THE SPECIAL COLOURS OF PLANTS: THEIR ORIGIN AND PURPOSE
The general colour relations of plants--Colours of fruits--The
meaning of nuts--Edible or attractive fruits--The colours of
flowers--Modes of securing cross-fertilisation--The
interpretation of the facts--Summary of additional facts bearing
on insect fertilisation--Fertilisation of flowers by
birds--Self-fertilisation of flowers--Difficulties and
contradictions--Intercrossing not necessarily
advantageous--Supposed evil results of close interbreeding--How
the struggle for existence acts among flowers--Flowers the
product of insect agency--Concluding remarks on colour in
nature.
The colours of plants are both less definite and less complex than are
those of animals, and their interpretation on the principle of utility
is, on the whole, more direct and more easy. Yet here, too, we find that
in our investigation of the uses of the various colours of fruits and
flowers, we are introduced to some of the most obscure recesses of
nature's workshop, and are confronted with problems of the deepest
interest and of the utmost complexity.
So much has been written on this interesting subject since Mr. Darwin
first called attention to it, and its main facts have become so
generally known by means of lectures, articles, and popular books, that
I shall give here a mere outline sketch, for the purpose of leading up
to a discussion of some of the more fundamental problems which arise out
of the facts, and which have hitherto received less attention than they
deserve.
_The General Colour Relations of Plants._
The green colour of the foliage of leafy plants is due to the existence
of a substance called chlorophyll, which is almost universally developed
in the leaves under the action of light. It is subject to definite
chemical changes during the processes of growth and of decay, and it is
owing to these changes that we have the delicate tints of spring
foliage, and the more varied, intense, and gorgeous hues of autumn. But
these all belong to the class of intrinsic or normal colours, due to the
chemical constitution of the organism; as colours they are unadaptive,
and appear to have no more relation to the wellbeing of the plants
themselves than have the colours of gems and minerals. We may also
include in the same category those algae and fungi which have bright
colours--the "red snow" of the arctic regions, the red, green, or purple
seaweeds, the brilliant scarlet, yellow, white, or black agarics, and
other fungi. All these colours are probably the direct results of
chemical composition or molecular structure, and, being thus normal
products of the vegetable organism, need no special explanation from our
present point of view; and the same remark will apply to the varied
tints of the bark of trunks, branches, and twigs, which are often of
various shades of brown and green, or even vivid reds or yellows.
There are, however, a few cases in which the need of protection, which
we have found to be so important an agency in modifying the colours of
animals, has also determined those of some of the smaller members of the
vegetable kingdom. Dr. Burchell found a mesembryanthomum in South Africa
like a curiously shaped pebble, closely resembling the stones among
which it grew;[136] and Mr. J.P. Mansel Weale states that in the same
country one of the Asclepiadeae has tubers growing above ground among
stones which they exactly resemble, and that, when not in leaf, they are
for this reason quite invisible.[137] It is clear that such resemblances
must be highly useful to these plants, inhabiting an arid country
abounding in herbivorous mammalia, which, in times of drought or
scarcity, will devour everything in the shape of a fleshy stem or tuber.
True mimicry is very rare in plants, though adaptation to like
conditions often produces in foliage and habit a similarity that is
deceiving. Euphorbias growing in deserts often closely resemble cacti.
Seaside plants and high alpine plants of different orders are often much
alike; and innumerable resemblances of this kind are recorded in the
names of plants, as Veronica epacridea (the veronica like an epacris),
Limnanthemum nymphaeoides (the limnanthemum like a nymphaea), the
resembling species in each case belonging to totally distinct families.
But in these cases, and in most others that have been observed, the
essential features of true mimicry are absent, inasmuch as the one plant
cannot be supposed to derive any benefit from its close resemblance to
the other, and this is still more certain from the fact that the two
species usually inhabit different localities. A few cases exist,
however, in which there does seem to be the necessary accordance and
utility. Mr. Mansel Weale mentions a labiate plant (Ajuga ophrydis), the
only species of the genus Ajuga in South Africa, which is strikingly
like an orchid of the same country; while a balsam (Impatiens capensis),
also a solitary species of the genus in that country, is equally like an
orchid, growing in the same locality and visited by the same insects. As
both these genera of plants are specialised for insect fertilisation,
and both of the plants in question are isolated species of their
respective genera, we may suppose that, when they first reached South
Africa they were neglected by the insects of the country; but, being
both remotely like orchids in form of flower, those varieties that
approached nearest to the familiar species of the country were visited
by insects and cross-fertilised, and thus a closer resemblance would at
length be brought about. Another case of close general resemblance, is
that of our common white dead-nettle (Lamium album) to the
stinging-nettle (Urtica dioica); and Sir John Lubbock thinks that this
is a case of true mimicry, the dead-nettle being benefited by being
mistaken by grazing animals for the stinging-nettle.[138]
_Colours of Fruits._
It is when we come to the essential parts of plants on which their
perpetuation and distribution depends, that we find colour largely
utilised for a distinct purpose in flowers and fruits. In the former we
find attractive colours and guiding marks to secure cross-fertilisation
by insects; in the latter attractive or protective coloration, the first
to attract birds or other animals when the fruits are intended to be
eaten, the second to enable them to escape being eaten when it would be
injurious to the species. The colour phenomena of fruits being much the
most simple will be considered first.
The perpetuation and therefore the very existence of each species of
flowering plant depend upon its seeds being preserved from destruction
and more or less effectually dispersed over a considerable area. The
dispersal is effected either mechanically or by the agency of animals.
Mechanical dispersal is chiefly by means of air-currents, and large
numbers of seeds are specially adapted to be so carried, either by being
clothed with down or pappus, as in the well-known thistle and dandelion
seeds; by having wings or other appendages, as in the sycamore, birch,
and many other trees; by being thrown to a considerable distance by the
splitting of the seed-vessel, and by many other curious devices.[139]
Very large numbers of seeds, however, are so small and light that they
can be carried enormous distances by gales of wind, more especially as
most of this kind are flattened or curved, so as to expose a large
surface in proportion to their weight. Those which are carried by
animals have their surfaces, or that of the seed-vessel, armed with
minute hooks, or some prickly covering which attaches itself to the hair
of mammalia or the feathers of birds, as in the burdock, cleavers, and
many other species. Others again are sticky, as in Plumbago europaea,
mistletoe, and many foreign plants.
All the seeds or seed-vessels which are adapted to be dispersed in any
of these ways are of dull protective tints, so that when they fall on
the ground they are almost indistinguishable; besides which, they are
usually small, hard, and altogether unattractive, never having any
soft, juicy pulp; while the edible seeds often bear such a small
proportion to the hard, dry envelopes or appendages, that few animals
would care to eat them.
_The Meaning of Nuts._
There is, however, another class of fruits or seeds, usually termed
nuts, in which there is a large amount of edible matter, often very
agreeable to the taste, and especially attractive and nourishing to a
large number of animals. But when eaten, the seed is destroyed and the
existence of the species endangered. It is evident, therefore, that it
is by a kind of accident that these nuts are eatable; and that they are
not intended to be eaten is shown by the special care nature seems to
have taken to conceal or to protect them. We see that all our common
nuts are green when on the tree, so as not easily to be distinguished
from the leaves; but when ripe they turn brown, so that when they fall
on to the ground they are equally indistinguishable among the dead
leaves and twigs, or on the brown earth. Then they are almost always
protected by hard coverings, as in hazel-nuts, which are concealed by
the enlarged leafy involucre, and in the large tropical brazil-nuts and
cocoa-nuts by such a hard and tough case as to be safe from almost every
animal. Others have an external bitter rind, as in the walnut; while in
the chestnuts and beech-nuts two or three fruits are enclosed in a
prickly involucre.
Notwithstanding all these precautions, nuts are largely devoured by
mammalia and birds; but as they are chiefly the product of trees or
shrubs of considerable longevity, and are generally produced in great
profusion, the perpetuation of the species is not endangered. In some
cases the devourers of nuts may aid in their dispersal, as they probably
now and then swallow the seed whole, or not sufficiently crushed to
prevent germination; while squirrels have been observed to bury nuts,
many of which are forgotten and afterwards grow in places they could not
have otherwise reached.[140] Nuts, especially the larger kinds which are
so well protected by their hard, nearly globular cases, have their
dispersal facilitated by rolling down hill, and more especially by
floating in rivers and lakes, and thus reaching other localities. During
the elevation of land areas this method would be very effective, as the
new land would always be at a lower level than that already covered with
vegetation, and therefore in the best position for being stocked with
plants from it.
