground. But the females occur in several
quite different forms and colourings, and one of these only, the
Abyssinian form, is like the male, while the other three or four are
_mimetic_, that is to say, they copy a butterfly of quite a different
family the Danaids, which are among the _immune_ forms. In each region
the females have thus copied two or three different immune species.
There is much that is interesting to be said in regard to these
species, but it would be out of keeping with the general tenor of this
paper to give details of this very complicated case of polymorphism in
_P. Dardanus_. Anyone who is interested in the matter will find a full
and exact statement of the case in as far as we know it, in Poulton's
_Essays on Evolution_ (pp. 373-375[49]). I need only add that three
different mimetic female forms have been reared from the eggs of a
single female in South Africa. The resemblance of the forms to their
immune models goes so far that even the details of the _local_ forms
of the models are copied by the mimetic species.
It remains to be said that in Madagascar a butterfly,
_Papilio meriones_, occurs, of which both sexes are very similar in
form and markings to the non-mimetic male of _P. dardanus_, so that it
probably represents the ancestor of this latter species.
In face of such facts as these every attempt at another explanation
must fail. Similarly all the other details of the case fulfil the
preliminary postulates of selection, and leave no room for any other
interpretation. That the males do not take on the protective colouring
is easily explained, because they are in general more numerous, and
the females are more important for the preservation of the species,
and must also live longer in order to deposit their eggs. We find the
same state of things in many other species, and in one case (_Elymnias
undularis_) in which the male is also mimetically coloured, it copies
quite a differently coloured immune species from the model followed by
the female. This is quite intelligible when we consider that if there
were _too many_ false immune types, the birds would soon discover that
there were palatable individuals among those with unpalatable warning
colours. Hence the imitation of different immune species by _Papilio
dardanus_!
I regret that lack of space prevents my bringing forward more examples
of mimicry and discussing them fully. But from the case of _Papilio
dardanus_ alone there is much to be learnt which is of the highest
importance for our understanding of transformations. It shows us
chiefly what I once called, somewhat strongly perhaps, _the
omnipotence of natural selection_ in answer to an opponent who had
spoken of its "inadequacy." We here see that one and the same species
is capable of producing four or five different patterns of colouring
and marking; thus the colouring and marking are not, as has often been
supposed, a necessary outcome of the specific nature of the species,
but a true adaptation, which cannot arise as a direct effect of
climatic conditions, but solely through what I may call the sorting
out of the variations produced by the species, according to their
utility. That caterpillars may be either green or brown is already
something more than could have been expected according to the old
conception of species, but that one and the same butterfly should be
now pale yellow, with black; now red with black and pure white; now
deep black with large, pure white spots; and again black with a large
ocheous-yellow spot, and many small white and yellow spots; that in
one sub-species it may be tailed like the ancestral form, and in
another tailless like its Danaid model,--all this shows a far-reaching
capacity for variation and adaptation that we could never have
expected if we did not see the facts before us. How it is possible
that the primary colour-variations should thus be intensified and
combined remains a puzzle even now; we are reminded of the modern
three-colour printing,--perhaps similar combinations of the primary
colours take place in this case; in any case the direction of these
primary variations is determined by the artist whom we know as natural
selection, for there is no other conceivable way in which the model
could affect the butterfly that is becoming more and more like it. The
same climate surrounds all four forms of female; they are subject to
the same conditions of nutrition. Moreover, _Papilio dardanus_ is by
no means the only species of butterfly which exhibits different kinds
of colour-pattern on its wings. Many species of the Asiatic genus
Elymnias have on the upper surface a very good imitation of an immune
Euploeine (Danainae), often with a steel-blue ground-colour, while the
under surface is well concealed when the butterfly is at rest,--thus
there are two kinds of protective coloration each with a different
meaning! The same thing may be observed in many non-mimetic
butterflies, for instance in all our species of Vanessa, in which the
under side shows a grey-brown or brownish-black protective coloration,
but we do not yet know with certainty what may be the biological
significance of the gaily coloured upper surface.
In general it may be said that mimetic butterflies are comparatively
rare species, but there are exceptions, for instance _Limenitis
archippus_ in North America, of which the immune model (_Danaida
plexippus_) also occurs in enormous numbers.
