that the mimics of one model, belonging to different genera, often
secure the same effect in quite different ways. Thus the glass-like
transparency of the wings in the Heliconiidæ of the genus _Methona_
depends on a considerable reduction of the size of the scales, which
ordinarily cover both sides of the wing as thickly as the tiles on a
roof, and produce the colour. In another quite similar species, also
transparent-winged, the Danaid _Ituna ilione_, the transparency is due
to the absence of most of the scales, and in a third mimic, _Castnia
linus_, var. _heliconoides_, the scales are not altered either in size
or number, but have become absolutely unpigmented and transparent.
In a fourth mimic, a Pierid, _Dismorphia crise_, the scales have not
decreased in number, but have become quite minute, while in a fifth
case, the nocturnal _Hyelosia heliconoides_ Swains., the same thing
has happened as in _Castnia_, but the scales are also fewer in number.
Thus in each of the mimics the changes which have taken place in the
scales are quite different, but they bring about the same effect, the
glass-like transparency of the wings, on which the resemblance to the
model depends: what we have before us is, therefore, not a similarity
of variation, but only an appearance of similarity in external features.
In the face of such facts there can be no further question of the often
repeated objection, that the resemblance of model and copy depend on
the similarity of external influences upon species living in the same
latitude, even if that were not already sufficiently refuted by the
frequent restriction of the mimicry to the female. And that mimicry
should be a mere matter of chance is negatived even by the single
fact that model and copy always live in the same area, and that the
local varieties of the model are faithfully followed by the mimic.
An interesting example of this is furnished by _Elymnias undularis_,
already mentioned, for in this case the female (Pl. II, Fig. 23)
mimics the brown-yellow _Danais plexippus_ (Pl. II, Fig. 22), not
wherever _E. undularis_ occurs, but only in Ceylon and British India.
In Burmah, where another Danaid, _D. hegesippus_, is common, it mimics
that; and in Malacca it does not copy a Danaid at all, but resembles
the male of its own species, which in India is very different from
it, since there the female mimics one of the blue iridescent EuplϾ
(Pl. III, Fig. 24). It cannot therefore be a matter of 'chance,' and
we should have to give up all attempt at a scientific interpretation
if we were not prepared to accept that of natural selection. Even the
interference of a purposeful Power can hardly be seriously considered
in this case, even by those who are inclined to such a view, for the
_gradual_ approximation to the model, which is a matter of course in a
process of evolution, could only appear, if referred to the benevolent
intelligence of a Creator, as an unworthy trick, designed to lead
humanity astray in its strivings after knowledge. On the other hand,
this gradual increase of resemblance, which becomes apparent when we
compare several mimetic species--this carrying over, step by step,
from the female to the male--and many other facts point to the working
of natural forces according to law, and, if there is to be found
anywhere in living nature a complicated process of self-regulation,
it certainly lies before us here, clearer and less open to objections
than almost anywhere else. I do not mean to say, however, that we
can verify it statistically in detail, as has been demanded by the
fanatical opponents of natural selection. A direct testing of natural
selection is, as has been already shown, nowhere possible: we can never
exactly estimate how great the advantage is which a species requiring
protection derives from a slight increase in the resemblance to an
immune model; and I for one do not know how we could even definitely
prove that a certain species needed a greater degree of protection than
it had previously enjoyed in order to ensure its persistence in the
struggle. It would be necessary to know the total number of individuals
living on a certain area for many generations. If it appeared that
there was a progressive diminution in the number of individuals, we
should be justified in concluding that the species had not an adequate
power of persistence, and that it therefore required a more effective
protection. But it is impossible for us to collect such exact data
for any species living under natural conditions, although we can
often say approximately that a species is progressively decreasing in
numbers. Even this, however, we can usually do only in cases which
are influenced directly or indirectly by the interference of Man in
nature, and in which the falling off in the species occurs so rapidly
that there is no time for the slow counteractive influence of natural
selection. We shall see later that in this way many species have been
eliminated even within historic times.