The other modes of dispersal of seeds are so clearly adapted to their
special wants, that we feel sure they must have been acquired by the
process of variation and natural selection. The hooked and sticky seeds
are always those of such herbaceous plants as are likely, from their
size, to come in contact with the wool of sheep or the hair of cattle;
while seeds of this kind never occur on forest trees, on aquatic plants,
or even on very dwarf creepers or trailers. The winged seed-vessels or
seeds, on the other hand, mostly belong to trees and to tall shrubs or
climbers. We have, therefore, a very exact adaptation to conditions in
these different modes of dispersal; while, when we come to consider
individual cases, we find innumerable other adaptations, some of which
the reader will find described in the little work by Sir John Lubbock
already referred to.
_Edible or Attractive Fruits._
It is, however, when we come to true fruits (in a popular sense) that we
find varied colours evidently intended to attract animals, in order that
the fruits may be eaten, while the seeds pass through the body
undigested and are then in the fittest state for germination. This end
has been gained in a great variety of ways, and with so many
corresponding adaptations as to leave no doubt as to the value of the
result. Fruits are pulpy or juicy, and usually sweet, and form the
favourite food of innumerable birds and some mammals. They are always
coloured so as to contrast with the foliage or surroundings, red being
the most common as it is certainly the most conspicuous colour, but
yellow, purple, black, or white being not uncommon. The edible portion
of fruits is developed from different parts of the floral envelopes, or
of the ovary, in the various orders and genera. Sometimes the calyx
becomes enlarged and fleshy, as in the apple and pear tribe; more often
the integuments of the ovary itself are enlarged, as in the plum, peach,
grape, etc.; the receptacle is enlarged and forms the fruit of the
strawberry; while the mulberry, pineapple, and fig are examples of
compound fruits formed in various ways from a dense mass of flowers.
In all cases the seeds themselves are protected from injury by various
devices. They are small and hard in the strawberry, raspberry, currant,
etc., and are readily swallowed among the copious pulp. In the grape
they are hard and bitter; in the rose (hip) disagreeably hairy; in the
orange tribe very bitter; and all these have a smooth, glutinous
exterior which facilitates their being swallowed. When the seeds are
larger and are eatable, they are enclosed in an excessively hard and
thick covering, as in the various kinds of "stone" fruit (plums,
peaches, etc.), or in a very tough core, as in the apple. In the nutmeg
of the Eastern Archipelago we have a curious adaptation to a single
group of birds. The fruit is yellow, somewhat like an oval peach, but
firm and hardly eatable. This splits open and shows the glossy black
covering of the seed or nutmeg, over which spreads the bright scarlet
arillus or "mace," an adventitious growth of no use to the plant except
to attract attention. Large fruit pigeons pluck out this seed and
swallow it entire for the sake of the mace, while the large nutmeg
passes through their bodies and germinates; and this has led to the wide
distribution of wild nutmegs over New Guinea and the surrounding
islands.
In the restriction of bright colour to those edible fruits the eating of
which is beneficial to the plant, we see the undoubted result of natural
selection; and this is the more evident when we find that the colour
never appears till the fruit is ripe--that is, till the seeds within it
are fully matured and in the best state for germination. Some
brilliantly coloured fruits are poisonous, as in our bitter-sweet
(Solanum dulcamara), cuckoo-pint (Arum) and the West Indian manchineel.
Many of these are, no doubt, eaten by animals to whom they are harmless;
and it has been suggested that even if some animals are poisoned by them
the plant is benefited, since it not only gets dispersed, but finds, in
the decaying body of its victim, a rich manure heap.[141] The particular
colours of fruits are not, so far as we know, of any use to them other
than as regards conspicuousness, hence a tendency to _any_ decided
colour has been preserved and accumulated as serving to render the fruit
easily visible among its surroundings of leaves or herbage. Out of 134
fruit-bearing plants in Mongredien's _Trees and Shrubs_, and Hooker's
_British Flora_, the fruits of no less than sixty-eight, or rather more
than half, are red, forty-five are black, fourteen yellow, and seven
white. The great prevalence of red fruits is almost certainly due to
their greater conspicuousness having favoured their dispersal, though it
may also have arisen in part from the chemical changes of chlorophyll
during ripening and decay producing red tints as in many fading leaves.
Yet the comparative scarcity of yellow in fruits, while it is the most
common tint of fading leaves, is against this supposition.
There are, however, a few instances of coloured fruits which do not seem
to be intended to be eaten; such are the colocynth plant (Cucumis
colocynthus), which has a beautiful fruit the size and colour of an
orange, but nauseous beyond description to the taste. It has a hard
rind, and may perhaps be dispersed by being blown along the ground, the
colour being an adventitious product; but it is quite possible,
notwithstanding its repulsiveness to us, that it may be eaten by some
animals. With regard to the fruit of another plant, Calotropis procera,
there is less doubt, as it is dry and full of thin, flat-winged seeds,
with fine silky filaments, eminently adapted for wind-dispersal; yet it
is of a bright yellow colour, as large as an apple, and therefore very
conspicuous. Here, therefore, we seem to have colour which is a mere
byproduct of the organism and of no use to it; but such cases are
exceedingly rare, and this rarity, when compared with the great
abundance of cases in which there is an obvious purpose in the colour,
adds weight to the evidence in favour of the theory of the attractive
coloration of edible fruits in order that birds and other animals may
assist in their dispersal. Both the above-named plants are natives of
Palestine and the adjacent arid countries.[142]
_The Colours of Flowers._
Flowers are much more varied in their colours than fruits, as they are
more complex and more varied in form and structure; yet there is some
parallelism between them in both respects. Flowers are frequently
adapted to attract insects as fruits are to attract birds, the object
being in the former to secure cross-fertilisation, in the latter
dispersal; while just as colour is an index of the edibility of fruits
which supply pulp or juice to birds, so are the colours of flowers an
indication of the presence of nectar or of pollen which are devoured by
insects.
The main facts and many of the details, as to the relation of insects to
flowers, were discovered by Sprengel in 1793. He noticed the curious
adaptation of the structure of many flowers to the particular insects
which visit them; he proved that insects do cross-fertilise flowers, and
he believed that this was the object of the adaptations, while the
presence of nectar and pollen ensured the continuance of their visits;
yet he missed discovering the _use_ of this cross-fertilisation. Several
writers at a later period obtained evidence that cross-fertilisation of
plants was a benefit to them; but the wide generality of this fact and
its intimate connection with the numerous and curious adaptations
discovered by Sprengel, was first shown by Mr. Darwin, and has since
been demonstrated by a vast mass of observations, foremost among which
are his own researches on orchids, primulas, and other plants.[143]
By an elaborate series of experiments carried on for many years Mr.
Darwin demonstrated the great value of cross-fertilisation in increasing
the rapidity of growth, the strength and vigour of the plant, and in
adding to its fertility. This effect is produced immediately, not as he
expected would be the case, after several generations of crosses. He
planted seeds from cross-fertilised and self-fertilised plants on two
sides of the same pot exposed to exactly similar conditions, and in most
cases the difference in size and vigour was amazing, while the plants
from cross-fertilised parents also produced more and finer seeds. These
experiments entirely confirmed the experience of breeders of animals
already referred to (p. 160), and led him to enunciate his famous
aphorism, "Nature abhors perpetual self-fertilisation".[144] In this
principle we appear to have a sufficient reason for the various
contrivances by which so many flowers secure cross-fertilisation, either
constantly or occasionally. These contrivances are so numerous, so
varied, and often so highly complex and extraordinary, that they have
formed the subject of many elaborate treatises, and have also been amply
popularised in lectures and handbooks. It will be unnecessary,
therefore, to give details here, but the main facts will be summarised
in order to call attention to some difficulties of the theory which seem
to require further elucidation.