In another mimicry-category the imitators are often more numerous than
the models, namely in the case of the imitation of _dangerous insects_
by harmless species. Bees and wasps are dreaded for their sting, and
they are copied by harmless flies of the genera Eristalis and Syrphus,
and these mimics often occur in swarms about flowering plants without
damage to themselves or to their models; they are feared and are
therefore left unmolested.
In regard also to the _faithfulness of the copy_ the facts are quite
in harmony with the theory, according to which the resemblance must
have arisen and increased _by degrees_. We can recognise this in many
cases, for even now the mimetic species show very _varying degrees of
resemblance_ to their immune model. If we compare, for instance, the
many different imitators of _Danaida chrysippus_ we find that, with
their brownish-yellow ground-colour, and the position and size, and
more or less sharp limitation of their clear marginal spots, they have
reached very different degrees of nearness to their model. Or compare
the female of _Elymnias undularis_ with its model _Danaida genutia_;
there is a general resemblance, but the marking of the Danaida is very
roughly imitated in Elymnias.
Another fact that bears out the theory of mimicry is, that even when
the resemblance in colour-pattern is very great, the _wing-venation_,
which is so constant, and so important in determining the systematic
position of butterflies, is never affected by the variation. The
pursuers of the butterfly have no time to trouble about entomological
intricacies.
I must not pass over a discovery of Poulton's which is of great
theoretical importance--that mimetic butterflies may reach the same
effect by very different means.[50] Thus the glass-like transparency
of the wing of a certain Ithomiine (Methona) and its Pierine mimic
(_Dismorphia orise_) depends on a diminution in the size of the
scales; in the Danaine genus Itune it is due to the fewness of the
scales and in a third imitator, a moth (_Castnia linus var.
heliconoides_) the glass-like appearance of the wing is due neither to
diminution nor to absence of scales, but to their absolute
colourlessness and transparency, and to the fact that they stand
upright. In another moth mimic (Anthomyza) the arrangement of the
transparent scales is normal. Thus it is not some unknown external
influence that has brought about the transparency of the wing in these
five forms, as has sometimes been supposed. Nor is it a hypothetical
_internal_ evolutionary tendency, for all three vary in a different
manner. The cause of this agreement can only lie in selection, which
preserves and intensifies in each species the favourable variations
that present themselves. The great faithfulness of the copy is
astonishing in these cases, for it is not _the whole_ wing which is
transparent; certain markings are black in colour, and these contrast
sharply with the glass-like ground. It is obvious that the pursuers of
these butterflies must be very sharp-sighted, for otherwise the
agreement between the species could never have been pushed so far. The
less the enemies see and observe, the more defective must the
imitation be, and if they had been blind, no visible resemblance
between the species which required protection could ever have arisen.
A seemingly irreconcilable contradiction to the mimicry theory is
presented in the following cases, which were known to Bates, who,
however, never succeeded in bringing them into line with the principle
of mimicry.
In South America there are, as we have already said, many mimics of
the immune Ithomiinae (or as Bates called them Heliconidae). Among
these there occur not merely species which are edible, and thus
require the protection of a disguise, but others which are rejected on
account of their unpalatableness. How could the Ithomiine dress have
developed in their case, and of what use is it, since the species
would in any case be immune? In Eastern Brazil, for instance, there
are four butterflies, which bear a most confusing resemblance to one
another in colour, marking, and form of wing, and all four are
unpalatable to birds. They belong to four different genera and three
sub-families, and we have to inquire: Whence came this resemblance and
what end does it serve? For a long time no satisfactory answer could
be found, but Fritz Mueller,[51] seventeen years after Bates, offered a
solution to the riddle, when he pointed out that young birds could not
have an instinctive knowledge of the unpalatableness of the
Ithomiines, but must learn by experience which species were edible and
which inedible. Thus each young bird must have tasted at least one
individual of each inedible species and discovered its unpalatability,
before it learnt to avoid, and thus to spare the species. But if the
four species resemble each other very closely the bird will regard
them all as of the same kind, and avoid them all. Thus there developed
a process of selection which resulted in the survival of the
Ithomiine-like individuals, and in so great an increase of resemblance
between the four species, that they are difficult to distinguish one
from another even in a collection. The advantage for the four species,
living side by side as they do e.g. in Bahia, lies in the fact that
only one individual from the _mimicry-ring_ ("inedible association")
need be tasted by a young bird, instead of at least four individuals,
as would otherwise be the case. As the number of young birds is great,
this makes a considerable difference in the ratio of elimination. The
four Brazilian species are _Lycorea halia_ (Danainae), _Heliconius
narcaea_ (_eucrate_) (Heliconinae), _Melinaea ethra_, and _Mechanitis
lysimnia_ (Ithomiinae).
