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

eggshell. As we have seen, however, that every part of the organism

appears to be varying independently, at the same time, though to

different amounts, there seems no reason to believe that the necessity

for two or more coincident variations would prevent the required change

from taking place.

_The Continued Existence of Low Forms of Life._

superiority over other species or enabling them to occupy fresh places

in nature, it may be asked--Why do any low forms continue to exist? Why

have they not long since been improved and developed into higher forms?

The answer, probably, is, that these low forms occupy places in nature

which cannot be filled by higher forms, and that they have few or no

competitors; they therefore continue to exist. Thus, earthworms are

adapted to their mode of life better than they would be if more highly

organised. So, in the ocean, the minute foraminifera and infusoria, and

the larger sponges and corals, occupy places which more highly developed

creatures could not fill. They form, as it were, the base of the great

structure of animal life, on which the next higher forms rest; and

though in the course of ages they may undergo some changes, and

diversification of form and structure, in accordance with changed

conditions, their essential nature has probably remained the same from

the very dawn of life on the earth. The low aquatic diatomaceae and

confervae, together with the lowest fungi and lichens, occupy a similar

position in the vegetable kingdom, filling places in nature which would

be left vacant if only highly organised plants existed. There is,

therefore, no motive power to destroy or seriously to modify them; and

they have thus probably persisted, under slightly varying forms, through

all geological time.

_Extinction of Lower Types among the Higher Animals._
So soon; however, as we approach the higher and more fully developed

groups, we see indications of the often repeated extinction of lower by

higher forms. This is shown by the great gaps that separate the

mammalia, birds, reptiles, and fishes from each other; while the lowest

forms of each are always few in number and confined to limited areas.

Such are the lowest mammals--the echidna and ornithorhynchus of

Australia; the lowest birds--the apteryx of New Zealand and the

cassowaries of the New Guinea region; while the lowest fish--the

amphioxus or lancelet, is completely isolated, and has apparently

survived only by its habit of burrowing in the sand. The great

distinctness of the carnivora, ruminants, rodents, whales, bats, and

other orders of mammalia; of the accipitres, pigeons, and parrots, among

birds; and of the beetles, bees, flies, and moths, among insects, all

indicate an enormous amount of extinction among the comparatively low

forms by which, on any theory of evolution, these higher and more

specialised groups must have been preceded.

_Circumstances favourable to the Origin of New Species by Natural

Selection._

organic conditions of a country, the effect of natural selection is to

keep all the species inhabiting it in a state of perfect health and full

development, and to preserve the balance that already exists between the

different groups of organisms. But, whenever the physical or organic

conditions change, to however small an extent, some corresponding change

will be produced in the flora and fauna, since, considering the severe

struggle for existence and the complex relations of the various

organisms, it is hardly possible that the change should not be

beneficial to some species and hurtful to others. The most common

effect, therefore, will be that some species will increase and others

will diminish; and in cases where a species was already small in numbers

a further diminution might lead to extinction. This would afford room

for the increase of other species, and thus a considerable readjustment

of the proportions of the several species might take place. When,

however, the change was of a more important character, directly

affecting the existence of many species so as to render it difficult for

them to maintain themselves without some considerable change in

structure or habits, that change would, in some cases, be brought about

by variation and natural selection, and thus new varieties or new

species might be formed. We have to consider, then, which are the

species that would be most likely to be so modified, while others, not

becoming modified, would succumb to the changed conditions and become

extinct.
The most important condition of all is, undoubtedly, that variations

should occur of sufficient amount, of a sufficiently diverse character,

and in a large number of individuals, so as to afford ample materials

for natural selection to act upon; and this, we have seen, does occur in

most, if not in all, large, wide-ranging, and dominant species. From

some of these, therefore, the new species adapted to the changed

conditions would usually be derived; and this would especially be the

case when the change of conditions was rather rapid, and when a

correspondingly rapid modification could alone save some species from

extinction. But when the change was very gradual, then even less

abundant and less widely distributed species might become modified into

new forms, more especially if the extinction of many of the rarer

species left vacant places in the economy of nature.