_Modes of securing Cross-Fertilisation._
When we examine the various modes in which the cross-fertilisation of
flowers is brought about, we find that some are comparatively simple in
their operation and needful adjustments, others highly complex. The
simple methods belong to four principal classes:--(1) By dichogamy--that
is, by the anthers and the stigma becoming mature or in a fit state for
fertilisation at slightly different times on the same plant. The result
of this is that, as plants in different stations, on different soils, or
exposed to different aspects flower earlier or later, the mature pollen
of one plant can only fertilise some plant exposed to somewhat different
conditions or of different constitution, whose stigma will be mature at
the same time; and this difference has been shown by Darwin to be that
which is adapted to secure the fullest benefit of cross-fertilisation.
This occurs in Geranium pratense, Thymus serpyllum, Arum maculatum, and
many others. (2) By the flower being self-sterile with its own pollen,
as in the crimson flax. This absolutely prevents self-fertilisation. (3)
By the stamens and anthers being so placed that the pollen cannot fall
upon the stigma, while it does fall upon a visiting insect which carries
it to the stigma of another flower. This effect is produced in a variety
of very simple ways, and is often aided by the motion of the stamens
which bend down out of the way of the stigmas before the pollen is ripe,
as in Malva sylvestris (see Fig. 28). (4) By the male and female flowers
being on different plants, forming the class Dioecia of Linnaeus. In
these cases the pollen may be carried to the stigmas either by the wind
or by the agency of insects.
[Illustration: FIG. 28.
Malva sylvestris, adapted for insect-fertilisation.
Malva rotundifolia, adapted for self-fertilisation.]
Now these four methods are all apparently very simple, and easily
produced by variation and selection. They are applicable to flowers of
any shape, requiring only such size and colour as to attract insects,
and some secretion of nectar to ensure their repeated visits, characters
common to the great majority of flowers. All these methods are common,
except perhaps the second; but there are many flowers in which the
pollen from another plant is prepotent over the pollen from
fertilisation, the same flower, and this has nearly the same effect as
self-sterility if the flowers are frequently crossed by insects. We
cannot help asking, therefore, why have other and much more elaborate
methods been needed? And how have the more complex arrangements of so
many flowers been brought about? Before attempting to answer these
questions, and in order that the reader may appreciate the difficulty of
the problem and the nature of the facts to be explained, it will be
necessary to give a summary of the more elaborate modes of securing
cross-fertilisation.
(1) We first have dimorphism and heteromorphism, the phenomena of which
have been already sketched in our seventh chapter.
Here we have both a mechanical and a physiological modification, the
stamens and pistil being variously modified in length and position,
while the different stamens in the same flower have widely different
degrees of fertility when applied to the same stigma,--a phenomenon
which, if it were not so well established, would have appeared in the
highest degree improbable. The most remarkable case is that of the three
different forms of the loosestrife (Lythrum salicaria) here figured
(Fig. 29 on next page).
(2) Some flowers have irritable stamens which, when their bases are
touched by an insect, spring up and dust it with pollen. This occurs in
our common berberry.
[Illustration: FIG. 29.--Lythrum salicaria (Purple loosestrife).]
(3) In others there are levers or processes by which the anthers are
mechanically brought down on to the head or back of an insect entering
the flower, in such a position as to be carried to the stigma of the
next flower it visits. This may be well seen in many species of Salvia
and Erica.
(4) In some there is a sticky secretion which, getting on to the
proboscis of an insect, carries away the pollen, and applies it to the
stigma of another flower. This occurs in our common milkwort (Polygala
vulgaris).
(5) In papilionaceous plants there are many complex adjustments, such as
the squeezing out of pollen from a receptacle on to an insect, as in
Lotus corniculatus, or the sudden springing out and exploding of the
anthers so as thoroughly to dust the insect, as in Medicago falcata,
this occurring after the stigma has touched the insect and taken off
some pollen from the last flower.
(6) Some flowers or spathes form closed boxes in which insects find
themselves entrapped, and when they have fertilised the flower, the
fringe of hairs opens and allows them to escape. This occurs in many
species of Arum and Aristolochia.
(7) Still more remarkable are the traps in the flower of Asclepias which
catch flies, butterflies, and wasps by the legs, and the wonderfully
complex arrangements of the orchids. One of these, our common Orchis
pyramidalis, may be briefly described to show how varied and beautiful
are the arrangements to secure cross-fertilisation. The broad trifid lip
of the flower offers a support to the moth which is attracted by its
sweet odour, and two ridges at the base guide the proboscis with
certainty to the narrow entrance of the nectary. When the proboscis has
reached the end of the spur, its basal portion depresses the little
hinged rostellum that covers the saddle-shaped sticky glands to which
the pollen masses (pollinia) are attached. On the proboscis being
withdrawn, the two pollinia stand erect and parallel, firmly attached to
the proboscis. In this position, however, they would be useless, as they
would miss the stigmatic surface of the next flower visited by the moth.
But as soon as the proboscis is withdrawn, the two pollen masses begin
to diverge till they are exactly as far apart as are the stigmas of the
flower; and then commences a second movement which brings them down
till they project straight forward nearly at right angles to their first
position, so as exactly to hit against the stigmatic surfaces of the
next flower visited on which they leave a portion of their pollen. The
whole of these motions take about half a minute, and in that time the
moth will usually have flown to another plant, and thus effect the most
beneficial kind of cross-fertilisation.[145] This description will be
better understood by referring to the illustration opposite, from
Darwin's _Fertilisation of Orchids_(Fig. 30).
[Illustration: FIG. 30.--Orchis pyramidalis.]
_The Interpretation of these Facts._
Having thus briefly indicated the general character of the more complex
adaptations for cross-fertilisation, the details of which are to be
found in any of the numerous works on the subject,[146] we find
ourselves confronted with the very puzzling question--Why were these
innumerable highly complex adaptations produced, when the very same
result may be effected--and often is effected--by extremely simple
means? Supposing, as we must do, that all flowers were once of simple
and regular forms, like a buttercup or a rose, how did such irregular
and often complicated flowers as the papilionaceous or pea family, the
labiates or sage family, and the infinitely varied and fantastic orchids
ever come into existence? No cause has yet been suggested but the need
of attracting insects to cross-fertilise them; yet the attractiveness of
regular flowers with bright colours and an ample supply of nectar is
equally great, and cross-fertilisation can be quite as effectively
secured in these by any of the four simple methods already described.
Before attempting to suggest a possible solution of this difficult
problem, we have yet to pass in review a large body of curious
adaptations connected with insect fertilisation, and will first call
attention to that portion of the phenomena which throw some light upon
the special colours of flowers in their relation to the various kinds of
insects which visit them. For these facts we are largely indebted to
the exact and long-continued researches of Professor Hermann Müller.
_Summary of Additional Facts bearing on Insect Fertilisation._
1. That the size and colour of a flower are important factors in
determining the visits of insects, is shown by the general fact of more
insects visiting conspicuous than inconspicuous flowers. As a single
instance, the handsome Geranium palustre was observed by Professor
Müller to be visited by sixteen different species of insects, the
equally showy G. pratense by thirteen species, while the smaller and
much less conspicuous G. molle was visited by eight species, and G.
pusillum by only one. In many cases, however, a flower may be very
attractive to only a few species of insects; and Professor Müller
states, as the result of many years' assiduous observation, that "a
species of flower is the more visited by insects the more conspicuous it
is."
2. Sweet odour is usually supplementary to the attraction of colour.
Thus it is rarely present in the largest and most gaudily coloured
flowers which inhabit open places, such as poppies, paeonies,
sunflowers, and many others; while it is often the accompaniment of
inconspicuous flowers, as the mignonette; of such as grow in shady
places, as the violet and primrose; and especially of white or yellowish
flowers, as the white jasmine, clematis, stephanotis, etc.
3. White flowers are often fertilised by moths, and very frequently give
out their scent only by night, as in our butterfly-orchis (Habenaria
chlorantha); and they sometimes open only at night, as do many of the
evening primroses and other flowers. These flowers are often long tubed
in accordance with the length of the moths' probosces, as in the genus
Pancratium, our butterfly orchis, white jasmine, and a host of others.
4. Bright red flowers are very attractive to butterflies, and are
sometimes specially adapted to be fertilised by them, as in many pinks
(Dianthus deltoides, D. superbus, D. atrorubens), the corn-cockle
(Lychnis Githago), and many others. Blue flowers are especially
attractive to bees and other hymenoptera (though they frequent flowers
of all colours), no less than sixty-seven species of this order having
been observed to visit the common "sheep's-bit" (Jasione montana). Dull
yellow or brownish flowers, some of which smell like carrion, are
attractive to flies, as the Arum and Aristolochia; while the dull
purplish flowers of the Scrophularia are specially attractive to wasps.
