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

domesticated animals and cultivated plants. A few examples will

therefore be adduced showing that such constitutional variation does

occur.
Among animals the cases are not numerous, because no systematic attempt

has been made to select varieties for this special quality. It has,

however, been observed that, though no European dogs thrive well in

India, the Newfoundland dog, originating from a severe climate, can

hardly be kept alive. A better case, perhaps, is furnished by merino

sheep, which, when imported directly from England, do not thrive, while

those which have been bred in the intermediate climate of the Cape of

Good Hope do much better. When geese were first introduced into Bogota,

they laid few eggs at long intervals, and few of the young survived. By

degrees, however, the fecundity improved, and in about twenty years

became equal to what it is in Europe. According to Garcilaso, when fowls

were first introduced into Peru they were not fertile, whereas now they

are as much so as in Europe.

in their catalogues varieties of fruit-trees which are more or less

hardy, and this is especially the case in America, where certain

varieties only will stand the severe climate of Canada. There is one

variety of pear, the Forelle, which both in England and France withstood

frosts that killed the flowers and buds of all other kinds of pears.

Wheat, which is grown over so large a portion of the world, has become

adapted to special climates. Wheat imported from India and sown in good

wheat soil in England produced the most meagre ears; while wheat taken

from France to the West Indian Islands produced either wholly barren

spikes or spikes furnished with two or three miserable seeds, while West

Indian seed by its side yielded an enormous harvest. The orange was very

tender when first introduced into Italy, and continued so as long as it

was propagated by grafts, but when trees were raised from seed many of

these were found to be hardier, and the orange is now perfectly

acclimatised in Italy. Sweet-peas (Lathyrus odoratus) imported from

England to the Calcutta Botanic Gardens produced few blossoms and no

seed; those from France flowered a little better, but still produced no

seed, but plants raised from seed brought from Darjeeling in the

Himalayas, but originally derived from England, flower and seed

profusely in Calcutta.[36]
An observation by Mr. Darwin himself is perhaps even more instructive.

He says: "On 24th May 1864 there was a severe frost in Kent, and two

rows of scarlet runners (Phaseolus multiflorus) in my garden, containing

390 plants of the same age and equally exposed, were all blackened and

killed except about a dozen plants. In an adjoining row of Fulmer's

dwarf bean (Phaseolus vulgaris) one single plant escaped. A still more

severe frost occurred four days afterwards, and of the dozen plants

which had previously escaped only three survived; these were not taller

or more vigorous than the other young plants, but they escaped

completely, with not even the tips of their leaves browned. It was

impossible to behold these three plants, with their blackened, withered,

and dead brethren all around them, and not see at a glance that they

differed widely in their constitutional power of resisting frost."
The preceding sketch of the variation that occurs among domestic animals

and cultivated plants shows how wide it is in range and how great in

amount; and we have good reason to believe that similar variation

extends to all organised beings. In the class of fishes, for example, we

have one kind which has been long domesticated in the East, the gold

and silver carps; and these present great variation, not only of colour

but in the form and structure of the fins and other external organs. In

like manner, the only domesticated insects, hive bees and silkworm

moths, present numbers of remarkable varieties which have been produced

by the selection of chance variations just as in the case of plants and

the higher animals.

_Circumstances favourable to Selection by Man._

employed for the purpose of improving the races of animals or plants

useful to man is of comparatively recent origin, though some of the

different races are known to have been in existence in very early times.

But Mr. Darwin has pointed out, that unconscious selection must have

begun to produce an effect as soon as plants were cultivated or animals

domesticated by man. It would have been very soon observed that animals

and plants produced their like, that seed of early wheat produced early

wheat, that the offspring of very swift dogs were also swift, and as

every one would try to have a good rather than a bad sort this would

necessarily lead to the slow but steady improvement of all useful plants

and animals subject to man's care. Soon there would arise distinct

breeds, owing to the varying uses to which the animals and plants were

put. Dogs would be wanted chiefly to hunt one kind of game in one part

of the country and another kind elsewhere; for one purpose scent would

be more important, for another swiftness, for another strength and

courage, for yet another watchfulness and intelligence, and this would

soon lead to the formation of very distinct races. In the case of

vegetables and fruits, different varieties would be found to succeed

best in certain soils and climates; some might be preferred on account

of the quantity of food they produced, others for their sweetness and

tenderness, while others might be more useful on account of their

ripening at a particular season, and thus again distinct varieties would

be established. An instance of unconscious selection leading to distinct

results in modern times is afforded by two flocks of Leicester sheep

which both originated from the same stock, and were then bred pure for

upwards of fifty years by two gentlemen, Mr. Buckley and Mr. Burgess.