I have just spoken of the 'need of protection,' and I have a few
remarks to add on that subject. It is a mistake to believe that every
'rare' species, that is, one represented by few individuals, is
already in process of disappearing. It is not the absolute number of
individuals that determines the survival of a species, but the fact
of the number remaining the same. It is equally mistaken to suppose
that an amelioration of the conditions of existence for any species
by natural selection is possible only when its persistence is already
threatened; that is, when the number of individuals (the 'normal
number') is steadily decreasing. On the contrary, it is of the essence
of natural selection that every favourable variation which crops up is,
_ceteris paribus_, preserved, and becomes the common possession of the
species, quite independently of whether this improvement is absolutely
necessary to its preservation or not. In the latter case it will simply
become a commoner species instead of a rare one; and every species
is, so to speak, striving to become common and widely distributed,
since every advantageous variation that can possibly be produced is
accumulated and made the common property of the species. But this has
its limits, not only in the constitution and the structure of each
species, but also in the external conditions of its life. If a species
of butterfly be restricted, in the caterpillar stage, to a single, rare
species of plant, its normal number will be, and must remain, a small
one. But if there arise within it a variation in the food-instinct
whereby a second and it may be a commoner plant becomes available, then
the normal number of the species will rise, and perhaps the original
number of individuals may be more than doubled. It is, however, by no
means necessary to assume that the species was previously in process of
decadence; on the contrary its normal number may have remained quite
constant.
So, in the case of the mimetic butterflies, we do not need to assume
that they all previously required protection in the sense that they
would have become extinct had they not assumed a likeness to an
immune species. We may indeed conclude, on other grounds, that it
was the rarer species which increased their number of individuals
by the mimetic protection, and in doing so they certainly enhanced
at the same time their chance of survival as a species. In the more
abundant species mimetic resemblance to species whose unpalatability
rendered them immune could not have been evolved, as it would have been
disadvantageous, not only for the model, but for the mimicking species
itself, while in species less rich in individuals, such resemblance
would necessarily have a protective value, no matter whether the
species was in danger of extinction or not. The process of selection
must have started simply because the mimetic individuals survived more
frequently than the others, and the mimetic resemblance must have
gone on increasing as long as the increase brought with it a more
effective protection. It is, therefore, a fallacious objection to say
that a species, whose existence was threatened, would, considering the
slowness of the process of selection, have died out altogether before
it could have acquired effective protection by mimicking an immune
species. The assumption is false--the widespread, hazy idea that the
process of natural selection can only begin when the existence of the
species is threatened. On the contrary, every species utilizes every
possibility of improvement; and every improvement for which variation
supplies the necessary material is possible. The augmentation of the
profitable variations follows as a necessity from the more frequent
survival of the best-adapted individuals, and this 'more frequent
survival' will be not only a relative one, due to the fact that the
better adapted individuals will be less decimated, it will also be
absolute, because more individuals of the species will survive than
before. Of this _Papilio merope_ may serve as an example; in Madagascar
it now flies about only slightly varied from the original form, var.
_meriones_. Here, therefore, the species is maintained, without the aid
of mimetic protection. We do not know if the reason for this lies in
the absence of an immune model, or in the non-appearance of suitable
mimetic variants, or in other conditions; but we know that without
mimicry the species holds its own against its enemies. But if, in
Abyssinia, a female of this butterfly exhibited variations which would
make her resemble, in any degree, the unpalatable _Danias chrysippus_,
these mimetic variants would be less decimated than the original form
of female, and would, therefore, gain stability, and gradually increase
both in mimetic resemblance and in the number of individuals. But is
this any reason why the original form of the female should diminish
in numbers? In itself, certainly not; the red mimetic females could
increase in number without causing any decrease of the yellow ones,
for the red are in no way in conflict with the yellow, and we must not
think of the number of individuals as so fixed for each species that it
cannot increase. On the contrary, it _must_ increase, as soon as the
conditions of existence are permanently improved, and this happens, in
this case, through the mimetic protection of the red female. We can
thus easily understand how mimetic and non-mimetic females can live
side by side in Abyssinia.