5. Some flowers have neither scent nor nectar, and yet attract insects
by sham nectaries! In the herb-paris (Paris quadrifolia) the ovary
glistens as if moist, and flies alight on it and carry away pollen to
another flower; while in grass of parnassus (Parnassia palustris) there
are a number of small stalked yellow balls near the base of the flower,
which look like drops of honey but are really dry. In this case there is
a little nectar lower down, but the special attraction is a sham; and as
there are fresh broods of insects every year, it takes time for them to
learn by experience, and thus enough are always deceived to effect
cross-fertilisation.[147] This is analogous to the case of the young
birds, which have to learn by experience the insects that are inedible,
as explained at page 253.
6. Many flowers change their colour as soon as fertilised; and this is
beneficial, as it enables bees to avoid wasting time in visiting those
blossoms which have been already fertilised and their nectar exhausted.
The common lungwort (Pulmonaria officinalis), is at first red, but later
turns blue; and H. Müller observed bees visiting many red flowers in
succession, but neglecting the blue. In South Brazil there is a species
of Lantana, whose flowers are yellow the first day, orange the second,
and purple the third; and Dr. Fritz Müller observed that many
butterflies visited the yellow flowers only, some both the yellow and
the orange flowers, but none the purple.
7. Many flowers have markings which serve as guides to insects; in some
cases a bright central eye, as in the borage and forget-me-not; or lines
or spots converging to the centre, as in geraniums, pinks, and many
others. This enables insects to go quickly and directly to the opening
of the flower, and is equally important in aiding them to obtain a
better supply of food, and to fertilise a larger number of flowers.
8. Flowers have been specially adapted to the kinds of insects that
most abound where they grow. Thus the gentians of the lowlands are
adapted to bees, those of the high alps to butterflies only; and while
most species of Rhinanthus (a genus to which our common "yellow rattle"
belongs) are bee-flowers, one high alpine species (R. alpinus) has been
also adapted for fertilisation by butterflies only. The reason of this
is, that in the high alps butterflies are immensely more plentiful than
bees, and flowers adapted to be fertilised by bees can often have their
nectar extracted by butterflies without effecting cross-fertilisation.
It is, therefore, important to have a modification of structure which
shall make butterflies the fertilisers, and this in many cases has been
done.[148]
9. Economy of time is very important both to the insects and the
flowers, because the fine working days are comparatively few, and if no
time is wasted the bees will get more honey, and in doing so will
fertilise more flowers. Now, it has been ascertained by several
observers that many insects, bees especially, keep to one kind of flower
at a time, visiting hundreds of blossoms in succession, and passing over
other species that may be mixed with them. They thus acquire quickness
in going at once to the nectar, and the change of colour in the flower,
or incipient withering when fertilised, enables them to avoid those
flowers that have already had their honey exhausted. It is probably to
assist the insects in keeping to one flower at a time, which is of vital
importance to the perpetuation of the species, that the flowers which
bloom intermingled at the same season are usually very distinct both in
form and colour. In the sandy districts of Surrey, in the early spring,
the copses are gay with three flowers--the primrose, the wood-anemone,
and the lesser celandine, forming a beautiful contrast, while at the
same time the purple and the white dead-nettles abound on hedge banks. A
little later, in the same copses, we have the blue wild hyacinth (Scilla
nutans), the red campion (Lychnis dioica), the pure white great starwort
(Stellaria Holosteum), and the yellow dead-nettle (Lamium Galeobdolon),
all distinct and well-contrasted flowers. In damp meadows in summer we
have the ragged robin (Lychnis Floscuculi), the spotted orchis (O.
maculata), and the yellow rattle (Rhinanthus Crista-galli); while in
drier meadows we have cowslips, ox-eye daisies, and buttercups, all very
distinct both in form and colour. So in cornfields we have the scarlet
poppies, the purple corn-cockle, the yellow corn-marygold, and the blue
cornflower; while on our moors the purple heath and the dwarf gorse make
a gorgeous contrast. Thus the difference of colour which enables the
insect to visit with rapidity and unerring aim a number of flowers of
the same kind in succession, serves to adorn our meadows, banks, woods,
and heaths with a charming variety of floral colour and form at each
season of the year.[149]
_Fertilisation of Flowers by Birds._
In the temperate regions of the Northern Hemisphere, insects are the
chief agents in cross-fertilisation when this is not effected by the
wind; but in warmer regions, and in the Southern hemisphere, birds are
found to take a considerable part in the operation, and have in many
cases led to modifications in the form and colour of flowers. Each part
of the globe has special groups of birds which are flower-haunters.
America has the humming-birds (Trochilidae), and the smaller group of
the sugar-birds (Caerebidae). In the Eastern tropics the sun-birds
(Nectarineidae) take the place of the humming-birds, and another small
group, the flower-peckers (Dicaeidae), assist them. In the Australian
region there are also two flower-feeding groups, the Meliphagidae, or
honey-suckers, and the brush-tongued lories (Trichoglossidae). Recent
researches by American naturalists have shown that many flowers are
fertilised by humming-birds, such as passion-flowers, trumpet-flowers,
fuchsias, and lobelias; while some, as the Salvia splendens of Mexico,
are specially adapted to their visits. We may thus perhaps explain the
number of very large tubular flowers in the tropics, such as the huge
brugmansias and bignonias; while in the Andes and in Chile, where
humming-birds are especially plentiful, we find great numbers of red
tubular flowers, often of large size and apparently adapted to these
little creatures. Such are the beautiful Lapageria and Philesia, the
grand Pitcairneas, and the genera Fuchsia, Mitraria, Embothrium,
Escallonia, Desfontainea, Eccremocarpus, and many Gesneraceae. Among the
most extraordinary modifications of flower structure adapted to bird
fertilisation are the species of Marcgravia, in which the pedicels and
bracts of the terminal portion of a pendent bunch of flowers have been
modified into pitchers which secrete nectar and attract insects, while
birds feeding on the nectar, or insects, have the pollen of the
overhanging flowers dusted on their backs, and, carrying it to other
flowers, thus cross-fertilise them (see Illustration).
[Illustration: FIG. 31.--Humming-bird fertilising Marcgravia
nepenthoides.]
In Australia and New Zealand the fine "glory peas" (Clianthus), the
Sophora, Loranthus, many Epacrideae and Myrtaceae, and the large flowers
of the New Zealand flax (Phormium tenax), are cross-fertilised by
birds; while in Natal the fine trumpet-creeper (Tecoma capensis) is
fertilised by Nectarineas.
The great extent to which insect and bird agency is necessary to flowers
is well shown by the case of New Zealand. The entire country is
comparatively poor in species of insects, especially in bees and
butterflies which are the chief flower fertilisers; yet according to the
researches of local botanists no less than one-fourth of all the
flowering plants are incapable of self-fertilisation, and, therefore,
wholly dependent on insect or bird agency for the continuance of the
species.
The facts as to the cross-fertilisation of flowers which have now been
very briefly summarised, taken in connection with Darwin's experiments
proving the increased vigour and fertility given by cross-fertilisation,
seem amply to justify his aphorism that "Nature abhors
self-fertilisation," and his more precise statement, that, "No plant is
perpetually self-fertilised;" and this view has been upheld by
Hildebrand, Delpino, and other botanists.[150]
_Self-Fertilisation of Flowers._
But all this time we have been only looking at one side of the question,
for there exists an abundance of facts which seem to imply, just as
surely, the utter uselessness of cross-fertilisation. Let us, then, see
what these facts are before proceeding further.
1. An immense variety of plants are habitually self-fertilised, and
their numbers probably far exceed those which are habitually
cross-fertilised by insects. Almost all the very small or obscure
flowered plants with hermaphrodite flowers are of this kind. Most of
these, however, may be insect fertilised occasionally, and may,
therefore, come under the rule that no species are perpetually
self-fertilised.