_Probable Origin of the Dippers._
An excellent example of how a limited group of species has been able to

maintain itself by adaptation to one of these "vacant places" in nature,

is afforded by the curious little birds called dippers or water-ouzels,

forming the genus Cinclus and the family Cinclidae of naturalists. These

birds are something like small thrushes, with very short wings and tail,

and very dense plumage. They frequent, exclusively, mountain torrents in

the northern hemisphere, and obtain their food entirely in the water,

consisting, as it does, of water-beetles, caddis-worms and other

insect-larvae, as well as numerous small freshwater shells. These birds,

although not far removed in structure from thrushes and wrens, have the

extraordinary power of flying under water; for such, according to the

best observers, is their process of diving in search of their prey,

their dense and somewhat fibrous plumage retaining so much air that the

water is prevented from touching their bodies or even from wetting their

feathers to any great extent. Their powerful feet and long curved claws

enable them to hold on to stones at the bottom, and thus to retain their

position while picking up insects, shells, etc. As they frequent

chiefly the most rapid and boisterous torrents, among rocks, waterfalls,

and huge boulders, the water is never frozen over, and they are thus

able to live during the severest winters. Only a very few species of

dipper are known, all those of the old world being so closely allied to

our British bird that some ornithologists consider them to be merely

local races of one species; while in North America and the northern

Andes there are two other species.

affinity to the smaller typical perching birds, but which has departed

from all its allies in its habits and mode of life, and has secured for

itself a place in nature where it has few competitors and few enemies.

We may well suppose, that, at some remote period, a bird which was

perhaps the common and more generalised ancestor of most of our

thrushes, warblers, wrens, etc., had spread widely over the great

northern continent, and had given rise to numerous varieties adapted to

special conditions of life. Among these some took to feeding on the

borders of clear streams, picking out such larvae and molluscs as they

could reach in shallow water. When food became scarce they would attempt

to pick them out of deeper and deeper water, and while doing this in

cold weather many would become frozen and starved. But any which

possessed denser and more hairy plumage than usual, which was able to

keep out the water, would survive; and thus a race would be formed which

would depend more and more on this kind of food. Then, following up the

frozen streams into the mountains, they would be able to live there

during the winter; and as such places afforded them much protection from

enemies and ample shelter for their nests and young, further adaptations

would occur, till the wonderful power of diving and flying under water

was acquired by a true land-bird.
That such habits might be acquired under stress of need is rendered

highly probable by the facts stated by the well-known American

naturalist, Dr. Abbott. He says that "the water-thrushes (Seiurus sp.)

all wade in water, and often, seeing minute mollusca on the bottom of

the stream, plunge both head and neck beneath the surface, so that

often, for several seconds, a large part of the body is submerged. Now

these birds still have the plumage pervious to water, and so are liable

to be drenched and sodden; but they have also the faculty of giving

these drenched feathers such a good shaking that flight is practicable a

moment after leaving the water. Certainly the water-thrushes (Seiurus

ludovicianus, S. auricapillus, and S. noveboracensis) have taken many

preliminary steps to becoming as aquatic as the dipper; and the

winter-wren, and even the Maryland yellow-throat are not far

behind."[40]

to occupy new places in nature, is afforded by the various animals which

inhabit the water-vessels formed by the leaves of many epiphytal species

of Bromelia. Fritz Müller has described a caddis-fly larva which lives

among these leaves, and which has been modified in the pupa state in

accordance with its surroundings. The pupae of caddis-flies inhabiting

streams have fringes of hair on the tarsi to enable them to reach the

surface on leaving their cases. But in the species inhabiting bromelia

leaves there is no need for swimming, and accordingly we find the tarsi

entirely bare. In the same plants are found curious little Entomostraca,

very abundant there but found nowhere else. These form a new genus, but

are most nearly allied to Cythere, a marine type. It is believed that

the transmission of this species from one tree to another must be

effected by the young crustacea, which are very minute, clinging to

beetles, many of which, both terrestrial and aquatic, also inhabit the

bromelia leaves; and as some water-beetles are known to frequent the

sea, it is perhaps by these means that the first emigrants established

themselves in this strange new abode. Bromeliae are often very abundant

on trees growing on the water's edge, and this would facilitate the

transition from a marine to an arboreal habitat. Fritz Müller has also

found, among the bromelia leaves, a small frog bearing its eggs on its

back, and having some other peculiarities of structure. Several

beautiful little aquatic plants of the genus Utricularia or bladder-wort

also inhabit bromelia leaves; and these send runners out to neighbouring

plants and thus spread themselves with great rapidity.

