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

many species must either become modified or cease to exist. When the

vegetation has changed in character the herbivorous animals must become

able to live on new and perhaps less nutritious food; while the change

from a damp to a dry climate may necessitate migration at certain

periods to escape destruction by drought. This will expose the species

to new dangers, and require special modifications of structure to meet

them. Greater swiftness, increased cunning, nocturnal habits, change of

colour, or the power of climbing trees and living for a time on their

foliage or fruit, may be the means adopted by different species to bring

themselves into harmony with the new conditions; and by the continued

survival of those individuals, only, which varied sufficiently in the

right direction, the necessary modifications of structure or of function

would be brought about, just as surely as man has been able to breed the

greyhound to hunt by sight and the foxhound by scent, or has produced

from the same wild plant such distinct forms as the cauliflower and the

brussels sprouts.
We will now consider the special characteristics of the changes in

species that are likely to be effected, and how far they agree with what

we observe in nature.

_Divergence of Character._

cause some individuals or groups of individuals to adopt new habits in

order to seize upon vacant places in nature where the struggle is less

severe. Some, living among extensive marshes, may adopt a more aquatic

mode of life; others, living where forests abound, may become more

arboreal. In either case we cannot doubt that the changes of structure

needed to adapt them to their new habits would soon be brought about,

because we know that variations in all the external organs and all their

separate parts are very abundant and are also considerable in amount.

That such divergence of character has actually occurred we have some

direct evidence. Mr. Darwin informs us that in the Catskill Mountains in

the United States there are two varieties of wolves, one with a light

greyhound-like form which pursues deer, the other more bulky with

shorter legs, which more frequently attacks sheep.[37] Another good

example is that of the insects in the island of Madeira, many of which

have either lost their wings or have had them so much reduced as to be

useless for flight, while the very same species on the continent of

Europe possess fully developed wings. In other cases the wingless

Madeira species are distinct from, but closely allied to, winged species

of Europe. The explanation of this change is, that Madeira, like many

oceanic islands in the temperate zone, is much exposed to sudden gales

of wind, and as most of the fertile land is on the coast, insects which

flew much would be very liable to be blown out to sea and lost. Year

after year, therefore, those individuals which had shorter wings, or

which used them least, were preserved; and thus, in time, terrestrial,

wingless, or imperfectly winged races or species have been produced.

That this is the true explanation of this singular fact is proved by

much corroborative evidence. There are some few flower-frequenting

insects in Madeira to whom wings are essential, and in these the wings

are somewhat larger than in the same species on the mainland. We thus

see that there is no general tendency to the abortion of wings in

Madeira, but that it is simply a case of adaptation to new conditions.

Those insects to whom wings were not absolutely essential escaped a

serious danger by not using them, and the wings therefore became reduced

or were completely lost. But when they were essential they were enlarged

and strengthened, so that the insect could battle against the winds and

save itself from destruction at sea. Many flying insects, not varying

fast enough, would be destroyed before they could establish themselves,

and thus we may explain the total absence from Madeira of several whole

families of winged insects which must have had many opportunities of

reaching the islands. Such are the large groups of the tiger-beetles

(Cicindelidae), the chafers (Melolonthidae), the click-beetles

(Elateridae), and many others.
But the most curious and striking confirmation of this portion of Mr.

Darwin's theory is afforded by the case of Kerguelen Island. This island

was visited by the _Transit of Venus_ expedition. It is one of the

stormiest places on the globe, being subject to almost perpetual gales,

while, there being no wood, it is almost entirely without shelter. The

Rev. A.E. Eaton, an experienced entomologist, was naturalist to the

expedition, and he assiduously collected the few insects that were to be

found. All were incapable of flight, and most of them entirely without

wings. They included a moth, several flies, and numerous beetles. As

these insects could hardly have reached the islands in a wingless state,

even if there were any other known land inhabited by them--which there

is not--we must assume that, like the Madeiran insects, they were

originally winged, and lost their power of flight because its possession

was injurious to them.

exposed islands have their wings reduced in size, as is strikingly the

case with the small tortoise-shell butterfly (Vanessa urticae)

inhabiting the Isle of Man, which is only about half the size of the

same species in England or Ireland; and Mr. Wollaston notes that Vanessa

callirhoe--a closely allied South European form of our red-admiral

butterfly--is permanently smaller in the small and bare island of Porto

Santo than in the larger and more wooded adjacent island of Madeira.

accordance with new conditions of life, is afforded by our red grouse.