2. There are many plants, however, in which special arrangements exist
to secure self-fertilisation. Sometimes the corolla closes and brings
the anthers and stigma into contact; in others the anthers cluster round
the stigmas, both maturing together, as in many buttercups, stitchwort
(Stellaria media), sandwort (Spergula), and some willow-herbs
(Epilobium); or they arch over the pistil, as in Galium aparine and
Alisma Plantago. The style is also modified to bring it into contact
with the anthers, as in the dandelion, groundsel, and many other
plants.[151] All these, however, may be occasionally cross-fertilised.
3. In other cases precautions are taken to prevent cross-fertilisation,
as in the numerous cleistogamous or closed flowers. These occur in no
less than fifty-five different genera, belonging to twenty-four natural
orders, and in thirty-two of these genera the normal flowers are
irregular, and have therefore been specially modified for insect
fertilisation.[152] These flowers appear to be degradations of the
normal flowers, and are closed up by various modifications of the petals
or other parts, so that it is impossible for insects to reach the
interior, yet they produce seed in abundance, and are often the chief
means by which the species is continued. Thus, in our common dog-violet
the perfect flowers rarely produce seed, while the rudimentary
cleistogamic flowers do so in abundance. The sweet violet also produces
abundance of seed from its cleistogamic flowers, and few from its
perfect flowers; but in Liguria it produces only perfect flowers which
seed abundantly. No case appears to be known of a plant which has
cleistogamic flowers only, but a small rush (Juncus bufonius) is in this
condition in some parts of Russia, while in other parts perfect flowers
are also produced.[153] Our common henbit dead-nettle (Lamium
amplexicaule) produces cleistogamic flowers, as do also some orchids.
The advantage gained by the plant is great economy of specialised
material, since with very small flowers and very little expenditure of
pollen an abundance of seed is produced.
4. A considerable number of plants which have evidently been specially
modified for insect fertilisation have, by further modification, become
quite self-fertile. This is the case with the garden-pea, and also with
our beautiful bee-orchis, in which the pollen-masses constantly fall on
to the stigmas, and the flower, being thus self-fertilised, produces
abundance of capsules and of seed. Yet in many of its close allies
insect agency is absolutely required; but in one of these, the
fly-orchis, comparatively very little seed is produced, and
self-fertilisation would therefore be advantageous to it. When
garden-peas were artificially cross-fertilised by Mr. Darwin, it seemed
to do them no good, as the seeds from these crosses produced less
vigorous plants than seed from those which were self-fertilised; a fact
directly opposed to what usually occurs in cross-fertilised plants.
5. As opposed to the theory that there is any absolute need for
cross-fertilisation, it has been urged by Mr. Henslow and others that
many self-fertilised plants are exceptionally vigorous, such as
groundsel, chickweed, sow-thistle, buttercups, and other common weeds;
while most plants of world-wide distribution are self-fertilised, and
these have proved themselves to be best fitted to survive in the battle
of life. More than fifty species of common British plants are very
widely distributed, and all are habitually self-fertilised.[154] That
self-fertilisation has some great advantage is shown by the fact that it
is usually the species which have the smallest and least conspicuous
flowers which have spread widely, while the large and showy flowered
species of the same genera or families, which require insects to
cross-fertilise them, have a much more limited distribution.
6. It is now believed by some botanists that many inconspicuous and
imperfect flowers, including those that are wind-fertilised, such as
plantains, nettles, sedges, and grasses, do not represent primitive or
undeveloped forms, but are degradations from more perfect flowers which
were once adapted to insect fertilisation. In almost every order we find
some plants which have become thus reduced or degraded for wind or
self-fertilisation, as Poterium and Sanguisorba among the Rosaceae;
while this has certainly been the case in the cleistogamic flowers. In
most of the above-mentioned plants there are distinct rudiments of
petals or other floral organs, and as the chief use of these is to
attract insects, they could hardly have existed in primitive
flowers.[155] We know, moreover, that when the petals cease to be
required for the attraction of insects, they rapidly diminish in size,
lose their bright colour or almost wholly disappear.[156]
_Difficulties and Contradictions._
The very bare summary that has now been given of the main facts relating
to the fertilisation of flowers, will have served to show the vast
extent and complexity of the inquiry, and the extraordinary
contradictions and difficulties which it presents. We have direct proof
of the beneficial results of intercrossing in a great number of cases;
we have an overwhelming mass of facts as to the varied and complex
structure of flowers evidently adapted to secure this intercrossing by
insect agency; yet we see many of the most vigorous plants which spread
widely over the globe, with none of these adaptations, and evidently
depending on self-fertilisation for their continued existence and
success in the battle of life. Yet more extraordinary is it to find
numerous cases in which the special arrangements for cross-fertilisation
appear to have been a failure, since they have either been supplemented
by special means for self-fertilisation, or have reverted back in
various degrees to simpler forms in which self-fertilisation becomes the
rule. There is also a further difficulty in the highly complex modes by
which cross-fertilisation is often brought about; for we have seen that
there are several very effective yet very simple modes of securing
intercrossing, involving a minimum of change in the form and structure
of the flower; and when we consider that the result attained with so
much cost of structural modification is by no means an unmixed good, and
is far less certain in securing the perpetuation of the species than is
self-fertilisation, it is most puzzling to find such complex methods
resorted to, sometimes to the extent of special precautions against the
possibility of self-fertilisation ever taking place. Let us now see
whether any light can be thrown on these various anomalies and
contradictions.
_Intercrossing not necessarily Advantageous._
No one was more fully impressed than Mr. Darwin with the beneficial
effects of intercrossing on the vigour and fertility of the species or
race, yet he clearly saw that it was not always and necessarily
advantageous. He says: "The most important conclusion at which I have
arrived is, that the mere act of intercrossing by itself does no good.
The good depends on the individuals which are crossed differing slightly
in constitution, owing to their progenitors having been subjected during
several generations to slightly different conditions. This conclusion,
as we shall hereafter see, is closely connected with various important
physiological problems, such as the benefit derived from slight changes
in the conditions of life."[157] Mr. Darwin has also adduced much direct
evidence proving that slight changes in the conditions of life are
beneficial to both animals and plants, maintaining or restoring their
vigour and fertility in the same way as a favourable cross seems to
restore it.[158] It is, I believe, by a careful consideration of these
two classes of facts that we shall find the clue to the labyrinth in
which this subject has appeared to involve us.
_Supposed Evil Results of Close Interbreeding._
Just as we have seen that intercrossing is not necessarily good, we
shall be forced to admit that close interbreeding is not necessarily
bad. Our finest breeds of domestic animals have been thus produced, and
by a careful statistical inquiry Mr. George Darwin has shown that the
most constant and long-continued intermarriages among the British
aristocracy have produced no prejudicial results. The rabbits on Porto
Santo are all the produce of a single female; they have lived on the
same small island for 470 years, and they still abound there and appear
to be vigorous and healthy (see p. 161).
We have, however, on the other hand, overwhelming evidence that in many
cases, among our domestic animals and cultivated plants, close
interbreeding does produce bad results, and the apparent contradiction
may perhaps be explained on the same general principles, and under
similar limitations, as were found to be necessary in defining the value
of intercrossing. It appears probable, then, that it is not
interbreeding in itself that is hurtful, but interbreeding without
rigid selection or some change of conditions. Under nature, as in the
case of the Porto Santo rabbits, the rapid increase of these animals
would in a very few years stock the island with a full population, and
thereafter natural selection would act powerfully in the preservation
only of the healthiest and the most fertile, and under these conditions
no deterioration would occur. Among the aristocracy there has been a
constant selection of beauty, which is generally synonymous with health,
while any constitutional infertility has led to the extinction of the
family. With domestic animals the selection practised is usually neither
severe enough nor of the right kind. There is no natural struggle for
existence, but certain points of form and colour characteristic of the
breed are considered essential, and thus the most vigorous or the most
fertile are not always those which are selected to continue the stock.
In nature, too, the species always extends over a larger area and
consists of much greater numbers, and thus a difference of constitution
soon arises in different parts of the area, which is wanting in the
limited numbers of pure bred domestic animals. From a consideration of
these varied facts we conclude that an occasional disturbance of the
organic equilibrium is what is essential to keep up the vigour and
fertility of any organism, and that this disturbance may be equally well
produced either by a cross between individuals of somewhat different
constitutions, or by occasional slight changes in the conditions of
life. Now plants which have great powers of dispersal enjoy a constant
change of conditions, and can, therefore, exist permanently, or at all
events, for very long periods, without intercrossing; while those which
have limited powers of dispersal, and are restricted to a comparatively
small and uniform area, need an occasional cross to keep up their
fertility and general vigour. We should, therefore, expect that those
groups of plants which are adapted both for cross-and
self-fertilisation, which have showy flowers and possess great powers of
seed-dispersal, would be the most abundant and most widely distributed;
and this we find to be the case, the Compositae possessing all these
characteristics in the highest degree, and being the most generally
abundant group of plants with conspicuous flowers in all parts of the
world.