_The Importance of Isolation._

the fact that islands so often present a number of peculiar species; and

the same thing is seen on the two sides of a great mountain range or on

opposite coasts of a continent. The importance of isolation is twofold.

In the first place, it leads to a body of individuals of each species

being limited in their range and thus subjected to uniform conditions

for long spaces of time. Both the direct action of the environment and

the natural selection of such varieties only as are suited to the

conditions, will, therefore, be able to produce their full effect. In

the second place, the process of change will not be interfered with by

intercrossing with other individuals which are becoming adapted to

somewhat different conditions in an adjacent area. But this question of

the swamping effects of intercrossing will be considered in another

chapter.
Mr. Darwin was of opinion that, on the whole, the largeness of the area

occupied by a species was of more importance than isolation, as a factor

in the production of new species, and in this I quite agree with him. It

must, too, be remembered, that isolation will often be produced in a

continuous area whenever a species becomes modified in accordance with

varied conditions or diverging habits. For example, a wide-ranging

species may in the northern and colder part of its area become modified

in one direction, and in the southern part in another direction; and

though for a long time an intermediate form may continue to exist in the

intervening area, this will be likely soon to die out, both because its

numbers will be small, and it will be more or less pressed upon in

varying seasons by the modified varieties, each better able to endure

extremes of climate. So, when one portion of a terrestrial species takes

to a more arboreal or to a more aquatic mode of life, the change of

habit itself leads to the isolation of each portion. Again, as will be

more fully explained in a future chapter, any difference of habits or of

haunts usually leads to some modification of colour or marking, as a

means of concealment from enemies; and there is reason to believe that

this difference will be intensified by natural selection as a means of

identification and recognition by members of the same variety or

incipient species. It has also been observed that each differently

coloured variety of wild animals, or of domesticated animals which have

run wild, keep together, and refuse to pair with individuals of the

other colours; and this must of itself act to keep the races separate as

completely as physical isolation.

_On the Advance of Organisation by Natural Selection._
As natural selection acts solely by the preservation of useful

variations, or those which are beneficial to the organism under the

conditions to which it is exposed, the result must necessarily be that

each species or group tends to become more and more improved in relation

to its conditions. Hence we should expect that the larger groups in each

class of animals and plants--those which have persisted and have been

abundant throughout geological ages--would, almost necessarily, have

arrived at a high degree of organisation, both physical and mental.

Illustrations of this are to be seen everywhere. Among mammalia we have

the carnivora, which from Eocene times have been becoming more and more

specialised, till they have culminated in the cat and dog tribes, which

have reached a degree of perfection both in structure and intelligence

fully equal to that of any other animals. In another line of

development, the herbivora have been specialised for living solely on

vegetable food till they have culminated in the sheep, the cattle, the

deer, and the antelopes. The horse tribe, commencing with an early

four-toed ancestor in the Eocene age, has increased in size and in

perfect adaptation of feet and teeth to a life on open plains, and has

reached its highest perfection in the horse, the ass, and the zebra. In

birds, also, we see an advance from the imperfect tooth-billed and

reptile-tailed birds of the secondary epoch, to the wonderfully

developed falcons, crows, and swallows of our time. So, the ferns,

lycopods, conifers, and monocotyledons of the palaeozoic and mesozoic

rocks, have developed into the marvellous wealth of forms of the higher

dicotyledons that now adorn the earth.
But this remarkable advance in the higher and larger groups does not

