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

as being so abundant, that from twelve to fifteen hundred have been seen

within an hour at one spot; and they range over the whole country from

the Himalayas to Ceylon. Why, in allied species, the development of

accessory plumes has taken different forms, we are unable to say, except

that it may be due to that individual variability which has served as

the starting-point for so much of what seems to us strange in form, or

fantastic in colour, both in the animal and vegetable world.

_Development of Accessory Plumes and their Display._
If we have found a _vera causa_ for the origin of ornamental appendages

of birds and other animals in a surplus of vital energy, leading to

abnormal growths in those parts of the integument where muscular and

nervous action are greatest, the continuous development of these

appendages will result from the ordinary action of natural selection in

preserving the most healthy and vigorous individuals, and the still

further selective agency of sexual struggle in giving to the very

strongest and most energetic the parentage of the next generation. And,

as all the evidence goes to show that, so far as female birds exercise

any choice, it is of "the most vigorous, defiant, and mettlesome male,"

this form of sexual selection will act in the same direction, and help

to carry on the process of plume development to its culmination. That

culmination will be reached when the excessive length or abundance of

the plumes begins to be injurious to the bearer of them; and it may be

this check to the further lengthening of the peacock's train that has

led to the broadening of the feathers at the ends, and the consequent

production of the magnificent eye-spots which now form its crowning

ornament.

to their production. Just in proportion as the feathers themselves

increased in length and abundance, the skin-muscles which serve to

elevate them would increase also; and the nervous development as well as

the supply of blood to these parts being at a maximum, the erection of

the plumes would become a habit at all periods of nervous or sexual

excitement. The display of the plumes, like the existence of the plumes

themselves, would be the chief external indication of the maturity and

vigour of the male, and would, therefore, be necessarily attractive to

the female. We have, thus, no reason for imputing to her any of those

aesthetic emotions which are excited in us, by the beauty of form,

colour, and pattern of these plumes; or the still more improbable

aesthetic tastes, which would cause her to choose her mate on account of

minute differences in their forms, colours, or patterns.

I have elsewhere suggested[134] that crests and other erectile feathers

may have been useful in making the bird more formidable in appearance,

and thus serving to frighten away enemies; while long tail or wing

feathers might serve to distract the aim of a bird of prey. But though

this might be of some use in the earlier stages of their development, it

is probably of little importance compared with the vigour and pugnacity

of which the plumes are the indication, and which enable most of their

possessors to defend themselves against the enemies which are dangerous

to weaker and more timid birds. Even the tiny humming-birds are said to

attack birds of prey that approach too near to their nests.

_The Effect of Female Preference will be Neutralised by Natural

Selection._
The various facts and arguments now briefly set forth, afford an

explanation of the phenomena of male ornament, as being due to the

general laws of growth and development, and make it unnecessary to call

to our aid so hypothetical a cause as the cumulative action of female

preference. There remains, however, a general argument, arising from the

action of natural selection itself, which renders it almost

inconceivable that female preference could have been effective in the

way suggested; while the same argument strongly supports the view here

set forth. Natural selection, as we have seen in our earlier chapters,

acts perpetually and on an enormous scale in weeding out the "unfit" at

every stage of existence, and preserving only those which are in all

respects the very best. Each year, only a small percentage of young

birds survive to take the place of the old birds which die; and the

survivors will be those which are best able to maintain existence from

the egg onwards, an important factor being that their parents should be

well able to feed and protect them, while they themselves must in turn

be equally able to feed and protect their own offspring. Now this

extremely rigid action of natural selection must render any attempt to

select mere ornament utterly nugatory, unless the most ornamented always

coincide with "the fittest" in every other respect; while, if they do so

coincide, then any selection of ornament is altogether superfluous. If

the most brightly coloured and fullest plumaged males are _not_ the most

healthy and vigorous, have _not_ the best instincts for the proper

construction and concealment of the nest, and for the care and

protection of the young, they are certainly not the fittest, and will

not survive, or be the parents of survivors. If, on the other hand,

there _is_ generally this correlation--if, as has been here argued,

ornament is the natural product and direct outcome of superabundant

health and vigour, then no other mode of selection is needed to account

for the presence of such ornament. The action of natural selection does

not indeed disprove the existence of female selection of ornament as

ornament, but it renders it entirely ineffective; and as the direct

evidence for any such female selection is almost _nil_, while the

objections to it are certainly weighty, there can be no longer any

reason for upholding a theory which was provisionally useful in calling

attention to a most curious and suggestive body of facts, but which is

now no longer tenable. The term "sexual selection" must, therefore, be

restricted to the direct results of male struggle and combat. This is

really a form of natural selection, and is a matter of direct

observation; while its results are as clearly deducible as those of any

of the other modes in which selection acts. And if this restriction of

the term is needful in the case of the higher animals it is much more so

with the lower. In butterflies the weeding out by natural selection

takes place to an enormous extent in the egg, larva, and pupa states;

