there is usually a great abundance of individuals. Thus _Danais
chrysippus_, which is distributed over the whole of Africa, is a very
common butterfly wherever it can live at all; and in North America,
in which country there are only two widely distributed species of
_Danais_, these often occur in enormous numbers. The beautiful large
_Danais erippus_ Cramer (Pl. I, Fig. 8), is distributed over almost all
America, and in many places is not only frequent, but occurs in great
swarms. Usually it peoples the broad, open stretches of the western
prairies of the United States, but when violent winds blow, as they
do there in September especially, the insects are driven together
into the small wooded spots of the prairie, and then they cover the
trees in incredibly large crowds, often so thickly that the leaves are
entirely hidden, and the trees look brown instead of green. Millions of
butterflies go to make up such swarms, which have been observed in many
parts of the United States, even quite in the East, in New Jersey, and
elsewhere.
Considering this extraordinary abundance of the immune species, it
is not surprising that its palatable copy, _Limenitis archippus_
(Pl. I, Fig. 9), should also be widely distributed in North America,
and in many places it is not rare, but even abundant. The enormous
majority of _Danais erippus_ will protect the species which resembles
it so closely, even though it is not rare. Any doubt as to this being
a case of mimicry disappears in face of the fact that, in Florida,
there flies a second very similar but much darker brown North American
_Danais_, and that it is accompanied there by an equally dark variety
of _Limenitis archippus_ (_L. eros_).
To prove the correctness of the hypothesis of an actual process of
selection--which we assume in our interpretation of mimicry--I mean
the assumption that the disguise of the species seeking protection
really deceives the enemy, and thus actually affords protection, I
need only cite the evidence of an acute and experienced entomologist
who was himself deceived by it. Seitz[6], to whom we owe many valuable
biological observations on butterflies, relates that, while he was
collecting in the neighbourhood of the town of Bahia, he was surrounded
by swarms of _Catopsiliæ_, similar to our lemon butterfly, especially
the common _Catopsilia argante_, but he took no notice of these, as he
'had already collected as many of them as he wanted.' It was only when
he saw a pair _in copula_ that he caught them in his net. But to his
extreme surprise he found that he had not caught a _Catopsilia_, but a
butterfly of the family Nymphalidæ, one of those _Anææ_ whose numerous
species are distributed over South America. These _Anææ_ are dark, or
beautifully bright on the upper surface, but on the under side are
leaf-coloured, and one of them bears the name _Anæa opalina_, because
it is quite clear and pale, and of opal-like brilliance. The captive
was nearly related to this species. Seitz was so much surprised
by the discovery that the male, which had quickly detached itself from
the female, escaped him, and he could only make out that, 'as it flew
away, it unfolded dark wings, which certainly bore little resemblance
to those of the lemon butterfly.' In the hope of securing more of
this rare booty he then hunted only for _Catopsilia argante_, without
however securing another coveted specimen--he caught no more _AnϾ_,
which shows that in this case, too, the mimetic species was much rarer.
[6] In citing this observation of Seitz, I do not mean to assert that
there is true mimicry between _Anæa opalina_, or its allied species
in Bahia, and the _Catopsilia_, though I regard this as extremely
probable, because of the marked dimorphism between the male and the
female, in conjunction with the very striking resemblance of the female
to the _Catopsilia_. The example was given only to show how very
deceptive such resemblances may be. To assert with confidence that it
is a case of mimicry we should require to know that _Catopsilia_ is
immune, and on that point we have as yet no information.
PLATE II
FIG.
12-15 REPRESENT A 'MIMICRY-RING' COMPOSED OF FOUR IMMUNE SPECIES
BELONGING TO THREE DIFFERENT FAMILIES AND FOUR DIFFERENT GENERA.
12. HELICONIUS EUCRATE, BAHIA.
13. LYCOREA HALIA, BAHIA.
14. MECHANITIS LYSIMNIA, BAHIA.
15. MELINÆA ETHRA, BAHIA.
16, 17. PERHYBRIS PYRRHA, MALE AND FEMALE, S. AMERICAN 'WHITES'
(PIERIDÆ). THE FEMALE MIMICS AN IMMUNE HELICONIID, WHILE THE MALE
SHOWS ONLY AN INDICATION OF THE MIMETIC COLOURING ON THE UNDER SURFACE.