This bird, the Lagopus scoticus of naturalists, is entirely confined to

the British Isles. It is, however, very closely allied to the willow

grouse (Lagopus albus), a bird which ranges all over Europe, Northern

Asia, and North America, but which, unlike our species, changes to white

in winter. No difference in form or structure can be detected between

the two birds, but as they differ so decidedly in colour--our species

being usually rather darker in winter than in summer, while there are

also slight differences in the call-note and in habits,--the two species

are generally considered to be distinct. The differences, however, are

so clearly adaptations to changed conditions that we can hardly doubt

that, during the early part of the glacial period, when our islands were

united to the continent, our grouse was identical with that of the rest

of Europe. But when the cold passed away and our islands became

permanently separated from the mainland, with a mild and equable climate

and very little snow in winter, the change to white at that season

became hurtful, rendering the birds more conspicuous instead of serving

as a means of concealment. The colour was, therefore, gradually changed

by the process of variation and natural selection; and as the birds

obtained ample shelter among the heather which clothes so many of our

moorlands, it became useful for them to assimilate with its brown and

dusky stems and withered flowers rather than with the snow of the higher

mountains. An interesting confirmation of this change having really

occurred is afforded by the occasional occurrence in Scotland of birds

with a considerable amount of white in the winter plumage. This is

considered to be a case of reversion to the ancestral type, just as the

slaty colours and banded wings of the wild rock-pigeon sometimes

reappear in our fancy breeds of domestic pigeons.[38]
The principle of "divergence of character" pervades all nature from the

lowest groups to the highest, as may be well seen in the class of birds.

Among our native species we see it well marked in the different species

of titmice, pipits, and chats. The great titmouse (Parus major) by its

larger size and stronger bill is adapted to feed on larger insects, and

is even said sometimes to kill small and weak birds. The smaller and

weaker coal titmouse (Parus ater) has adopted a more vegetarian diet,

eating seeds as well as insects, and feeding on the ground as well as

among trees. The delicate little blue titmouse (Parus coeruleus), with

its very small bill, feeds on the minutest insects and grubs which it

extracts from crevices of bark and from the buds of fruit-trees. The

marsh titmouse, again (Parus palustris), has received its name from the

low and marshy localities it frequents; while the crested titmouse

(Parus cristatus) is a northern bird frequenting especially pine

forests, on the seeds of which trees it partially feeds. Then, again,

our three common pipits--the tree-pipit (Anthus arboreus), the

meadow-pipit (Anthus pratensis), and the rock-pipit or sea-lark (Anthus

obscurus) have each occupied a distinct place in nature to which they

have become specially adapted, as indicated by the different form and

size of the hind toe and claw in each species. So, the stone-chat

(Saxicola rubicola), the whin-chat (S. rubetra), and the wheat-ear (S.

oenanthe) are more or less divergent forms of one type, with

modifications in the shape of the wing, feet, and bill adapting them to

slightly different modes of life. The whin-chat is the smallest, and

frequents furzy commons, fields, and lowlands, feeding on worms,

insects, small molluscs, and berries; the stone-chat is next in size,

and is especially active and lively, frequenting heaths and uplands, and

is a permanent resident with us, the two other species being migrants;

while the larger and more conspicuous wheat-ear, besides feeding on

grubs, beetles, etc., is able to capture flying insects on the wing,

something after the manner of true flycatchers.
These examples sufficiently indicate how divergence of character has

acted, and has led to the adaptation of numerous allied species, each to

a more or less special mode of life, with the variety of food, of

habits, and of enemies which must necessarily accompany such diversity.