and perhaps not more than one in a hundred of the eggs laid produces a

perfect insect which lives to breed. Here, then, the impotence of female

selection, if it exist, must be complete; for, unless the most

brilliantly coloured males are those which produce the best protected

eggs, larvae, and pupae, and unless the particular eggs, larvae, and

pupae, which are able to survive, are those which produce the most

brilliantly coloured butterflies, any choice the female might make must

be completely swamped. If, on the other hand, there _is_ this

correlation between colour development and perfect adaptation to

conditions in all stages, then this development will necessarily proceed

by the agency of natural selection and the general laws which determine

the production of colour and of ornamental appendages.[135]

_General Laws of Animal Coloration._
The condensed account which has now been given of the phenomena of

colour in the animal world will sufficiently show the wonderful

complexity and extreme interest of the subject; while it affords an

admirable illustration of the importance of the great principle of

utility, and of the effect of the theories of natural selection and

development in giving a new interest to the most familiar facts of

nature. Much yet remains to be done, both in the observation of new

facts as to the relations between the colours of animals and their

habits or economy, and, more especially, in the elucidation of the laws

of growth which determine changes of colour in the various groups; but

so much is already known that we are able, with some confidence, to

formulate the general principles which have brought about all the beauty

and variety of colour which everywhere delight us in our contemplation

of animated nature. A brief statement of these principles will fitly

conclude our exposition of the subject.
1. Colour may be looked upon as a necessary result of the highly complex

chemical constitution of animal tissues and fluids. The blood, the bile,

the bones, the fat, and other tissues have characteristic, and often

brilliant colours, which we cannot suppose to have been determined for

any special purpose, as colours, since they are usually concealed. The

external organs, with their various appendages and integuments, would,

by the same general laws, naturally give rise to a greater variety of

colour.
2. We find it to be the fact that colour increases in variety and

intensity as external structures and dermal appendages become more

differentiated and developed. It is on scales, hair, and especially on

the more highly specialised feathers, that colour is most varied and

beautiful; while among insects colour is most fully developed in those

whose wing membranes are most expanded, and, as in the lepidoptera, are

clothed with highly specialised scales. Here, too, we find an additional

mode of colour production in transparent lamellae or in fine surface

striae which, by the laws of interference, produce the wonderful

metallic hues of so many birds and insects.
3. There are indications of a progressive change of colour, perhaps in

some definite order, accompanying the development of tissues or

appendages. Thus spots spread and fuse into bands, and when a lateral or

centrifugal expansion has occurred--as in the termination of the

peacocks' train feathers, the outer web of the secondary quills of the

Argus pheasant, or the broad and rounded wings of many butterflies--into

variously shaded or coloured ocelli. The fact that we find gradations of

colour in many of the more extensive groups, from comparatively dull or

simple to brilliant and varied hues, is an indication of some such law

of development, due probably to progressive local segregation in the

tissues of identical chemical or organic molecules, and dependent on

laws of growth yet to be investigated.

have been modified in innumerable ways for the benefit of each species.

The most general modification has been in such directions as to favour

concealment when at rest in the usual surroundings of the species,

sometimes carried on by successive steps till it has resulted in the

most minute imitation of some inanimate object or exact mimicry of some

other animal. In other cases bright colours or striking contrasts have

been preserved, to serve as a warning of inedibility or of dangerous

powers of attack. Most frequent of all has been the specialisation of

each distinct form by some tint or marking for purposes of easy

recognition, especially in the case of gregarious animals whose safety

largely depends upon association and mutual defence.

higher animals there appears a tendency to deeper or more intense

colouring in the male, due probably to his greater vigour and

excitability. In many groups in which this superabundant vitality is at

a maximum, the development of dermal appendages and brilliant colours

has gone on increasing till it has resulted in a great diversity between

the sexes; and in most of these cases there is evidence to show that

natural selection has caused the female to retain the primitive and more

sober colours of the group for purposes of protection.