In all the rest of Africa, however, there are only mimetic females
of _Papilio merope_, and none of the colour of the male; these last,
therefore, have been crowded out by the mimetic form, not actively, but
through the more frequent survival of the mimetic form, so that those
like the male became gradually rarer, and finally died out--that is,
ceased to occur. The matter is not so simple as it seems, and we shall
best understand it by thinking of the dimorphism of the caterpillars
of our hawk-moths, which we discussed before, in which the green form
in the full-grown caterpillar is less well protected than the brown.
In many species the brown form has crowded out the green, in others
brown and green occur side by side, but the green is less abundant, and
in some species very rare. This must be regarded as the simple result
of the circumstance that a higher percentage of the green than of the
brown caterpillars fall victims to enemies, and thus, in the course of
generations, the green form becomes slowly but steadily rarer. This
will be the case even if the newer and better adaptation raises the
number of individuals (the 'normal number') in the species, for this
increase must always be a limited one, even if it be very great, which
is hardly likely in this case. For the normal number is not determined
by the mortality at one stage, but by that at all the stages of life
taken together. Thus a normal number always persists, notwithstanding
the improved conditions for the species, and, on this assumption, the
form under less favourable conditions cannot permanently hold its own
with that under better conditions, but must gradually disappear. We
can understand, then, that the primitive form of the _Papilio merope_
female may persist even for a long time side by side with the mimetic
form in certain habitats. It is, probably, not a mere chance, that
this should have happened just in Abyssinia, for, in that region, the
mimetic female is still tailed--that is, she has not yet reached the
highest degree of resemblance to her immune model. In the whole of the
rest of Africa the process of the transformation of the female has
already reached its highest point, and on the east and west coasts,
as well as in South Africa, the primitive form of the species is now
represented only by the male.
The gradual dying out of the less favourably conditioned forms of a
species is a law which follows as a logical necessity from the essence
of the process of selection, but its reality may be inferred from the
phenomena themselves. On it depends, as far at least as adaptations are
concerned, the transformation of species.
A beautiful example of the crowding out of a less favoured form of a
species by a more favoured one is afforded by a butterfly of North
America, of which the two female forms have long been known, although
the reason for their dimorphism was not understood. A yellow butterfly,
_Papilio turnus_, not unlike our swallow-tail, has yellow females in
the north and east of the United States, but black ones in the south
and west. There was much guessing as to what the cause of this striking
phenomenon might be, and it was for a time thought that this difference
was directly due to the influence of climate, and, later, the black
form of female was regarded as protectively coloured, because of the
supposed greater persecution by birds in the south, since the female
would be less easily recognized if of a dark colour, and would thus be
better protected. This last explanation could hardly be looked upon as
satisfactory, for a black butterfly in flight would be very easily seen
by sharp-sighted birds; indeed, against a light background, it would be
even more readily seen than a light one.
Since we have acquired a more exact knowledge of the immune species
of _Papilio_ this case has become clear to us. For on those stretches
of country on which the black female of _Papilio turnus_ lives there
occurs another _Papilio_ which is black in both sexes, _Papilio
philenor_, and this is one of those species which are protected by
their unpleasant taste and odour. Here, therefore, we have a case of
mimicry, the female of _Papilio turnus_ imitates the immune _Papilio
philenor_, and thereby secures protection for itself; but as the immune
model only occurs in the southern half of the distribution of _Papilio
turnus_ a somewhat sharp separation of the two forms of female
has been evolved; the black, mimetic form, being the most fit, has
completely crowded the primitive yellow form out of the area inhabited
by _Papilio philenor_, while beyond this area, to the north and west,
the yellow form alone prevails. The extensive and careful studies of
Edwards have shown that the two forms occur together only in a very
narrow transition region.