Mr. Youatt, one of the greatest authorities on breeding domestic

animals, says: "There is not a suspicion existing in the mind of any one

at all acquainted with the subject that the owner of either of them has

deviated in any one instance from the pure blood of Mr. Bakewell's

original flock, and yet the difference between the sheep possessed by

these two gentlemen is so great that they have the appearance of being

quite different varieties." In this case there was no desire to deviate

from the original breed, and the difference must have arisen from some

slight difference of taste or judgment in selecting, each year, the

parents for the next year's stock, combined perhaps with some direct

effect of the slight differences of climate and soil on the two farms.

from the earliest seats of civilisation in Western Asia or Egypt, and

have therefore been the subjects of human care and selection for some

thousands of years, the result being that, in many cases, we do not know

the wild stock from which they originally sprang. The horse, the camel,

and the common bull and cow are nowhere found in a wild state, and they

have all been domesticated from remote antiquity. The original of the

domestic fowl is still wild in India and the Malay Islands, and it was

domesticated in India and China before 1400 B.C. It was introduced into

Europe about 600 B.C. Several distinct breeds were known to the Romans

about the commencement of the Christian era, and they have since spread

all over the civilised world and been subjected to a vast amount of

conscious and unconscious selection, to many varieties of climate and to

differences of food; the result being seen in the wonderful diversity of

breeds which differ quite as remarkably as do the different races of

pigeons already described.

unknown as truly wild plants; and the same is the case with many

vegetables, for De Candolle states that out of 157 useful cultivated

plants thirty-two are quite unknown in a wild state, and that forty more

are of doubtful origin. It is not improbable that most of these do exist

wild, but they have been so profoundly changed by thousands of years of

cultivation as to be quite unrecognisable. The peach is unknown in a

wild state, unless it is derived from the common almond, on which point

there is much difference of opinion among botanists and horticulturists.
The immense antiquity of most of our cultivated plants sufficiently

explains the apparent absence of such useful productions in Australia

and the Cape of Good Hope, notwithstanding that they both possess an

exceedingly rich and varied flora. These countries having been, until a

comparatively recent period, inhabited only by uncivilised men, neither

cultivation nor selection has been carried on for a sufficiently long

time. In North America, however, where there was evidently a very

ancient if low form of civilisation, as indicated by the remarkable

mounds, earthworks, and other prehistoric remains, maize was cultivated,

though it was probably derived from Peru; and the ancient civilisation

of that country and of Mexico has given rise to no fewer than

thirty-three useful cultivated plants.

_Conditions favourable to the production of Variations._
In order that plants and animals may be improved and modified to any

considerable extent, it is of course essential that suitable variations

should occur with tolerable frequency. There seem to be three conditions

which are especially favourable to the production of variations: (1)

That the particular species or variety should be kept in very large

numbers; (2) that it should be spread over a wide area and thus

subjected to a considerable diversity of physical conditions; and (3)

