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

of the leaves, in the form, colour, or markings of the flowers, or in

the size, form, colour, or flavour of the fruit. These differences are

usually small, but are yet easily seen, and in their extremes are very

considerable; and they have this important quality, that they have a

tendency to be reproduced, and thus by careful breeding any particular

variation or group of variations can be increased to an enormous

extent--apparently to any extent not incompatible with the life, growth,

and reproduction of the plant or animal.

important to understand this process and its results. Suppose we have a

plant with a small edible seed, and we want to increase the size of that

seed. We grow as large a quantity of it as possible, and when the crop

is ripe we carefully choose a few of the very largest seeds, or we may

by means of a sieve sort out a quantity of the largest seeds. Next year

we sow only these large seeds, taking care to give them suitable soil

and manure, and the result is found to be that the _average_ size of the

seeds is larger than in the first crop, and that the largest seeds are

now somewhat larger and more numerous. Again sowing these, we obtain a

further slight increase of size, and in a very few years we obtain a

greatly improved race, which will always produce larger seeds than the

unimproved race, even if cultivated without any special care. In this

way all our fine sorts of vegetables, fruits, and flowers have been

obtained, all our choice breeds of cattle or of poultry, our wonderful

race-horses, and our endless varieties of dogs. It is a very common but

mistaken idea that this improvement is due to crossing and feeding in

the case of animals, and to improved cultivation in the case of plants.

Crossing is occasionally used in order to obtain a combination of

qualities found in two distinct breeds, and also because it is found to

increase the constitutional vigour; but every breed possessing any

exceptional quality is the result of the selection of variations

occurring year after year and accumulated in the manner just described.

Purity of breed, with repeated selection of the best varieties of that

breed, is the foundation of all improvement in our domestic animals and

cultivated plants.

_Proofs of the Generality of Variation._
Another very common error is, that variation is the exception, and

rather a rare exception, and that it occurs only in one direction at a

time--that is, that only one or two of the numerous possible modes of

variation occur at the same time. The experience of breeders and

cultivators, however, proves that variation is the rule instead of the

exception, and that it occurs, more or less, in almost every direction.

This is shown by the fact that different species of plants and animals

have required different _kinds_ of modification to adapt them to our

use, and we have never failed to meet with variation _in that particular

direction_, so as to enable us to accumulate it and so to produce

ultimately a large amount of change in the required direction. Our

gardens furnish us with numberless examples of this property of plants.

In the cabbage and lettuce we have found variation in the size and mode

of growth of the leaf, enabling us to produce by selection the almost

innumerable varieties, some with solid heads of foliage quite unlike any

plant in a state of nature, others with curiously wrinkled leaves like

the savoy, others of a deep purple colour used for pickling. From the

very same species as the cabbage (Brassica oleracea) have arisen the

broccoli and cauliflower, in which the leaves have undergone little

alteration, while the branching heads of flowers grow into a compact

mass forming one of our most delicate vegetables. The brussels sprouts

are another form of the same plant, in which the whole mode of growth

has been altered, numerous little heads of leaves being produced on the

stem. In other varieties the ribs of the leaves are thickened so as to

become themselves a culinary vegetable; while, in the Kohlrabi, the stem

grows into a turnip-like mass just above ground. Now all these

extraordinarily distinct plants come from one original species which

still grows wild on our coasts; and it must have varied in all these

directions, otherwise variations could not have been accumulated to the

extent we now see them. The flowers and seeds of all these plants have

remained nearly stationary, because no attempt has been made to

accumulate the slight variations that no doubt occur in them.

and potatoes, we find that the roots or underground tubers have been

wonderfully enlarged and improved, and also altered in shape and colour,

while the stems, leaves, flowers, and fruits have remained almost

unchanged. In the various kinds of peas and beans it is the pod or fruit

and the seed that has been subjected to selection, and therefore greatly

modified; and it is here very important to notice that while all these

plants have undergone cultivation in a great variety of soils and

climates, with different manures and under different systems, yet the

flowers have remained but little altered, those of the broad bean, the

scarlet-runner, and the garden-pea, being nearly the same in all the

varieties. This shows us how little change is produced by mere

cultivation, or even by variety of soil and climate, if there is no

_selection_ to preserve and accumulate the small variations that are

continually occurring. When, however, a great amount of modification has

been effected in one country, change to another country produces a

decided effect. Thus it has been found that some of the numerous

varieties of maize produced and cultivated in the United States change

considerably, not only in their size and colour, but even in the shape

of the seed when grown for a few successive years in Germany.[31] In all

our cultivated fruit trees the fruits vary immensely in shape, size,

colour, flavour, time of ripening, and other qualities, while the leaves

and flowers usually differ so little that they are hardly

distinguishable except to a very close observer.

