Darwinism (1889) by Alfred Russel Wallace

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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 Mueller.

_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
Mueller 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 Mueller
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. Mueller 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 Mueller 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]