18, 19. DISMORPHIA ASTYNOME, MALE AND FEMALE, ALSO BELONGING TO THE
FAMILY OF 'WHITES,' AND MIMICKING IMMUNE HELICONIIDS; A WHITE SPOT ON
THE POSTERIOR WING OF THE MALE IS ALL THAT REMAINS OF THE ORIGINAL
'WHITE' COLORATION.
20. ELYMNIAS PHEGEA, W. AFRICA, OF THE FAMILY SATYRIDES, MIMICS THE
FOREGOING SPECIES.
21. ACRÆA GEA, AN IMMUNE W. AFRICAN SPECIES.
22. DANAIS GENUTIA, AN IMMUNE DANAID FROM CEYLON.
23. PLYMNIAS UNDULARIS, FEMALE, ONE OF THE MIMICS OF FIG. 22. THE
MALE, WHICH IS QUITE DIFFERENT, IS FIGURED ON PLATE III (FIG. 24).
_To face Plate II_
[Illustration: PLATE II. LONDON: EDWARD ARNOLD.]
We see, then, that the need for protection in butterflies has a great
influence on their external appearance, especially as regards their
colour and marking. First, because the resting insect frequently has
the visible surfaces sympathetically coloured, and also, because there
are numerous species, indeed whole families, which contain nauseous,
perhaps even actually poisonous, juices, and these have been subject
to a double process of selection, directed towards the increase of the
nauseousness, and at the same time towards acquiring as conspicuous
a dress as possible. Thus the whole surface of these butterflies
became gaily coloured, and often--as in many of the tropical nocturnal
Lepidoptera which fly by day, the Agaristidæ, Euschemidæ, and
Glaucopidæ--quite glaringly bright. We thus understand the striking
or at least readily recognizable colours of the Heliconiidæ, the
Euplœæ, the Danaidæ, and the Acræidæ. Finally, the unpalatable species
influence many others which are edible, since the latter strive to
resemble an immune species; and how considerable the variations and
colour transformations thus induced can be is shown by the Whites of
the genus _Perhybris_ (Pl. II, Figs. 16 and 17) and _Archonias_, in
which the male has wholly or partially retained the primitive dress of
the Whites, and in which, side by side with wholly mimetic species,
other species occur in which both sexes exhibit the garb of the Whites
unaltered. Such cases tell decidedly against the often expressed view
that mimetic species must have had from the outset a great resemblance
to the model; they show rather that very great deviations in form,
but more especially in colour, have been brought about solely by the
necessity for mimetic adaptation, and that they have come about only
slowly and step by step, as the different grades of resemblance to the
model in different species of the same genus clearly show.
Lepidoptera are by no means the only insects which exhibit the
phenomenon of mimicry, nor are insects the only animals in which it
occurs; and unpleasant taste and odour are not the only protective
characters; there are many others, as, for instance, among insects, the
hardness of the chitinous cuticle.
One of the most beautiful examples of mimicry was discovered by
Gerstäcker, not in free nature, but in the entomological collection at
Berlin. There he found beside a green, metallic weevil-beetle, one of
the Pachyrhynchidæ from the Philippines, two other insects with the
same metallic sheen and very similar form of body. They had been put in
beside the weevil as duplicates, but more careful observation showed
that they were delicate Gryllidæ, which mimicked the hard beetles
so deceptively that even the practised eye of the entomologist was
misled by them. Later on it was shown that these Gryllids live in the
Philippines beside the weevils, and even on the same leaves with them,
and that the beetles are protected from the attacks of birds and other
enemies by the extraordinary hardness of their cuticle. The case is
especially remarkable because in general the Gryllidæ have no metallic
shimmer, and the form of body must have been considerably altered to
make them resemble the beetle. The usually broad head of the Gryllids
is in this case narrower, the usually flat wing-covers are arched and
pear-shaped, and the legs have become quite beetle-like. The security
enjoyed by the weevil must be very perfect, for it is mimicked by three
other species of beetle in the Philippines.