_Variations of Apples and of Melons._
The most remarkable varieties are afforded by the apple and the melon,

and some account of these will be given as illustrating the effects of

slight variations accumulated by selection. All our apples are known to

have descended from the common crab of our hedges (Pyrus malus), and

from this at least a thousand distinct varieties have been produced.

These differ greatly in the size and form of the fruit, in its colour,

and in the texture of the skin. They further differ in the time of

ripening, in their flavour, and in their keeping properties; but apple

trees also differ in many other ways. The foliage of the different

varieties can often be distinguished by peculiarities of form and

colour, and it varies considerably in the time of its appearance; in

some hardly a leaf appears till the tree is in full bloom, while others

produce their leaves so early as almost to hide the flowers. The flowers

differ in size and colour, and in one case in structure also, that of

the St. Valery apple having a double calyx with ten divisions, and

fourteen styles with oblique stigmas, but without stamens or corolla.

The flowers, therefore, have to be fertilised with the pollen from other

varieties in order to produce fruit. The pips or seeds differ also in

shape, size, and colour; some varieties are liable to canker more than

others, while the Winter Majetin and one or two others have the strange

constitutional peculiarity of never being attacked by the mealy bug even

when all the other trees in the same orchard are infested with it.

melo) exceeds them all. A French botanist, M. Naudin, devoted six years

to their study. He found that previous botanists had described thirty

distinct species, as they thought, which were really only varieties of

melons. They differ chiefly in their fruits, but also very much in

foliage and mode of growth. Some melons are only as large as small

plums, others weigh as much as sixty-six pounds. One variety has a

scarlet fruit. Another is not more than an inch in diameter, but

sometimes more than a yard in length, twisting about in all directions

like a serpent. Some melons are exactly like cucumbers; and an Algerian

variety, when ripe, cracks and falls to pieces, just as occurs in a

wild gourd (C. momordica).[32]

_Variations of Flowers._
Turning to flowers, we find that in the same genus as our currant and

gooseberry, which we have cultivated for their fruits, there are some

ornamental species, as the Ribes sanguinea, and in these the flowers

have been selected so as to produce deep red, pink, or white varieties.

When any particular flower becomes fashionable and is grown in large

quantities, variations are always met with sufficient to produce great

varieties of tint or marking, as shown by our roses, auriculas, and

geraniums. When varied leaves are required, it is found that a number of

plants vary sufficiently in this direction also, and we have zonal

geraniums, variegated ivies, gold and silver marked hollies, and many

others.

_Variations of Domestic Animals._

cases; and it appears as if any special quality or modification in an

animal can be obtained if we only breed it in sufficient quantity, watch

carefully for the required variations, and carry on selection with

patience and skill for a sufficiently long period. Thus, in sheep we

have enormously increased the wool, and have obtained the power of

rapidly forming flesh and fat; in cows we have increased the production

of milk; in horses we have obtained strength, endurance, or speed, and

have greatly modified size, form, and colour; in poultry we have secured

various colours of plumage, increase of size, and almost perpetual

egg-laying. But it is in dogs and pigeons that the most marvellous

changes have been effected, and these require our special attention.

