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

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.

existence under the conditions we have seen to exist.

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.

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.

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._

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.

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.

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."

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