_How the Struggle for Existence Acts among Flowers._
Let us now consider what will be the action of the struggle for
existence under the conditions we have seen to exist.
Everywhere and at all times some species of plants will be dominant and
aggressive; while others will be diminishing in numbers, reduced to
occupy a smaller area, and generally having a hard struggle to maintain
themselves. Whenever a self-fertilising plant is thus reduced in numbers
it will be in danger of extinction, because, being limited to a small
area, it will suffer from the effects of too uniform conditions which
will produce weakness and infertility. But while this change is in
progress, any crosses between individuals of slightly different
constitution will be beneficial, and all variations favouring either
insect agency on the one hand, or wind-dispersal of pollen on the other,
will lead to the production of a somewhat stronger and more fertile
stock. Increased size or greater brilliancy of the flower, more abundant
nectar, sweeter odour, or adaptations for more effectual
cross-fertilisation would all be preserved, and thus would be initiated
some form of specialisation for insect agency in cross-fertilisation;
and in every different species so circumstanced the result would be
different, depending as it would on many and complex combinations of
variation of parts of the flower, and of the insect species which most
abounded in the district.
Species thus favourably modified might begin a new era of development,
and, while spreading over a somewhat wider area, give rise to new
varieties or species, all adapted in various degrees and modes to secure
cross-fertilisation by insect agency. But in course of ages some change
of conditions might prove adverse. Either the insects required might
diminish in numbers or be attracted by other competing flowers, or a
change of climate might give the advantage to other more vigorous
plants. Then self-fertilisation with greater means of dispersal might be
more advantageous; the flowers might become smaller and more numerous;
the seeds smaller and lighter so as to be more easily dispersed by the
wind, while some of the special adaptations for insect fertilisation
being useless would, by the absence of selection and by the law of
economy of growth, be reduced to a rudimentary form. With these
modifications the species might extend its range into new districts,
thereby obtaining increased vigour by the change of conditions, as
appears to have been the case with so many of the small flowered
self-fertilised plants. Thus it might continue to exist for a long
series of ages, till under other changes--geographical or biological--it
might again suffer from competition or from other adverse circumstances,
and be at length again confined to a limited area, or reduced to very
scanty numbers.
But when this cycle of change had taken place, the species would be very
different from the original form. The flower would have been at one time
modified to favour the visits of insects and to secure
cross-fertilisation by their aid, and when the need for this passed
away, some portions of these structures would remain, though in a
reduced or rudimentary condition. But when insect agency became of
importance a second time, the new modifications would start from a
different or more advanced basis, and thus a more complex result might
be produced. Owing to the unequal rates at which the reduction of the
various parts might occur, some amount of irregularity in the flower
might arise, and on a second development towards insect
cross-fertilisation this irregularity, if useful, might be increased by
variation and selection.
The rapidity and comparative certainty with which such changes as are
here supposed do really take place, are well shown by the great
differences in floral structure, as regards the mode of fertilisation,
in allied genera and species, and even in some cases in varieties of the
same species. Thus in the Ranunculaceae we find the conspicuous part of
the flower to be the petals in Ranunculus, the sepals in Helleborus,
Anemone, etc., and the stamens in most species of Thalictrum. In all
these we have a simple regular flower, but in Aquilegia it is made
complex by the spurred petals, and in Delphinium and Aconitum it becomes
quite irregular. In the more simple class self-fertilisation occurs
freely, but it is prevented in the more complex flowers by the stamens
maturing before the pistil. In the Caprifoliaceae we have small and
regular greenish flowers, as in the moschatel (Adoxa); more conspicuous
regular open flowers without honey, as in the elder (Sambucus); and
tubular flowers increasing in length and irregularity, till in some,
like our common honeysuckle, they are adapted for fertilisation by moths
only, with abundant honey and delicious perfume to attract them. In the
Scrophulariaceae we find open, almost regular flowers, as Veronica and
Verbascum, fertilised by flies and bees, but also self-fertilised;
Scrophularia adapted in form and colour to be fertilised by wasps; and
the more complex and irregular flowers of Linaria, Rhinanthus,
Melampyrum, Pedicularis, etc., mostly adapted to be fertilised by bees.
In the genera Geranium, Polygonum, Veronica, and several others there is
a gradation of forms from large and bright to small and obscure coloured
flowers, and in every case the former are adapted for insect
fertilisation, often exclusively, while in the latter self-fertilisation
constantly occurs. In the yellow rattle (Rhinanthus Crista-galli) there
are two forms (which have been named _major_ and _minor_), the larger
and more conspicuous adapted to insect fertilisation only, the smaller
capable of self-fertilisation; and two similar forms exist in the
eyebright (Euphrasia officinalis). In both these cases there are special
modifications in the length and curvature of the style as well as in the
size and shape of the corolla; and the two forms are evidently becoming
each adapted to special conditions, since in some districts the one, in
other districts the other is most abundant.[159]
These examples show us that the kind of change suggested above is
actually going on, and has presumably always been going on in nature
throughout the long geological epochs during which the development of
flowers has been progressing. The two great modes of gaining increased
vigour and fertility--intercrossing and dispersal over wider areas--have
been resorted to again and again, under the pressure of a constant
struggle for existence and the need for adaptation to ever-changing
conditions. During all the modifications that ensued, useless parts were
reduced or suppressed, owing to the absence of selection and the
principle of economy of growth; and thus at each fresh adaptation some
rudiments of old structures were re-developed, but not unfrequently in
a different form and for a distinct purpose.
The chief types of flowering plants have existed during the millions of
ages of the whole tertiary period, and during this enormous lapse of
time many of them may have been modified in the direction of insect
fertilisation, and again into that of self-fertilisation, not once or
twice only, but perhaps scores or even hundreds of times; and at each
such modification a difference in the environment may have led to a
distinct line of development. At one epoch the highest specialisation of
structure in adaptation to a single species or group of insects may have
saved a plant from extinction; while, at other times, the simplest mode
of self-fertilisation, combined with greater powers of dispersal and a
constitution capable of supporting diverse physical conditions, may have
led to a similar result. With some groups the tendency seems to have
been almost continuously to greater and greater specialisation, while
with others a tendency to simplification and degradation has resulted in
such plants as the grasses and sedges.
We are now enabled dimly to perceive how the curious anomaly of very
simple and very complex methods of securing cross-fertilisation--both
equally effective--may have been brought about. The simple modes may be
the result of a comparatively direct modification from the more
primitive types of flowers, which were occasionally, and, as it were,
accidentally visited and fertilised by insects; while the more complex
modes, existing for the most part in the highly irregular flowers, may
result from those cases in which adaptation to insect-fertilisation, and
partial or complete degradation to self-fertilisation or to
wind-fertilisation, have again and again recurred, each time producing
some additional complexity, arising from the working up of old rudiments
for new purposes, till there have been reached the marvellous flower
structures of the papilionaceous tribes, of the asclepiads, or of the
orchids.
We thus see that the existing diversity of colour and of structure in
flowers is probably the ultimate result of the ever-recurring struggle
for existence, combined with the ever-changing relations between the
vegetable and animal kingdoms during countless ages. The constant
variability of every part and organ, with the enormous powers of
increase possessed by plants, have enabled them to become again and
again readjusted to each change of condition as it occurred, resulting
in that endless variety, that marvellous complexity, and that exquisite
colouring which excite our admiration in the realm of flowers, and
constitute them the perennial charm and crowning glory of nature.