distinct wild species, because in every part of the world the native

dogs resemble some wild dogs or wolves of the same country. Thus perhaps

several species of wolves and jackals were domesticated in very early

times, and from breeds derived from these, crossed and improved by

selection, our existing dogs have descended. But this intermixture of

distinct species will go a very little way in accounting for the

peculiarities of the different breeds of dogs, many of which are totally

unlike any wild animal. Such is the case with greyhounds, bloodhounds,

bulldogs, Blenheim spaniels, terriers, pugs, turnspits, pointers, and

many others; and these differ so greatly in size, shape, colour, and

habits, as well as in the form and proportions of all the different

parts of the body, that it seems impossible that they could have

descended from any of the known wild dogs, wolves, or allied animals,

none of which differ nearly so much in size, form, and proportions. We

have here a remarkable proof that variation is not confined to

superficial characters--to the colour, hair, or external appendages,

when we see how the entire skeletons of such forms as the greyhound and

the bulldog have been gradually changed in opposite directions till they

are both completely unlike that of any known wild animal, recent or

extinct. These changes have been the result of some thousands of years

of domestication and selection, different breeds being used and

preserved for different purposes; but some of the best breeds are known

to have been improved and perfected in modern times. About the middle of

the last century a new and improved kind of foxhound was produced; the

greyhound was also greatly improved at the end of the last century,

while the true bulldog was brought to perfection about the same period.

The Newfoundland dog has been so much changed since it was first

imported that it is now quite unlike any existing native dog in that

island.[33]

_Domestic Pigeons._
The most remarkable and instructive example of variation produced by

human selection is afforded by the various races and breeds of domestic

pigeons, not only because the variations produced are often most

extraordinary in amount and diverse in character, but because in this

case there is no doubt whatever that all have been derived from one wild

species, the common rock-pigeon (Columba livia). As this is a very

important point it is well to state the evidence on which the belief is

founded. The wild rock-pigeon is of a slaty-blue colour, the tail has a

dark band across the end, the wings have two black bands, and the outer

tail-feathers are edged with white at the base. No other wild pigeon in

the world has this combination of characters. Now in every one of the

domestic varieties, even the most extreme, all the above marks, even to

the white edging of the outer tail-feathers, are sometimes found

perfectly developed. When birds belonging to two distinct breeds are

crossed one or more times, neither of the parents being blue, or having

any of the above-named marks, the mongrel offspring are very apt to

acquire some of these characters. Mr. Darwin gives instances which he

observed himself. He crossed some white fantails with some black barbs,

and the mongrels were black, brown, or mottled. He also crossed a barb

with a spot, which is a white bird with a red tail and red spot on the

forehead, and the mongrel offspring were dusky and mottled. On now

crossing these two sets of mongrels with each other, he obtained a bird

of a beautiful blue colour, with the barred and white edged tail, and

double-banded wings, so as almost exactly to resemble a wild

rock-pigeon. This bird was descended in the second generation from a

pure white and pure black bird, both of which when unmixed breed their

kind remarkably true. These facts, well known to experienced

pigeon-fanciers, together with the habits of the birds, which all like

to nest in holes, or dovecots, not in trees like the great majority of

wild pigeons, have led to the general belief in the single origin of all

the different kinds.
In order to afford some idea of the great differences which exist among

domesticated pigeons, it will be well to give a brief abstract of Mr.

Darwin's account of them. He divides them into eleven distinct races,

most of which have several sub-races.

enlarged crop, which can be so inflated in some birds as almost to

conceal the beak. They are very long in the body and legs and stand

almost upright, so as to present a very distinct appearance. Their

skeleton has become modified, the ribs being broader and the vertebrae

more numerous than in other pigeons.

pointed beak, and the eyes surrounded with a naked carunculated skin or

wattle, which is also largely developed at the base of the beak. The

opening of the mouth is unusually wide. There are several sub-races, one

being called Dragons.
RACE III. _Runts_.--These are very large-bodied, long-beaked pigeons,

with naked skin round the eyes. The wings are usually very long, the

legs long, and the feet large, and the skin of the neck is often red.

There are several sub-races, and these differ very much, forming a

series of links between the wild rock-pigeon and the carrier.
RACE IV. _Barbs_.--These are remarkable for their very short and thick

beak, so unlike that of most pigeons that fanciers compare it with that

of a bullfinch. They have also a naked carunculated skin round the eyes,

and the skin over the nostrils swollen.

an enormously developed tail, consisting usually of from fourteen to

forty feathers instead of twelve, the regular number in all other

pigeons, wild and tame. The tail spreads out like a fan and is usually

carried erect, and the bird bends back its slender neck, so that in

highly-bred varieties the head touches the tail. The feet are small, and

they walk stiffly.
RACE VI. _Turbits and Owls_.--These are characterised by the feathers of

the middle of neck and breast in front spreading out irregularly so as

to form a frill. The Turbits also have a crest on the head, and both

have the beak exceedingly short.

they are specially distinguished by the singular habit of tumbling over

backwards during flight. One of the sub-races, the Indian Lotan or

Ground tumbler, if slightly shaken and placed on the ground, will

immediately begin tumbling head over heels until taken up and soothed.