And when we extend our inquiries to higher groups we find the same

indications of divergence and special adaptation, often to a still more

marked extent. Thus we have the larger falcons, which prey upon birds,

while some of the smaller species, like the hobby (Falco subbuteo), live

largely on insects. The true falcons capture their prey in the air,

while the hawks usually seize it on or near the ground, feeding on

hares, rabbits, squirrels, grouse, pigeons, and poultry. Kites and

buzzards, on the other hand, seize their prey upon the ground, and the

former feed largely on reptiles and offal as well as on birds and

quadrupeds. Others have adopted fish as their chief food, and the osprey

snatches its prey from the water with as much facility as a gull or a

petrel; while the South American caracaras (Polyborus) have adopted the

habits of vultures and live altogether on carrion. In every great group

there is the same divergence of habits. There are ground-pigeons,

rock-pigeons, and wood-pigeons,--seed-eating pigeons and fruit-eating

pigeons; there are carrion-eating, insect-eating, and fruit-eating

crows. Even kingfishers are, some aquatic, some terrestrial in their

habits; some live on fish, some on insects, some on reptiles. Lastly,

among the primary divisions of birds we find a purely terrestrial

group--the Ratitae, including the ostriches, cassowaries, etc.; other

great groups, including the ducks, cormorants, gulls, penguins, etc.,

are aquatic; while the bulk of the Passerine birds are aerial and

arboreal. The same general facts can be detected in all other classes of

animals. In the mammalia, for example, we have in the common rat a

fish-eater and flesh-eater as well as a grain-eater, which has no doubt

helped to give it the power of spreading over the world and driving away

the native rats of other countries. Throughout the Rodent tribe we find

everywhere aquatic, terrestrial, and arboreal forms. In the weasel and

cat tribes some live more in trees, others on the ground; squirrels have

diverged into terrestrial, arboreal, and flying species; and finally, in

the bats we have a truly aerial, and in the whales a truly aquatic order

of mammals. We thus see that, beginning with different varieties of the

same species, we have allied species, genera, families, and orders, with

similarly divergent habits, and adaptations to different modes of life,

indicating some general principle in nature which has been operative in

the development of the organic world. But in order to be thus operative

it must be a generally useful principle, and Mr. Darwin has very clearly

shown us in what this utility consists.

_Divergence leads to a Maximum of Organic Forms in each Area._
Divergence of character has a double purpose and use. In the first place

it enables a species which is being overcome by rivals, or is in

process of extinction by enemies, to save itself by adopting new habits

or by occupying vacant places in nature. This is the immediate and

obvious effect of all the numerous examples of divergence of character

which we have pointed out. But there is another and less obvious result,

which is, that the greater the diversity in the organisms inhabiting a

country or district the greater will be the total amount of life that

can be supported there. Hence the continued action of the struggle for

existence will tend to bring about more and more diversity in each area,

which may be shown to be the case by several kinds of evidence. As an

example, a piece of turf, three feet by four in size, was found by Mr.

Darwin to contain twenty species of plants, and these twenty species

belonged to eighteen genera and to eight orders, showing how greatly

they differed from each other. Farmers find that a greater quantity of

hay is obtained from ground sown with a variety of genera of grasses,

clover, etc., than from similar land sown with one or two species only;

and the same principle applies to rotation of crops, plants differing

very widely from each other giving the best results. So, in small and

uniform islands, and in small ponds of fresh water, the plants and

insects, though few in number, are found to be wonderfully varied in

character.

by man's agency in distant lands, for the species that thrive best and

establish themselves permanently are not only very varied among

themselves but differ greatly from the native inhabitants. Thus, in the

Northern United States there are, according to Dr. Asa Gray, 260

naturalised flowering plants which belong to no less than 162 genera;

and of these, 100 genera are not natives of the United States. So, in

Australia, the rabbit, though totally unlike any native animal, has

increased so much that it probably outnumbers in individuals all the

native mammals of the country; and in New Zealand the rabbit and the pig

have equally multiplied. Darwin remarks that this "advantage of

diversification of structure in the inhabitants of the same region is,

in fact, the same as that of the physiological division of labour in the

organs of the same body. No physiologist doubts that a stomach adapted

to digest vegetable matter alone, or flesh alone, draws more nutriment

from these substances. So, in the general economy of any land, the more

widely and perfectly the animals and plants are diversified for

different habits of life, so will a greater number of individuals be

capable of there supporting themselves."[39]

_The most closely allied Species inhabit distinct Areas._

nature is, that the most closely allied species--those whose differences

though often real and important are hardly perceptible to any one but a

naturalist--are usually not found in the same but in widely separated

countries. Thus, the nearest allies to our European golden plover are

found in North America and East Asia; the nearest ally of our European

jay is found in Japan, although there are several other species of jays

in Western Asia and North Africa; and though we have several species of

titmice in England they are not very closely allied to each other. The

form most akin to our blue tit is the azure tit of Central Asia (Parus

azureus); the Parus ledouci of Algeria is very near our coal tit, and

the Parus lugubris of South-Eastern Europe and Asia Minor is nearest to

our marsh tit. So, our four species of wild pigeons--the ring-dove,

stock-dove, rock-pigeon, and turtle-dove--are not closely allied to each

other, but each of them belongs, according to some ornithologists, to a

separate genus or subgenus, and has its nearest relatives in distant

parts of Asia and Africa. In mammalia the same thing occurs. Each

mountain region of Europe and Asia has usually its own species of wild

sheep and goat, and sometimes of antelope and deer; so that in each

region there is found the greatest diversity in this class of animals,

while the closest allies inhabit quite distinct and often distant areas.