_Flowers the Product of Insect Agency._
In his _Origin of Species_, Mr. Darwin first stated that flowers had
been rendered conspicuous and beautiful in order to attract insects,
adding: "Hence we may conclude that, if insects had not been developed
on the earth, our plants would not have been decked with beautiful
flowers, but would have produced only such poor flowers as we see on our
fir, oak, nut, and ash trees, on grasses, docks, and nettles, which are
all fertilised through the agency of the wind." The argument in favour
of this view is now much stronger than when he wrote; for not only have
we reason to believe that most of these wind-fertilised flowers are
degraded forms of flowers which have once been insect fertilised, but we
have abundant evidence that whenever insect agency becomes comparatively
ineffective, the colours of the flowers become less bright, their size
and beauty diminish, till they are reduced to such small, greenish,
inconspicuous flowers as those of the rupture-wort (Herniaria glabra),
the knotgrass (Polygonum aviculare), or the cleistogamic flowers of the
violet. There is good reason to believe, therefore, not only that
flowers have been developed in order to attract insects to aid in their
fertilisation, but that, having been once produced, in however great
profusion, if the insect races were all to become extinct, flowers (in
the temperate zones at all events) would soon dwindle away, and that
ultimately all floral beauty would vanish from the earth.
We cannot, therefore, deny the vast change which insects have produced
upon the earth's surface, and which has been thus forcibly and
beautifully delineated by Mr. Grant Allen: "While man has only tilled a
few level plains, a few great river valleys, a few peninsular mountain
slopes, leaving the vast mass of earth untouched by his hand, the insect
has spread himself over every land in a thousand shapes, and has made
the whole flowering creation subservient to his daily wants. His
buttercup, his dandelion, and his meadow-sweet grow thick in every
English field. His thyme clothes the hillside; his heather purples the
bleak gray moorland. High up among the alpine heights his gentian
spreads its lakes of blue; amid the snows of the Himalayas his
rhododendrons gleam with crimson light. Even the wayside pond yields him
the white crowfoot and the arrowhead, while the broad expanses of
Brazilian streams are beautified by his gorgeous water-lilies. The
insect has thus turned the whole surface of the earth into a boundless
flower-garden, which supplies him from year to year with pollen or
honey, and itself in turn gains perpetuation by the baits that it offers
for his allurement."[160]
_Concluding Remarks on Colour in Nature._
In the last four chapters I have endeavoured to give a general and
systematic, though necessarily condensed view of the part which is
played by colour in the organic world. We have seen in what infinitely
varied ways the need of concealment has led to the modification of
animal colours, whether among polar snows or sandy deserts, in tropical
forests or in the abysses of the ocean. We next find these general
adaptations giving way to more specialised types of coloration, by which
each species has become more and more harmonised with its immediate
surroundings, till we reach the most curiously minute resemblances to
natural objects in the leaf and stick insects, and those which are so
like flowers or moss or birds' droppings that they deceive the acutest
eye. We have learnt, further, that these varied forms of protective
colouring are far more numerous than has been usually suspected,
because, what appear to be very conspicuous colours or markings when the
species is observed in a museum or in a menagerie, are often highly
protective when the creature is seen under the natural conditions of its
existence. From these varied classes of facts it seems not improbable
that fully one-half of the species in the animal kingdom possess colours
which have been more or less adapted to secure for them concealment or
protection.
Passing onward we find the explanation of a distinct type of colour or
marking, often superimposed upon protective tints, in the importance of
easy recognition by many animals of their fellows, their parents, or
their mates. By this need we have been able to account for markings that
seem calculated to make the animal conspicuous, when the general tints
and well-known habits of the whole group demonstrate the need of
concealment. Thus also we are able to explain the constant symmetry in
the markings of wild animals, as well as the numerous cases in which the
conspicuous colours are concealed when at rest and only become visible
during rapid motion. In striking contrast to ordinary protective
coloration we have "warning colours," usually very conspicuous and often
brilliant or gaudy, which serve to indicate that their possessors are
either dangerous or uneatable to the usual enemies of their tribe. This
kind of coloration is probably more prevalent than has been hitherto
supposed, because in the case of many tropical animals we are quite
unacquainted with their special and most dangerous enemies, and are also
unable to determine whether they are or are not distasteful to those
enemies. As a kind of corollary to the "warning colours," we find the
extraordinary phenomena of "mimicry," in which defenceless species
obtain protection by being mistaken for those which, from any cause,
possess immunity from attack. Although a large number of instances of
warning colour and of mimicry are now recorded, it is probably still an
almost unworked field of research, more especially in tropical regions
and among the inhabitants of the ocean.
The phenomena of sexual diversities of coloration next engaged our
attention, and the reasons why Mr. Darwin's theory of "sexual
selection," as regards colour and ornament, could not be accepted were
stated at some length, together with the theory of animal coloration and
ornament we propose to substitute for it. This theory is held to be in
harmony with the general facts of animal coloration, while it entirely
dispenses with the very hypothetical and inadequate agency of female
choice in producing the detailed colours, patterns, and ornaments, which
in so many cases distinguish the male sex.
If my arguments on this point are sound, they will dispose also of Mr.
Grant Allen's view of the direct action of the colour sense on the
animal integuments.[161] He argues that the colours of insects and birds
reproduce generally the colours of the flowers they frequent or the
fruits they eat, and he adduces numerous cases in which flower-haunting
insects and fruit-eating birds are gaily coloured. This he supposes to
be due to the colour-taste, developed by the constant presence of bright
flowers and fruits, being applied to the selection of each variation
towards brilliancy in their mates; thus in time producing the gorgeous
and varied hues they now possess. Mr. Allen maintains that "insects are
bright where bright flowers exist in numbers, and dull where flowers are
rare or inconspicuous;" and he urges that "we can hardly explain this
wide coincidence otherwise than by supposing that a taste for colour is
produced through the constant search for food among entomophilous
blossoms, and that this taste has reacted upon its possessors through
the action of unconscious sexual selection."
The examples Mr. Allen quotes of bright insects being associated with
bright flowers seem very forcible, but are really deceptive or
erroneous; and quite as many cases could be quoted which prove the very
opposite. For example, in the dense equatorial forests flowers are
exceedingly scarce, and there is no comparison with the amount of floral
colour to be met with in our temperate meadows, woods, and hillsides.
The forests about Para in the lower Amazon are typical in this respect,
yet they abound with the most gorgeously coloured butterflies, almost
all of which frequent the forest depths, keeping near the ground, where
there is the greatest deficiency of brilliant flowers. In contrast with
this let us take the Cape of Good Hope--the most flowery region probably
that exists upon the globe,--where the country is a complete
flower-garden of heaths, pelargoniums, mesembryanthemus, exquisite
iridaceous and other bulbs, and numerous flowering shrubs and trees; yet
the Cape butterflies are hardly equal, either in number or variety, to
those of any country in South Europe, and are utterly insignificant when
compared with those of the comparatively flowerless forest-depths of the
Amazon or of New Guinea. Neither is there any relation between the
colours of other insects and their haunts. Few are more gorgeous than
some of the tiger-beetles and the carabi, yet these are all carnivorous;
while many of the most brilliant metallic buprestidae and longicorns are
always found on the bark of fallen trees. So with the humming-birds;
their brilliant metallic tints can only be compared with metals or gems,
and are totally unlike the delicate pinks and purples, yellows and reds
of the majority of flowers. Again, the Australian honey-suckers
(Meliphagidae) are genuine flower-haunters, and the Australian flora is
more brilliant in colour display than that of most tropical regions, yet
these birds are, as a rule, of dull colours, not superior on the average
to our grain-eating finches. Then, again, we have the grand pheasant
family, including the gold and the silver pheasants, the gorgeous
fire-backed and ocellated pheasants, and the resplendent peacock, all
feeding on the ground on grain or seeds or insects, yet adorned with the
most gorgeous colours.
There is, therefore, no adequate basis of facts for this theory to rest
upon, even if there were the slightest reason to believe that not only
birds, but butterflies and beetles, take any delight in colour for its
own sake, apart from the food-supply of which it indicates the presence.
All that has been proved or that appears to be probable is, that they
are able to perceive differences of colour, and to associate each colour
with the particular flowers or fruits which best satisfy their wants.
Colour being in its nature diverse, it has been beneficial for them to
be able to distinguish all its chief varieties, as manifested more
particularly in the vegetable kingdom, and among the different species
of their own group; and the fact that certain species of insects show
some preference for a particular colour may be explained by their having
found flowers of that colour to yield them a more abundant supply of
nectar or of pollen. In those cases in which butterflies frequent
flowers of their own colour, the habit may well have been acquired from
the protection it affords them.