If not taken up, some of them will go on tumbling till they die. Some

English tumblers are almost equally persistent. A writer, quoted by Mr.

Darwin, says that these birds generally begin to tumble almost as soon

as they can fly; "at three months old they tumble well, but still fly

strong; at five or six months they tumble excessively; and in the second

year they mostly give up flying, on account of their tumbling so much

and so close to the ground. Some fly round with the flock, throwing a

clean summersault every few yards till they are obliged to settle from

giddiness and exhaustion. These are called Air-tumblers, and they

commonly throw from twenty to thirty summersaults in a minute, each

clear and clean. I have one red cock that I have on two or three

occasions timed by my watch, and counted forty summersaults in the

minute. At first they throw a single summersault, then it is double,

till it becomes a continuous roll, which puts an end to flying, for if

they fly a few yards over they go, and roll till they reach the ground.

Thus I had one kill herself, and another broke his leg. Many of them

turn over only a few inches from the ground, and will tumble two or

three times in flying across their loft. These are called House-tumblers

from tumbling in the house. The act of tumbling seems to be one over

which they have no control, an involuntary movement which they seem to

try to prevent. I have seen a bird sometimes in his struggles fly a yard

or two straight upwards, the impulse forcing him backwards while he

struggles to go forwards."[34]

the power of tumbling, but are valued for possessing some other

characteristics in an extreme degree. They are very small, have almost

globular heads, and a very minute beak, so that fanciers say the head of

a perfect bird should resemble a cherry with a barleycorn stuck in it.

Some of these weigh less than seven ounces, whereas the wild rock-pigeon

weighs about fourteen ounces. The feet, too, are very short and small,

and the middle toe has twelve or thirteen instead of fourteen or fifteen

scutellae. They have often only nine primary wing-feathers instead of

ten as in all other pigeons.

and the feathers of the whole body are reversed or turn backwards.

enclosing the head and meeting in front of the neck. The wings and tail

are unusually long.
RACE X. _Trumpeter_.--Distinguished by a tuft of feathers curling

forwards over the beak, and the feet very much feathered. They obtain

their name from the peculiar voice unlike that of any other pigeon. The

coo is rapidly repeated, and is continued for several minutes. The feet

are covered with feathers so large as often to appear like little wings.
RACE XI. comprises _Laughers_, _Frill-backs_, _Nuns_, _Spots_, _and

Swallows_.--They are all very like the common rock-pigeon, but have each

some slight peculiarity. The Laughers have a peculiar voice, supposed to

resemble a laugh. The Nuns are white, with the head, tail, and primary

wing-feathers black or red. The Spots are white, with the tail and a

spot on the forehead red. The Swallows are slender, white in colour,

with the head and wings of some darker colour.
Besides these races and sub-races a number of other kinds have been

described, and about one hundred and fifty varieties can be

distinguished. It is interesting to note that almost every part of the

bird, whose variations can be noted and selected, has led to variations

of a considerable extent, and many of these have necessitated changes in

the plumage and in the skeleton quite as great as any that occur in the

numerous distinct species of large genera. The form of the skull and

beak varies enormously, so that the skulls of the Short-faced tumbler

and some of the Carriers differ more than any wild pigeons, even those

classed in distinct genera. The breadth and number of the ribs vary, as

well as the processes on them; the number of the vertebrae and the

length of the sternum also vary; and the perforations in the sternum

vary in size and shape. The oil gland varies in development, and is

sometimes absent. The number of the wing-feathers varies, and those of

the tail to an enormous extent. The proportions of the leg and feet and

the number of the scutellae also vary. The eggs also vary somewhat in

size and shape; and the amount of downy clothing on the young bird, when

first hatched, differs very considerably. Finally, the attitude of the

body, the manner of walking, the mode of flight, and the voice, all

exhibit modifications of the most remarkable kind.[35]

_Acclimatisation_.
A very important kind of variation is that constitutional change termed

acclimatisation, which enables any organism to become gradually adapted

to a different climate from the parent stock. As closely allied species

often inhabit different countries possessing very different climates, we

should expect to find cases illustrating this change among our