These interesting mimicry-rings (trusts), which have much significance
for the theory, have been the subject of numerous and careful
investigations, and at least their essential features are now fully
established. Mueller took for granted, without making any
investigations, that young birds only learn by experience to
distinguish between different kinds of victims. But Lloyd Morgan's[52]
experiments with young birds proved that this is really the case, and
at the same time furnished an additional argument against the
_Lamarckian principle_.
In addition to the mimicry-rings first observed in South America,
others have been described from Tropical India by Moore, and by
Poulton and Dixey from Africa, and we may expect to learn many more
interesting facts in this connection. Here again the preliminary
postulates of the theory are satisfied. And how much more that would
lead to the same conclusion might be added!
As in the case of mimicry many species have come to resemble one
another through processes of selection, so we know whole classes of
phenomena in which plants and animals have become adapted to one
another, and have thus been modified to a considerable degree. I refer
particularly to the relation between flowers and insects. Darwin has
shown that the originally inconspicuous blossoms of the phanerogams
were transformed into flowers through the visits of insects, and that,
conversely, several large orders of insects have been gradually
modified by their association with flowers, especially as regards the
parts of their body actively concerned. Bees and butterflies in
particular have become what they are through their relation to
flowers. In this case again all that is apparently contradictory to
the theory can, on closer investigation, be beautifully interpreted in
corroboration of it. Selection can give rise only to what is of use to
the organism actually concerned, never to what is of use to some other
organism, and we must therefore expect to find that in flowers only
characters of use to _themselves_ have arisen, never characters which
are of use to insects only, and conversely that in the insects
characters useful to them and not merely to the plants would have
originated. For a long time it seemed as if an exception to this rule
existed in the case of the fertilisation of the yucca blossoms by a
little moth, _Pronuba yuccasella_. This little moth has a
sickle-shaped appendage to its mouth-parts which occurs hi no other
Lepidopteron, and which is used for pushing the yellow pollen into the
opening of the pistil, thus fertilising the flower. Thus it appears as
if a new structure, which is useful only to the plant, has arisen in
the insect. But the difficulty is solved as soon as we learn that the
moth lays its eggs in the fruit-buds of the Yucca, and that the
larvae, when they emerge, feed on the developing seeds. In effecting
the fertilisation of the flower the moth is at the same time making
provision for its own offspring, since it is only after fertilisation
that the seeds begin to develop. There is thus nothing to prevent our
referring this structural adaptation in _Pronuba yuccasella_ to
processes of selection, which have gradually transformed the maxillary
palps of the female into the sickle-shaped instrument for collecting
the pollen, and which have at the same time developed in the insect
the instinct to press the pollen into the pistil.
In this domain, then, the theory of selection finds nothing but
corroboration, and it would be impossible to substitute for it any
other explanation, which now that the facts are so well known, could
be regarded as a serious rival to it. That selection is a factor, and
a very powerful factor in the evolution of organisms, can no longer be
doubted. Even although we cannot bring forward formal proofs of it _in
detail_, cannot calculate definitely the size of the variations which
present themselves, and their selection-value, cannot, in short,
reduce the whole process to a mathematical formula, yet we must assume
selection, because it is the only possible explanation applicable to
whole classes of phenomena, and because, on the other hand, it is made
up of factors which we know can be proved actually to exist, and
which, _if_ they exist, must of logical necessity cooeperate in the
manner required by the theory. _We must accept it because the
phenomena of evolution and adaptation must have a natural basis, and
because it is the only possible explanation of them._[53]
Many people are willing to admit that selection explains adaptations,
but they maintain that only a part of the phenomena are thus
explained, because everything does not depend upon adaptation. They
regard adaptation as, so to speak, a special effort on the part of
Nature, which she keeps in readiness to meet particularly difficult
claims of the external world on organisms. But if we look at the
matter more carefully we shall find that adaptations are by no means
exceptional, but that they are present everywhere in such enormous
numbers, that it would be difficult in regard to any structure
whatever, to prove that adaptation had _not_ played a part in its
evolution.