_Concluding Remarks._

to give some rational explanation of the wonderful amount of brilliant

colour which occurs among tropical animals. Looking on colour as a

normal product of organisation, which has either been allowed free play,

or has been checked and modified for the benefit of the species, we can

see at once that the luxuriant and perennial vegetation of the tropics,

by affording much more constant means of concealment, has rendered

brilliant colour less hurtful there than in the temperate and colder

regions. Again, this perennial vegetation supplies abundance of both

vegetable and insect food throughout the year, and thus a greater

abundance and greater variety of the forms of life are rendered

possible, than where recurrent seasons of cold and scarcity reduce the

possibilities of life to a minimum. Geology furnishes us with another

reason, in the fact, that throughout the tertiary period tropical

conditions prevailed far into the temperate regions, so that the

possibilities of colour development were still greater than they are at

the present time. The tropics, therefore, present to us the results of

animal development in a much larger area and under more favourable

conditions than prevail to-day. We see in them samples of the

productions of an earlier and a better world, from an animal point of

view; and this probably gives a greater variety and a finer display of

colour than would have been produced, had conditions always been what

they are now. The temperate zones, on the other hand, have recently

suffered the effects of a glacial period of extreme severity, with the

result that almost the only gay coloured birds they now possess are

summer visitors from tropical or sub-tropical lands. It is to the

unbroken and almost unchecked course of development from remote

geological times that has prevailed in the tropics, favoured by abundant

food and perennial shelter, that we owe such superb developments as the

frills and crests and jewelled shields of the humming-birds, the golden

plumes of the birds of paradise, and the resplendent train of the

peacock. This last exhibits to us the culmination of that marvel and

mystery of animal colour which is so well expressed by a poet-artist in

the following lines. The marvel will ever remain to the sympathetic

student of nature, but I venture to hope that in the preceding chapters

I have succeeded in lifting--if only by one of its corners--the veil of

mystery which has for long shrouded this department of nature.

_On a Peacock's Feather._

Of her poem but a word,

But a tint brushed from her palette,

This feather of a bird!

Yet set it in the sun glance,

Display it in the shine,

Take graver's lens, explore it,

Note filament and line,

Mark amethyst to sapphire,

And sapphire to gold,

And gold to emerald changing

The archetype unfold!

Tone, tint, thread, tissue, texture,

Through every atom scan,

Conforming still, developing,

Obedient to plan.

This but to form a pattern

On the garment of a bird!

What then must be the poem,

This but its lightest word!

Sit before it; ponder o'er it,

'Twill thy mind advantage more,

Than a treatise, than a sermon,

Than a library of lore.

chap. vii. in which these facts were first brought forward.]

Selection_, chap. v. i.]

introduction, p. xiii.]

and IV, pp. 30, 40, 42.]

Hutchinson's _Illustrations of Clinical Surgery_, quoted by Tylor.]

_Tropical Nature_, pp. 130, 213.]

views here advocated were first set forth, and the reader is referred

there for further details.]
[Footnote 135: The Rev. O. Pickard-Cambridge, who has devoted himself to

the study of spiders, has kindly sent me the following extract from a

letter, written in 1869, in which he states his views on this

question:--

"I myself doubt that particular application of the Darwinian

theory which attributes male peculiarities of form, structure,

colour, and ornament to female appetency or predilection. There

is, it seems to me, undoubtedly something in the male

organisation of a special, and sexual nature, which, of its own

vital force, develops the remarkable male peculiarities so

commonly seen, and of no imaginable use to that sex. In as far

as these peculiarities show a great vital power, they point out

to us the finest and strongest individuals of the sex, and show

us which of them would most certainly appropriate to themselves

the best and greatest number of females, and leave behind them

the strongest and greatest number of progeny. And here would

come in, as it appears to me, the proper application of Darwin's

theory of Natural Selection; for the possessors of greatest

vital power being those most frequently produced and reproduced,

the external signs of it would go on developing in an

ever-increasing exaggeration, only to be checked where it became

really detrimental in some respect or other to the individual."

This passage, giving the independent views of a close observer--one,

moreover, who has studied the species of an extensive group of animals

both in the field and in the laboratory--very nearly accords with my own

conclusions above given; and, so far as the matured opinions of a

competent naturalist have any weight, afford them an important support.]

CHAPTER XI

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._

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