The Project Gutenberg ebook of Darwinism (1889), by Alfred Russel Wallace

scarcity, will devour everything in the shape of a fleshy stem or tuber.

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._

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._

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.

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.

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._

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.

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.

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._

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._

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.

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.

have been already sketched in our seventh chapter.

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).

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).

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.

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.

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.