Animals can also be protected from attack by the possession of
dangerous weapons. To this class belong insects with poisonous
stings, like the bees, wasps, and ants, and in some degree also the
ichneumon-flies. We cannot wonder, therefore, that these dreaded
species find imitators. In this case it is not of so much importance
that the copy should be rarer than the model, for anything that looks
like a dangerous insect will be avoided, since close investigation is
in this case attended with danger. So we find that hornets, wasps,
and bees are frequently imitated by other insects, by beetles, flies,
and butterflies; and these must derive a certain advantage, even
when the resemblance is only a general one. Many Longicorns, which
visit flowers, are striped black and yellow, like a wasp, and so are
many flies, like the species of _Syrphus_, and so on. The Longicorn
_Necydalis major_ bears a strong resemblance to a large ichneumon-fly;
it has the same long-drawn-out body, the same swellings on the femur
and tibia, the curved antennæ, the glossy brown colour, and its
wing-covers are quite short, leaving the wings free, so that the
deception is very complete.
Bees, too, are sometimes so well imitated that they are hardly to be
distinguished from their mimics, not in flight only, but also when
visiting flowers. The best and commonest mimic of our honey-bee is
a perfectly harmless fly of the same size and colour, the drone-fly
(_Eristalis tenax_). The two are often to be seen together on the
same flowering shrub, as, for instance, in autumn, on the Japanese
buckwheat of our gardens (_Polygonum sieboldii_), both busily seeking
for honey. I once noticed a boy catching the flies with a net in order
to imprison them, but a bee stung him severely in the finger. He
immediately abandoned the chase, and gave up the flies, perceiving the
dangers of confusion. So the animal enemies of _Eristalis_ will often
prefer to leave it in peace rather than run the risk of being stung.
PLATE III
FIG.
24. ELYMNIAS UNDULARIS, MALE OF THE SPECIES OF WHICH THE MIMETIC
FEMALE IS DEPICTED IN FIG. 23.
25. EUPLŒA BINOTATA, IMMUNE INDIAN SPECIES, MIMICKED BY
26. ELYMNIAS LEUCOCYMA, MALE, OF WHICH
27. EUPLŒA MIDAMUS.
28. THE FEMALE MIMICS FAIRLY CLOSELY
29. DANAIS VULGARIS, IMMUNE INDIAN DANAID.
30. ELYMNIAS LAIS, MIMETIC OF THE FOREGOING SPECIES, BUT ONLY ON THE
UPPER SURFACE. THE LOWER SURFACE RETAINS THE ORIGINAL PROTECTIVE
COLOURING REPRESENTING A DECAYING LEAF.
31. TENARIS BIOCULATUS, FROM THE PAPUA REGION.
32. ELYMNIAS AGONDAS, MIMICS THE FOREGOING SPECIES FROM THE SAME
LOCALITY.
_To face Plate III_
[Illustration: PLATE III. LONDON: EDWARD ARNOLD.]
There is still another relation between two species which can be
induced by mimicry--namely, parasitism, when, for instance, the
so-called cuckoo-bees and parasitic humble-bees deceptively resemble
in colour, arrangement of hair, and form of body, the species into
whose nests they smuggle their eggs, to have them brought up at the
expense of the bee or humble-bee in question. In the same way, among
the numerous parasites of ant nests, there are some which copy the ants
themselves, and so secure themselves from molestation, although they
devour the ants' eggs and pupæ. Thus, among the hosts of South American
driver-ants (_Eciton prædator_) there lives a predaceous beetle of the
family Staphylinæ, which has received the name _Mimeciton_ because it
resembles the ant in form and in the nature of the external surface,
though not in colour, which is to be explained by the fact that this
ant has no compound eyes, and is therefore almost blind, or at any rate
cannot see colours.
I should never come to an end were I to attempt to exhibit the great
wealth of observations now available in regard to mimicry. But this
at least may be added, that isolated cases of mimicry have been found
even among Vertebrates. Thus, according to Wallace, the red-and-black
striped poisonous coral snake of South America (_Elaps_) is most
realistically imitated by a non-poisonous snake (_Erythrolampus_) of
the same region. Among birds, Wallace cites a few cases which may be
regarded as mimicry, but none are known among mammals, which is not
to be wondered at when we consider how very much less numerous in
individuals the species are which live together on one area, and how
much less likely it is that two species should be, to begin with, so
near each other in size, habit, and form that the process of natural
selection could bring about a deceptive degree of resemblance.
Without doubt it is among insects that the conditions for mimicry are
especially favourable, partly because of the enormous number of species
which live together and have interrelations on the same area, even in
our latitudes and much more so in the tropics, and also because of
their usually great fecundity, and their rapid multiplication, both of
which are factors favourable to starting and continuing the processes
of natural selection. Furthermore, we have to take into account the
hosts of enemies which depend wholly or in great part on insects for
food, and destroy them in enormous numbers, eliminating them in inverse
proportion to the perfection of their adaptation. Finally, there is the
extreme susceptibility of many insects to injury. This makes it very
desirable that they should have some disguise sufficient to protect
them from even the first attempt at an attack, since that would in many
cases prove fatal.