In plants we find the same phenomenon prevalent. Distinct species of

columbine are found in Central Europe (Aguilegia vulgaris), in Eastern

Europe, and Siberia (A. glandulosa), in the Alps (A. Alpina), in the

Pyrenees (A. pyrenaiea), in the Greek mountains (A. ottonis), and in

Corsica (A. Bernardi), but rarely are two species found in the same

area. So, each part of the world has its own peculiar forms of pines,

firs, and cedars, but the closely allied species or varieties are in

almost every case inhabitants of distinct areas. Examples are the deodar

of the Himalayas, the cedar of Lebanon, and that of North Africa, all

very closely allied but confined to distinct areas; and the numerous

closely allied species of true pine (genus Pinus), which almost always

inhabit different countries or occupy different stations. We will now

consider some other modes in which natural selection will act, to adapt

organisms to changed conditions.

_Adaptation to Conditions at Various Periods of Life._

occurring at any one period of life reappear in the offspring at the

same period, and can be perpetuated and increased by selection without

modifying other parts of the organisation. Thus, variations in the

caterpillar or the cocoon of the silkworm, in the eggs of poultry, and

in the seeds or young shoots of many culinary vegetables, have been

accumulated till those parts have become greatly modified and, for man's

purposes, improved. Owing to this fact it is easy for organisms to

become so modified as to avoid dangers that occur at any one period of

life. Thus it is that so many seeds have become adapted to various modes

of dissemination or protection. Some are winged, or have down or hairs

attached to them, so as to enable them to be carried long distances in

the air; others have curious hooks and prickles, which cause them to be

attached firmly to the fur of mammals or the feathers of birds; while

others are buried within sweet or juicy and brightly coloured fruits,

which are seen and devoured by birds, the hard smooth seeds passing

through their bodies in a fit state for germination. In the struggle for

existence it must benefit a plant to have increased means of dispersing

its seeds, and of thus having young plants produced in a greater variety

of soils, aspects, and surroundings, with a greater chance of some of

them escaping their numerous enemies and arriving at maturity. The

various differences referred to would, therefore, be brought about by

variation and survival of the fittest, just as surely as the length and

quality of cotton on the seed of the cotton-plant have been increased

by man's selection.
The larvae of insects have thus been wonderfully modified in order to

escape the numerous enemies to whose attacks they are exposed at this

period of their existence. Their colours and markings have become

marvellously adapted to conceal them among the foliage of the plant they

live upon, and this colour often changes completely after the last

moult, when the creature has to descend to the ground for its change to

the pupa state, during which period a brown instead of a green colour is

protective. Others have acquired curious attitudes and large ocelli,

which cause them to resemble the head of some reptile, or they have

curious horns or coloured ejectile processes which frighten away

enemies; while a great number have acquired secretions which render them

offensive to the taste of their enemies, and these are always adorned

with very conspicuous markings or brilliant colours, which serve as a

sign of inedibility and prevent their being needlessly attacked. This,

however, is a portion of the very large subject of organic colour and

marking, which will be fully discussed and illustrated in a separate

chapter.
In this way every possible modification of an animal or plant, whether

in colour, form, structure, or habits, which would be serviceable to it

or to its progeny at any period of its existence, may be readily brought

about. There are some curious organs which are used only once in a

creature's life, but which are yet essential to its existence, and thus

have very much the appearance of design by an intelligent designer. Such

are, the great jaws possessed by some insects, used exclusively for

opening the cocoon, and the hard tip to the beak of unhatched birds used

for breaking the eggshell. The increase in thickness or hardness of the

cocoons or the eggs being useful for protection against enemies or to

avoid accidents, it is probable that the change has been very gradual,

because it would be constantly checked by the necessity for a

corresponding change in the young insects or birds enabling them to

overcome the additional obstacle of a tougher cocoon or a harder