imply any universal law of progress in organisation, because we have at

the same time numerous examples (as has been already pointed out) of the

persistence of lowly organised forms, and also of absolute degradation

or degeneration. Serpents, for example, have been developed from some

lizard-like type which has lost its limbs; and though this loss has

enabled them to occupy fresh places in nature and to increase and

flourish to a marvellous extent, yet it must be considered to be a

retrogression rather than an advance in organisation. The same remark

will apply to the whale tribe among mammals; to the blind amphibia and

insects of the great caverns; and among plants to the numerous cases in

which flowers, once specially adapted to be fertilised by insects, have

lost their gay corollas and their special adaptations, and have become

degraded into wind-fertilised forms. Such are our plantains, our meadow

burnet, and even, as some botanists maintain, our rushes, sedges, and

grasses. The causes which have led to this degeneration will be

discussed in a future chapter; but the facts are undisputed, and they

show us that although variation and the struggle for existence may lead,

on the whole, to a continued advance of organisation; yet they also lead

in many cases to a retrogression, when such retrogression may aid in the

preservation of any form under new conditions. They also lead to the

persistence, with slight modifications, of numerous lowly organised

forms which are suited to places which higher forms could not fully

occupy, or to conditions under which they could not exist. Such are the

ocean depths, the soil of the earth, the mud of rivers, deep caverns,

subterranean waters, etc.; and it is in such places as these, as well as

in some oceanic islands which competing higher forms have not been able

to reach, that we find many curious relics of an earlier world, which,

in the free air and sunlight and in the great continents, have long

since been driven out or exterminated by higher types.

_Summary of the first Five Chapters._
We have now passed in review, in more or less detail, the main facts on

which the theory of "the origin of species by means of natural

selection" is founded. In future chapters we shall have to deal mainly

with the application of the theory to explain the varied and complex

phenomena presented by the organic world; and, also, to discuss some of

the theories put forth by modern writers, either as being more

fundamental than that of Darwin or as supplementary to it. Before doing

this, however, it will be well briefly to summarise the facts and

arguments already set forth, because it is only by a clear comprehension

of these that the full importance of the theory can be appreciated and

its further applications understood.
The theory itself is exceedingly simple, and the facts on which it

rests--though excessively numerous individually, and coextensive with

the entire organic world--yet come under a few simple and easily

understood classes. These facts are,--first, the enormous powers of

increase in geometrical progression possessed by all organisms, and the

inevitable struggle for existence among them; and, in the second place,

the occurrence of much individual variation combined with the hereditary

transmission of such variations. From these two great classes of facts,

which are universal and indisputable, there necessarily arises, as

Darwin termed it, the "preservation of favoured races in the struggle

for life," the continuous action of which, under the ever-changing

conditions both of the inorganic and organic universe, necessarily leads

to the formation or development of new species.
But, although this general statement is complete and indisputable, yet

to see its applications under all the complex conditions that actually

occur in nature, it is necessary always to bear in mind the tremendous

power and universality of the agencies at work. We must never for an

instant lose sight of the fact of the enormously rapid increase of all

organisms, which has been illustrated by actual cases, given in our

second chapter, no less than by calculations of the results of unchecked

increase for a few years. Then, never forgetting that the animal and

plant population of any country is, on the whole, stationary, we must be

always trying to realise the ever-recurring destruction of the enormous

annual increase, and asking ourselves what determines, in each

individual case, the death of the many, the survival of the few. We must

think over all the causes of destruction to each organism,--to the seed,

the young shoot, the growing plant, the full-grown tree, or shrub, or

herb, and again the fruit and seed; and among animals, to the egg or

new-born young, to the youthful, and to the adults. Then, we must always

bear in mind that what goes on in the case of the individual or family

group we may observe or think of, goes on also among the millions and

scores of millions of individuals which are comprised in almost every

species; and must get rid of the idea that _chance_ determines which

shall live and which die. For, although in many individual cases death

may be due to chance rather than to any inferiority in those which die

first, yet we cannot possibly believe that this can be the case on the

large scale on which nature works. A plant, for instance, cannot be

increased unless there are suitable vacant places its seeds can grow in,

or stations where it can overcome other less vigorous and healthy

plants. The seeds of all plants, by their varied modes of dispersal, may

be said to be seeking out such places in which to grow; and we cannot

doubt that, in the long run, those individuals whose seeds are the most

numerous, have the greatest powers of dispersal, and the greatest vigour

of growth, will leave more descendants than the individuals of the same

species which are inferior in all these respects, although now and then

some seed of an inferior individual may _chance_ to be carried to a spot

where it can grow and survive. The same rule will apply to every period

of life and to every danger to which plants or animals are exposed. The

best organised, or the most healthy, or the most active, or the best