LECTURE VI
PROTECTIVE ADAPTATIONS IN PLANTS
Protection against large animals--Poisons--Ethereal oils--Spines and
thorns--Sharp and stinging-hairs--Felt-hairs--Position of the thorns:
buckthorn--Tragacanth shrub--Prigana scrub--Alpine shrubs--Protection
against small enemies--Chemical substances--Mechanical protective
arrangements--Raphides--Conclusion.
WE have seen in how many different ways animals are able to adapt
themselves to the conditions of life, both protectively and
aggressively; how they approximate in their colour to that of their
surroundings so that they harmonize with it; how they copy lifeless
objects, or parts of plants, leaves, or twigs, or even mimic, in form
and colour, other animals which are in some way protected. When we
consider that by far the greater number of species find protection in
some degree through their colouring, and often through their form, and
when, at the same time, we remember how different this colouring is in
nearly related species, and even within the same species (dimorphism),
we can scarcely avoid the impression that the forms of life are made of
a plastic material, which, like the sculptor's clay, can be kneaded at
will into almost any desired form.
This impression is corroborated when we turn our attention to plants,
and consider the different ways in which they are able to protect
themselves from the attacks of animals.
That plants stand in need of some protection is obvious enough, since
their leaves and other green parts contain much nourishment, and an
endless army of animals, large and small, depends upon these alone for
sustenance. Indeed, the existence of animals depends altogether on the
occurrence of plants, for carnivorous and saprophytic animals could
only arise after vegetarian forms had been already in existence. But if
the green parts of the plants were left defenceless at the mercy of the
multitude of herbivorous animals, it would not be long before they were
exterminated from the face of the earth, for the animals would devour
unsparingly whatever was within their reach, and, as their increase
does not depend on their ratio of elimination alone, but also on their
fertility, and on their rapidity of multiplication, they would go on
increasing in numbers at the expense of the superabundant nourishment
until the plants on which they depended were themselves consumed.
When we inquire into the means whereby plants evade such a fate we are
astonished at the endless diversity of the devices employed.
Let us consider first of all the menace to plants from the larger
herbivores, from elephants and cattle down to the hare and the
roe-deer; we find that many plants are protected by poisons, which
develop in the sap of their stems, leaves, roots, and fruits. The
juicy and beautifully leaved Belladonna (_Atropa belladonna_) is never
touched by roe-deer, stags, or other herbivores, and the same is true
of the thorn-apple (_Datura stramonium_), the henbane (_Hyoscyamus
niger_), the spotted hemlock (_Conium maculatum_), the danewort of
our woods (_Sambucus ebulus_), and many others; they all contain a
poison. Like the unpalatable butterflies, these unpalatable plants are
also furnished with a warning sign of their undesirability, namely, a
disagreeable odour, perceptible even by man, which scares off animals
from touching them. The development of this through natural selection
presents no very serious difficulty.
But, strangely enough, there are not a few poisonous plants in which
we, at least, are unable to detect any such warning sign. Among these
are the blue aconite (_Aconitum_), the black hellebore (_Helleborus
niger_), the meadow-saffron (_Colchicum autumnale_), species of
Gentian, of spurge (_Euphorbia_), and others. Yet these are avoided by
deer, roe-deer, chamois, hares, and marmots, and our cattle, horses,
and sheep also usually leave them untouched. A case has, however, been
reported from the valley of the Aur, on the lower Rhine, which seems to
contradict this. On the rocky grass-slopes of the valley the poisonous
hellebore (_Helleborus viridis_) grows in great abundance, and the
sheep of that region, which were wont to graze on the slopes, avoided
these plants. But some sheep from another part were imported into the
valley, and these ate the hellebore, with the result that many died.
If these poisonous plants, then, were furnished with a warning sign
such as a disagreeable odour, not perceptible to us, we should have to
assume that the imported sheep had a less acute sense of smell than
the others, which is not impossible in domesticated animals. If there
were no such warning sign, then it must have been not an instinct but a
continuous _tradition_ which prevented the native sheep from touching
the inedible plants.