It appears to me that, in imputing to insects and birds the same love of
colour for its own sake and the same aesthetic tastes as we ourselves
possess, we may be as far from the truth as were those writers who held
that the bee was a good mathematician, and that the honeycomb was
constructed throughout to satisfy its refined mathematical instincts;
whereas it is now generally admitted to be the result of the simple
principle of economy of material applied to a primitive cylindrical
cell.[162]
In studying the phenomena of colour in the organic world we have been
led to realise the wonderful complexity of the adaptations which bring
each species into harmonious relation with all those which surround it,
and which thus link together the whole of nature in a network of
relations of marvellous intricacy. Yet all this is but, as it were, the
outward show and garment of nature, behind which lies the inner
structure--the framework, the vessels, the cells, the circulating
fluids, and the digestive and reproductive processes,--and behind these
again those mysterious chemical, electrical, and vital forces which
constitute what we term Life. These forces appear to be fundamentally
the same for all organisms, as is the material of which all are
constructed; and we thus find behind the outer diversities an inner
relationship which binds together the myriad forms of life.
Each species of animal or plant thus forms part of one harmonious whole,
carrying in all the details of its complex structure the record of the
long story of organic development; and it was with a truly inspired
insight that our great philosophical poet apostrophised the humble
weed--
Flower in the crannied wall,
I pluck you out of the crannies,
I hold you here, root and all, in my hand,
Little flower--but _if_ I could understand
What you are, root and all, and all in all,
I should know what God and man is.
FOOTNOTES:
[Footnote 136: Burchell's _Travels_, vol. i. p. 10.]
[Footnote 137: _Nature_, vol. iii. p. 507.]
[Footnote 138: _Flowers, Fruits, and Leaves_, p. 128 (Fig. 79).]
[Footnote 139: For a popular sketch of these, see Sir J. Lubbock's
_Flowers, Fruits, and Leaves_, or any general botanical work.]
[Footnote 140: _Nature_, vol. xv. p, 117.]
[Footnote 141: Grant Allen's _Colour Sense_, p. 113.]
[Footnote 142: Canon Tristram's _Natural History of the Bible_, pp. 483,
484.]
[Footnote 143: For a complete historical account of this subject with
full references to all the works upon it, see the Introduction to
Hermann Müller's _Fertilisation of Flowers_, translated by D'Arcy W.
Thompson.]
[Footnote 144: For the full detail of his experiments, see _Cross-and
Self-Fertilisation of Plants_, 1876.]
[Footnote 145: See Darwin's _Fertilisation of Orchids_ for the many
extraordinary and complex arrangements in these plants.]
[Footnote 146: The English reader may consult Sir John Lubbock's
_British Wild Flowers in Relation to Insects_, and H. Müller's great and
original work, _The Fertilisation of Flowers_.]
[Footnote 147: Müller's _Fertilisation of Flowers_, p. 248.]
[Footnote 148: "Alpenblumen," by D.H. Müller. See _Nature_, vol. xxiii.
p. 333.]
[Footnote 149: This peculiarity of local distribution of colour in
flowers may be compared, as regards its purpose, with the recognition
colours of animals. Just as these latter colours enable the sexes to
recognise each other, and thus avoid sterile unions of distinct species,
so the distinctive form and colour of each species of flower, as
compared with those that usually grow around it, enables the fertilising
insects to avoid carrying the pollen of one flower to the stigma of a
distinct species.]
[Footnote 150: See H. Müller's _Fertilisation of Flowers_, p. 18.]
[Footnote 151: The above examples are taken from Rev. G. Henslow's paper
on "Self-Fertilisation of Plants," in _Trans. Linn. Soc._ Second series,
_Botany_, vol. i. pp. 317-398, with plate. Mr. H.O. Forbes has shown
that the same thing occurs among tropical orchids, in his paper "On the
Contrivances for insuring Self-Fertilisation in some Tropical Orchids,"
_Journ. Linn. Soc._, xxi. p. 538.]
[Footnote 152: These are the numbers given by Darwin, but I am informed
by Mr. Hemsley that many additions have been since made to the list, and
that cleistogamic flowers probably occur in nearly all the natural
orders.]
[Footnote 153: For a full account of cleistogamic flowers, see Darwin's
_Forms of Flowers_, chap. viii.]
[Footnote 154: Henslow's "Self-Fertilisation," _Trans. Linn. Soc._
Second series, _Botany_, vol. i. p. 391.]
[Footnote 155: The Rev. George Henslow, in his _Origin of Floral
Structures_, says: "There is little doubt but that all wind-fertilised
angiosperms are degradations from insect-fertilised flowers....
_Poterium sanguisorba_ is anemophilous; and _Sanguisorba officinalis_
presumably was so formerly, but has reacquired an entomophilous habit;
the whole tribe Poterieae being, in fact, a degraded group which has
descended from Potentilleae. Plantains retain their corolla but in a
degraded form. Junceae are degraded Lilies; while Cyperaceae and
Gramineae among monocotyledons may be ranked with Amentiferae among
dicotyledons, as representing orders which have retrograded very far
from the entomophilous forms from which they were possibly and probably
descended" (p. 266).
"The genus Plantago, like _Thalictrum minus_, Poterium, and others, well
illustrate the change from an entomophilous to the anemophilous state.
_P. lanceolata_ has polymorphic flowers, and is visited by
pollen-seeking insects, so that it can be fertilised either by insects
or the wind. _P. media_ illustrates transitions in point of structure,
as the filaments are pink, the anthers motionless, and the pollen grains
aggregated, and it is regularly visited by _Bombus terrestris_. On the
other hand, the slender filaments, versatile anthers, powdery pollen,
and elongated protogynous style are features of other species indicating
anemophily; while the presence of a degraded corolla shows its ancestors
to have been entomophilous. _P. media_, therefore, illustrates, not a
primitive entomophilous condition, but a return to it; just as is the
case with _Sanguisorba officinalis_ and _Salix Caprea_; but these show
no capacity of restoring the corolla, the attractive features having to
be borne by the calyx, which is purplish in Sanguisorba, by the pink
filaments of Plantago, and by the yellow anthers in the Sallow willow"
(p. 271).
"The interpretation, then, I would offer of inconspicuousness and all
kinds of degradations is the exact opposite to that of conspicuousness
and great differentiations; namely, that species with minute flowers,
rarely or never visited by insects, and habitually self-fertilised, have
primarily arisen through the neglect of insects, and have in consequence
assumed their present floral structures" (p. 282).
In a letter just received from Mr. Henslow, he gives a few additional
illustrations of his views, of which the following are the most
important: "Passing to Incompletae, the orders known collectively as
'Cyclospermeae' are related to Caryophylleae; and to my mind are
degradations from it, of which Orache is anemophilous. Cupuliferae have
an inferior ovary and rudimentary calyx-limb on the top. These, as far
as I know, cannot be interpreted except as degradations. The whole of
Monocotyledons appear to me (from anatomical reasons especially) to be
degradations from Dicotyledons, and primarily through the agency of
growth in water. Many subsequently became terrestrial, but retained the
effects of their primitive habitat through heredity. The 3-merous [sic]
perianth of grasses, the parts of the flower being in whorls, point to a
degradation from a sub-liliaceous condition."
Mr. Henslow informs me that he has long held these views, but, as far as
he knows, alone. Mr. Grant Allen, however, set forth a similar theory in
his _Vignettes from Nature_ (p. 15) and more fully in _The Colours of
Flowers_ (chap. v.), where he develops it fully and uses similar
arguments to those of Mr. Henslow.]
[Footnote 156: H. Müller gives ample proof of this in his _Fertilisation
of Flowers_.]
[Footnote 157: _Cross-and Self-Fertilisation_, p. 27.]
[Footnote 158: _Animals and Plants_, vol. ii. p. 145.]
[Footnote 159: Müller's _Fertilisation of Flowers_, pp. 448, 455. Other
cases of recent degradation and readaptation to insect-fertilisation are
given by Professor Henslow (see footnote, p. 324).]
[Footnote 160: _The Colour Sense_, by Grant Allen, p. 95.]
[Footnote 161: _The Colour Sense_, chap. ix.]
[Footnote 162: See _Origin of Species_, sixth edition, p. 220.]