We thus see that the facts, wherever we scrutinize them carefully,
harmonize with the theory. Of course we can only penetrate to a
certain depth with the theory of selection, and we are still far from
having reached the fundamental causes of the phenomena. Indeed, our
understanding must in the meantime stop short before the causes of
variations and their accumulation, but up to that point the theory
gives us clearness, and discloses the causal connexion of phenomena
in the most beautiful way. Although we do not yet understand how the
southern female _Papilio turnus_ was able to produce the advantageous
black, we do see why a black variation, when it did occur, should
increase and be strengthened, until it crowded out the yellow form from
the area of the immune model, and we are able in a general way to refer
the whole complicated phenomena of mimicry to their proximate causes.
This is true also of other phenomena which have had no part in
establishing the theory, since attention was only directed to them
later, and it is true even of some which, at first sight, seem to
contradict the theory altogether. To this class belongs, for instance,
the phenomenon that immune species not unfrequently mimic each other,
as was first observed among the Heliconiid-like butterflies of South
America. In four different families, the Danaidæ, the Neotropidæ, the
Heliconiidæ, and the Acræidæ, there are species, distributed over the
same area, which resemble each other in their conspicuous colouring
and marking, and also in the peculiar shape of the wings. After what
has been said one might be inclined to regard one of these species as
the unpalatable model and the others as the palatable mimics, but they
are all unpalatable, and are not eaten by birds. The puzzle of this
apparent contradiction was solved by Fritz Müller[4], who pointed out
that the aversion to non-edible butterflies is not innate in birds, but
must be acquired. Each young bird has to learn from experience which
victim is good to eat, and which bad. If every inedible species had
its particular and distinctive colour-dress a considerable number of
individuals of each species would fall victims to the experiments of
young birds in each generation, for a butterfly which has once been
pecked at, or squeezed by the bill of a bird, is doomed to die. But if
two inedible species which resemble each other inhabit the same area
they will be regarded by the birds as one and the same, and if five or
more inedible species resemble each other all five will present the
same appearance to the bird, and it will not require to repeat on the
other four the experience of unpalatability it has gained from one.
Thus the total of five species will be no more severely decimated by
the young birds than each of them would have been if it had occurred
alone; the same number of victims of experiment, which are necessary
every year in the education of the young birds, will, when all five
species look alike, be divided among the whole 'mimicry ring,' as we
may say. The advantage of the resemblance is thus obvious, and we
can understand why a process of selection should develop among such
inedible species which should result in their being readily mistaken
for one another; we can understand why, in the neighbourhood of Fritz
Müller's home, Blumenau, in the province of Santa Catarina in South
Brazil, the Danaidæ, species of _Lycorea_; the Heliconiidæ, _Heliconius
eucrate_ and _Eueides isabella_; and the Neotropinæ, _Mechanitis
lysimnia_ and species of _Melinæa_, should all exhibit the same
colours, brown, black and yellow, in a similar pattern, on similarly
shaped wings. The agreement is by no means perfect in detail, but it
can be noticed in all parts of South America inhabited by species of
these genera, and the same differences which distinguish, for instance,
the two species of _Heliconius_ flying in two different regions,
also distinguish the two species of _Eueides_ and the two species
of _Mechanitis_. In Honduras we find the same mutually protective
company of inedible genera as in Santa Catarina, but represented by
other species, which all differ from the species in Santa Catarina
in the same characters, as, for instance, that they have two instead
of one pale yellow cross-stripe on the anterior wings. The species
are: _Lycorea atergatis_, _Heliconius telchinia_, _Eueides dynastes_,
_Mechanitis doryssus_, and _Melinæa imitata_[5]. In the environs of
Bahia this mimicry ring consists of the following species: _Heliconius
eucrate_, _Lycorea halia_, _Mechanitis lysimnia_, and _Melinæa ethra_,
as figured on Pl. II, Fig. 12, iv, and such a mutual assurance society
has always one or other edible species as mimic. The larger the mimetic
assurance company is, the less harm can mimics do to it. In the case
figured it is two Pieridæ already known to us that have fairly well
assumed the Heliconiid guise, namely, _Dismorphia astynome_ (Pl.