that it should be occasionally crossed with some distinct but closely

allied race. The first of these conditions is perhaps the most

important, the chance of variations of any particular kind being

increased in proportion to the quantity of the original stock and of its

annual offspring. It has been remarked that only those breeders who keep

large flocks can effect much improvement; and it is for the same reason

that pigeons and fowls, which can be so easily and rapidly increased,

and which have been kept in such large numbers by so great a number of

persons, have produced such strange and numerous varieties. In like

manner, nurserymen who grow fruit and flowers in large quantities have a

great advantage over private amateurs in the production of new

varieties.
Although I believe, for reasons which will be given further on, that

some amount of variability is a constant and necessary property of all

organisms, yet there appears to be good evidence to show that changed

conditions of life tend to increase it, both by a direct action on the

organisation and by indirectly affecting the reproductive system. Hence

the extension of civilisation, by favouring domestication under altered

conditions, facilitates the process of modification. Yet this change

does not seem to be an essential condition, for nowhere has the

production of extreme varieties of plants and flowers been carried

farther than in Japan, where careful selection continued for many

generations must have been the chief factor. The effect of occasional

crosses often results in a great amount of variation, but it also leads

to instability of character, and is therefore very little employed in

the production of fixed and well-marked races. For this purpose, in

fact, it has to be carefully avoided, as it is only by isolation and

pure breeding that any specially desired qualities can be increased by

selection. It is for this reason that among savage peoples, whose

animals run half wild, little improvement takes place; and the

difficulty of isolation also explains why distinct and pure breeds of

cats are so rarely met with. The wide distribution of useful animals and

plants from a very remote epoch has, no doubt, been a powerful cause of

modification, because the particular breed first introduced into each

country has often been kept pure for many years, and has also been

subjected to slight differences of conditions. It will also usually have

been selected for a somewhat different purpose in each locality, and

thus very distinct races would soon originate.

the part this plays in the economy of nature, will be explained in a

future chapter.

_Concluding Remarks._

indefinitely increased--will suffice to show that there is hardly an

organ or a quality in plants or animals which has not been observed to

vary; and further, that whenever any of these variations have been

useful to man he has been able to increase them to a marvellous extent

by the simple process of always preserving the best varieties to breed

from. Along with these larger variations others of smaller amount

occasionally appear, sometimes in external, sometimes in internal

characters, the very bones of the skeleton often changing slightly in

form, size, or number; but as these secondary characters have been of no

use to man, and have not been specially selected by him, they have,

usually, not been developed to any great amount except when they have

been closely dependent on those external characters which he has largely

modified.

his love of beauty, of novelty, or merely of something strange or

amusing, the variations he has thus produced have something of the

character of monstrosities. Not only are they often of no use to the

animals or plants themselves, but they are not unfrequently injurious to

them. In the Tumbler pigeons, for instance, the habit of tumbling is

sometimes so excessive as to injure or kill the bird; and many of our

highly-bred animals have such delicate constitutions that they are very

liable to disease, while their extreme peculiarities of form or

structure would often render them quite unfit to live in a wild state.

In plants, many of our double flowers, and some fruits, have lost the

power of producing seed, and the race can thus be continued only by

means of cuttings or grafts. This peculiar character of domestic

productions distinguishes them broadly from wild species and varieties,

which, as will be seen by and by, are necessarily adapted in every part

of their organisation to the conditions under which they have to live.

Their importance for our present inquiry depends on their demonstrating

the occurrence of incessant slight variations in all parts of an

organism, with the transmission to the offspring of the special

characteristics of the parents; and also, that all such slight

variations are capable of being accumulated by selection till they

present very large and important divergencies from the ancestral stock.

plants under domestication strikingly accords with that which we have

proved to exist in a state of nature. And it is not at all surprising

that it should be so, since all the species were in a state of nature

when first domesticated or cultivated by man, and whatever variations

occur must be due to purely natural causes. Moreover, on comparing the

variations which occur in any one generation of domesticated animals

with those which we know to occur in wild animals, we find no evidence

of greater individual variation in the former than in the latter. The

results of man's selection are more striking to us because we have

always considered the varieties of each domestic animal to be

essentially identical, while those which we observe in a wild state are

held to be essentially diverse. The greyhound and the spaniel seem

wonderful, as varieties of one animal produced by man's selection; while

we think little of the diversities of the fox and the wolf, or the horse

and the zebra, because we have been accustomed to look upon them as

radically distinct animals, not as the results of nature's selection of

the varieties of a common ancestor.

p. 322.]
[Footnote 32: These facts are taken from Darwin's _Domesticated Animals

and Cultivated Plants_, vol. i. pp. 359, 360, 392-401; vol. ii. pp. 231,

275, 330.]

vol. i. pp. 40-42.]

quoted by Darwin, _Animals and Plants under Domestication_, vol. i. p.