A more naïve interpretation of nature than that of our day would have
regarded the fragrant ethereal oils developed in the seeds of many
plants, as in those of fennel, cummin, and other Umbelliferous plants,
as a peculiarity designed for the use and profit of man. But these
ethereal substances are obviously a means of protection against the
depredations of seed-eating birds, for a sparrow which was allowed to
eat three or four seeds of cummin died very soon afterwards.
Many plants produce bitter substances in their green parts, and so
secure at least some measure of protection, as is the case with the
majority of mosses, the ferns, and species of _Plantago_ and _Linaria_.
Others, again, deposit silicic acid in their cell-walls, or develop in
addition a very thick epidermis, so that they afford at the best an
unpleasant food, e.g. many grasses, the horse-tails, the rhododendron,
and the bilberry. Others, again (_Alchemilla vulgaris_), have
cup-shaped leaves, which retain rain and dew for a long time, and this
protects them from grazing animals, which are unwilling to touch wet
grass and plants.
Especially widely distributed and diverse is the protection of plants
by sharp thorns and spines. It is extremely interesting to note in how
many different and advantageous ways this armature is disposed.
Obvious at once is the fact that thorns and spines only occur on
those parts which are naturally exposed to attack. Thus we find them
particularly strong in young plants, and on the lower parts of older
ones. The holly, for instance, has crenate, spinose leaves only to the
height to which grazing animals can reach; beyond that the leaves are
smooth-edged and spineless, like those of the camelia. It is almost the
same with some wild pear-trees, which are quite covered with thorns as
long as they are low, but afterwards grow a thornless crown.
Similarly, low bushes, when they are armed with thorns or the like at
all, are covered with them all over, like the rose-bush.
When the leaves of a plant are spinose the spines are disposed on
the parts usually attacked; and thus we understand why the enormous
floating leaves of _Victoria regia_ should have on their under surface
long, pointed spines which, especially at the upturned margin, attain a
length of several inches; it is from water animals--water snails--that
danger threatens them.
Thorns are developed in the most diverse ways. In many of the bushes
on the coast of the Mediterranean true leaves are wanting altogether,
the green branches and twigs being themselves the assimilating parts,
and these are so stiff and rigid, so like some kind of thorn, that they
suffice to scare off any greedy herbivore. Among our own bushes the
Broom (_Spartium scoparium_) may be taken as an example of this class.
In other cases the spines are found on the leaves themselves, but
there is great diversity in their mode of arrangement. In many
tropical plants, such as the Yucca and the Aloe, the point of the
long, reed-shaped leaf is transformed into a spine, and this is the
case in many of our native grasses. Kerner von Marilaun notes that, in
the Southern Alps, two such grasses, _Festuca alpestris_ and _Nardus
stricta_, occur frequently in certain localities, and they prick the
muzzles of the cattle so badly that they return bleeding from the
pasture. This prevents these Alpine runs from being made full use of,
so the grasses are as far as possible extirpated by man, and, curiously
enough, also by the cattle themselves, for they seize the grass at the
base of the tuft with their teeth, pull it out, and let it fall, so
that it withers. Kerner saw thousands of such pieces of turf which had
been pulled up by the cattle lying dried and bleached by the sun on
some of the Alpine grazing grounds in the Tyrolese Stubaithal.
Again, in many plants the whole leaf-edge is transformed into a spiny
wall, which may be enlarged by indentations and lobate projections,
as in the holly, and, in a much higher degree, in the thistles
(_Carduus_), in _Eryngium_, in _Acanthus_, and in many Solanaceæ.
Often, too, there are barbed hooks on the leaf-edge, which work like
a saw; or the leaf-edge, though without spines, may be made sharp by
deposits of silicic acid, as in the sedges, whose sharp edges are
moved to and fro in the mouths of ruminants, and thus injure the
mucous membrane. The hook-bristles of the fig-cactus (_Opuntia_),
which, though small, are abundantly provided with barbs, must also be
mentioned; for they are to be found in great numbers surrounding the
buds of these plants, and most effectively protect them from being
eaten away by animals (Fig. 19).
To this category, too, belong the short, prickly bristles of the
rough-leaved plants, which cover the whole plant as with an overcoat
of sharp needles; of these we may mention the adder's tongue (_Echium
vulgare_), the comfrey (_Symphytum officinale_), and the borage
(_Borago officinalis_).