II, Figs. 18 and 19) and _Perhybris pyrrha_ (Pl. II, Figs. 16 and 17).
In the latter of these the male still has, on the upper surface, just
the appearance of one of our common Garden-whites, while the female is
coloured quite like the Heliconiidæ, but without having lost the form
of wing of the Whites. The larger the mimetic company is the greater
will be the protection afforded to its palatable mimics, since they
will be the more rarely seized by way of experiment. It is, of course,
obvious that in this kind of mimicry--that is, in the imitation of an
unpalatable and rejected species for protection--it is presupposed as
a general postulate that the edible mimics are considerably in the
minority, as Darwin showed; for if it were otherwise their enemies
would soon discover that among the apparently unpalatable species there
were some which were pleasant to taste. Here, too, the facts bear out
the theory, although exceptions can easily be imagined, and do seem to
occur.
[4] _Kosmos_, vol. v, 1881, p. 260 onwards.
[5] According to Poulton's report in _Nature_, July 6, 1889, of 'Sykes,
Natural Selection in the Lepidoptera,' _Trans. Manchester Microscop.
Soc._ 1897, p. 54.
PLATE I
FIG
1. PAPILIO MEROPE, MALE, AFRICA.
2. THE SAME SPECIES, ONE FORM OF MIMETIC FEMALE.
3. DANAIS CHRYSIPPUS, AFRICA, IMMUNE MODEL OF FIG. 2.
4. PAPILIO MEROPE, SECOND FORM OF MIMETIC FEMALE, S. AFRICA.
5. AMAURIS NIAVIUS, S. AFRICA, IMMUNE MODEL OF FIG 4.
6. PAPILIO MEROPE, THIRD FORM OF MIMETIC FEMALE, S. AFRICA.
7. AMAURIS ECHERIA, S. AFRICA, IMMUNE MODEL OF FIG. 6.
8. DANAIS ERIPPUS, IMMUNE MODEL OF FIG. 9, CENTRAL N. AMERICA.
9. LIMENITIS ARCHIPPUS, CENTRAL N. AMERICA, MIMICS THE FOREGOING
SPECIES.
10. DANAIS ERIPPUS, (_a_) CATERPILLAR, (_b_) PUPA.
11. LIMENITIS ARCHIPPUS, (_a_) CATERPILLAR, (_b_) PUPA.
_To face Plate I_
[Illustration: PLATE I. LONDON: EDWARD ARNOLD.]
This comparative rarity is true of the imitators of the Heliconiidæ
and their great mimicry ring of unpalatable species, and is very
general. Thus, for instance, there is a series of palatable mimics of
the beautiful blue _EuplϾ_ of the Indo-Malayan region (Pl. III, Figs.
25 and 27), but each of these mimics is rare compared with the hosts
of the blue unpalatable company, for these immune butterflies also
occur in many species, all similar to _Euplœa midamus_ or _binotata_
(Pl. II, Figs. 1 and 3); and the same applies to the mimics of the
Indo-Malayan Danaidæ. There are a great many _Danais_ species, all of
them resembling _Danais vulgaris_ (Pl. III, Fig. 20), which, when they
occur together, form an inedible ring, and this ring is imitated by a
whole series of edible species, each of which is comparatively rare.
And there are no fewer than six species of _Papilio_ which resemble
these Danaids to the point of being easily mistaken for them, while
another rare _Papilio_ effectively copies the iridescence of the blue
_EuplϾ_--a coloration so unusual in the genus that the species has
received the name of _Papilio paradoxus_.