151.]
[Footnote 35: This account of domestic pigeons is greatly condensed from

Mr. Darwin's work already referred to.]

307-311.]

CHAPTER V
NATURAL SELECTION BY VARIATION AND SURVIVAL OF THE FITTEST

Effect of struggle for existence under unchanged conditions--The

effect under change of conditions--Divergence of character--In

insects--In birds--In mammalia--Divergence leads to a maximum of

life in each area--Closely allied species inhabit distinct

areas--Adaptation to conditions at various periods of life--The

continued existence of low forms of life--Extinction of low

types among the higher animals--Circumstances favourable to the

origin of new species--Probable origin of the dippers--The

importance of isolation--On the advance of organisation by

natural selection--Summary of the first five chapters.

In the preceding chapters we have accumulated a body of facts and

arguments which will enable us now to deal with the very core of our

subject--the formation of species by means of natural selection. We have

seen how tremendous is the struggle for existence always going on in

nature owing to the great powers of increase of all organisms; we have

ascertained the fact of variability extending to every part and organ,

each of which varies simultaneously and for the most part independently;

and we have seen that this variability is both large in its amount in

proportion to the size of each part, and usually affects a considerable

proportion of the individuals in the large and dominant species. And,

lastly, we have seen how similar variations, occurring in cultivated

plants and domestic animals, are capable of being perpetuated and

accumulated by artificial selection, till they have resulted in all the

wonderful varieties of our fruits, flowers, and vegetables, our domestic

animals and household pets, many of which differ from each other far

more in external characters, habits, and instincts than do species in a

state of nature. We have now to inquire whether there is any analogous

process in nature, by which wild animals and plants can be permanently

modified and new races or new species produced.

_Effect of Struggle for Existence under Unchanged Conditions._
Let us first consider what will be the effect of the struggle for

existence upon the animals and plants which we see around us, under

conditions which do not perceptibly vary from year to year or from

century to century. We have seen that every species is exposed to

numerous and varied dangers throughout its entire existence, and that it

is only by means of the exact adaptation of its organisation--including

its instincts and habits--to its surroundings that it is enabled to live

till it produces offspring which may take its place when it ceases to

exist. We have seen also that, of the whole annual increase only a very

small fraction survives; and though the survival in individual cases may

sometimes be due rather to accident than to any real superiority, yet we

cannot doubt that, in the long run, those survive which are best fitted

by their perfect organisation to escape the dangers that surround them.

This "survival of the fittest" is what Darwin termed "natural

selection," because it leads to the same results in nature as are

produced by man's selection among domestic animals and cultivated

plants. Its primary effect will, clearly, be to keep each species in the

most perfect health and vigour, with every part of its organisation in

full harmony with the conditions of its existence. It prevents any

possible deterioration in the organic world, and produces that

appearance of exuberant life and enjoyment, of health and beauty, that

affords us so much pleasure, and which might lead a superficial observer

to suppose that peace and quietude reigned throughout nature.

_The Effect under changed Conditions._

substantially the same, secures the continuance of each species of

animal or plant in its full perfection, will usually, under changed

conditions, bring about whatever change of structure or habits may be

necessitated by them. The changed conditions to which we refer are such

as we know have occurred throughout all geological time and in every

part of the world. Land and water have been continually shifting their

positions; some regions are undergoing subsidence with diminution of

area, others elevation with extension of area; dry land has been

converted into marshes, while marshes have been drained or have even

been elevated into plateaux. Climate too has changed again and again,

either through the elevation of mountains in high latitudes leading to

the accumulation of snow and ice, or by a change in the direction of

winds and ocean currents produced by the subsidence or elevation of

lands which connected continents and divided oceans. Again, along with

all these changes have come not less important changes in the

distribution of species. Vegetation has been greatly modified by changes

of climate and of altitude; while every union of lands before separated

has led to extensive migrations of animals into new countries,

disturbing the balance that before existed among its forms of life,

leading to the extermination of some species and the increase of others.