How often has the senseless objection been urged against selection
that it can create nothing, it can only reject. It is true that it
cannot create either the living substance or the variations of it;
both must be given. But in rejecting one thing it preserves another,
intensifies it, combines it, and in this way _creates_ what is new.
_Everything_ in organisms depends on adaptation; that is to say,
everything must be admitted through the narrow door of selection,
otherwise it can take no part in the building up of the whole. But, it
is asked, what of the direct effect of external conditions,
temperature, nutrition, climate and the like? Undoubtedly these can
give rise to variations, but they too must pass through the door of
selection, and if they cannot do this they are rejected, eliminated
from the constitution of the species.
It may, perhaps, be objected that such external influences are often
of a compelling power, and that every animal must submit to them, and
that thus selection has no choice and can neither select nor reject.
There may be such cases; let us assume for instance that the effect
of the cold of the Arctic regions was to make all the mammals become
black; the result would be that they would all be eliminated by
selection, and that no mammals would be able to live there at all. But
in most cases a certain percentage of animals resists these strong
influences, and thus selection secures a foothold on which to work,
eliminating the unfavourable variation, and establishing a useful
colouring, consistent with what is required for the maintenance of the
species.
Everything depends upon adaptation! We have spoken much of adaptation
in colouring, in connection with the examples brought into prominence
by Darwin, because these are conspicuous, easily verified, and at the
same time convincing for the theory of selection. But is it only
desert and polar animals whose colouring is determined through
adaptation? Or the leaf-butterflies, and the mimetic species, or the
terrifying markings, and "warning-colours" and a thousand other kinds
of sympathetic colouring? It is, indeed, never the colouring alone
which makes up the adaptation; the structure of the animal plays a
part, often a very essential part, in the protective disguise, and
thus _many_ variations may cooperate towards _one_ common end. And it
is to be noted that it is by no means only external parts that are
changed; internal parts are _always_ modified at the same time--for
instance, the delicate elements of the nervous system on which depend
the _instinct_ of the insect to hold its wings, when at rest, in a
perfectly definite position, which, in the leaf-butterfly, has the
effect of bringing the two pieces on which the marking occurs on the
anterior and posterior wing into the same direction, and thus
displaying as a whole the fine curve of the midrib on the seeming
leaf. But the wing-holding instinct is not regulated in the same way
in all leaf-butterflies; even our indigenous species of Vanessa, with
their protective ground-colouring, have quite a distinctive way of
holding their wings so that the greater part of the anterior wing is
covered by the posterior when the butterfly is at rest. But the
protective colouring appears on the posterior wing and on the tip of
the anterior, _to precisely the distance to which it is left
uncovered_. This occurs, as Standfuss has shown, in different degrees
in our two most nearly allied species, the uncovered portion being
smaller in _V. urticae_ than in _V. polychloros_. In this case, as in
most leaf-butterflies, the holding of the wing was probably the
primary character; only after that was thoroughly established did the
protective marking develop. In any case, the instinctive manner of
holding the wings is associated with the protective colouring, and
must remain as it is if the latter is to be effective. How greatly
instincts may change, that is to say, may be adapted, is shown by the
case of the Noctuid "shark" moth, _Xylina vetusta_. This form bears a
most deceptive resemblance to a piece of rotten wood, and the
appearance is greatly increased by the modification of the innate
impulse to flight common to so many animals, which has here been
transformed into an almost contrary instinct. This moth does not fly
away from danger, but "feigns death," that is, it draws antennae, legs
and wings close to the body, and remains perfectly motionless. It may
be touched, picked up, and thrown down again, and still it does not
move. This remarkable instinct must surely have developed
simultaneously with the wood-colouring; at all events, both
cooeperating variations are now present, and prove that both the
external and the most minute internal structure have undergone a
process of adaptation.
The case is the same with all structural variations of animal parts,
which are not absolutely insignificant. When the insects acquired
wings they must also have acquired the mechanism with which to move
them--the musculature, and the nervous apparatus necessary for its
automatic regulation. All instincts depend upon compound reflex
mechanisms and are just as indispensable as the parts they have to set
in motion, and all may have arisen through processes of selection if
the reasons which I have elsewhere given for this view are
correct.[54]