Darwinism (1889) by Alfred Russel Wallace

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Let us now consider a less extreme and more familiar case. We possess a
considerable number of birds which, like the redbreast, sparrow, the
four common titmice, the thrush, and the blackbird, stay with us all the
year round These lay on an average six eggs, but, as several of them
have two or more broods a year, ten will be below the average of the
year's increase. Such birds as these often live from fifteen to twenty
years in confinement, and we cannot suppose them to live shorter lives
in a state of nature, if unmolested; but to avoid possible exaggeration
we will take only ten years as the average duration of their lives. Now,
if we start with a single pair, and these are allowed to live and breed,
unmolested, till they die at the end of ten years,--as they might do if
turned loose into a good-sized island with ample vegetable and insect
food, but no other competing or destructive birds or quadrupeds--their
numbers would amount to more than twenty millions. But we know very well
that our bird population is no greater, on the average, now than it was
ten years ago. Year by year it may fluctuate a little according as the
winters are more or less severe, or from other causes, but on the whole
there is no increase. What, then, becomes of the enormous surplus
population annually produced? It is evident they must all die or be
killed, somehow; and as the increase is, on the average, about five to
one, it follows that, if the average number of birds of all kinds in our
islands is taken at ten millions--and this is probably far under the
mark--then about fifty millions of birds, including eggs as possible
birds, must annually die or be destroyed. Yet we see nothing, or almost
nothing, of this tremendous slaughter of the innocents going on all
around us. In severe winters a few birds are found dead, and a few
feathers or mangled remains show us where a wood-pigeon or some other
bird has been destroyed by a hawk, but no one would imagine that five
times as many birds as the total number in the country in early spring
die every year. No doubt a considerable proportion of these do not die
here but during or after migration to other countries, but others which
are bred in distant countries come here, and thus balance the account.
Again, as the average number of young produced is four or five times
that of the parents, we ought to have at least five times as many birds
in the country at the end of summer as at the beginning, and there is
certainly no such enormous disproportion as this. The fact is, that the
destruction commences, and is probably most severe, with nestling birds,
which are often killed by heavy rains or blown away by severe storms, or
left to die of hunger if either of the parents is killed; while they
offer a defenceless prey to jackdaws, jays, and magpies, and not a few
are ejected from their nests by their foster-brothers the cuckoos. As
soon as they are fledged and begin to leave the nest great numbers are
destroyed by buzzards, sparrow-hawks, and shrikes. Of those which
migrate in autumn a considerable proportion are probably lost at sea or
otherwise destroyed before they reach a place of safety; while those
which remain with us are greatly thinned by cold and starvation during
severe winters. Exactly the same thing goes on with every species of
wild animal and plant from the lowest to the highest. All breed at such
a rate, that in a few years the progeny of any one species would, if
allowed to increase unchecked, alone monopolise the land; but all alike
are kept within bounds by various destructive agencies, so that, though
the numbers of each may fluctuate, they can never permanently increase
except at the expense of some others, which must proportionately
decrease.

_Cases showing the Great Powers of Increase of Animals._

As the facts now stated are the very foundation of the theory we are
considering, and the enormous increase and perpetual destruction
continually going on require to be kept ever present in the mind, some
direct evidence of actual cases of increase must be adduced. That even
the larger animals, which breed comparatively slowly, increase
enormously when placed under favourable conditions in new countries, is
shown by the rapid spread of cattle and horses in America. Columbus, in
his second voyage, left a few black cattle at St. Domingo, and these ran
wild and increased so much that, twenty-seven years afterwards, herds of
from 4000 to 8000 head were not uncommon. Cattle were afterwards taken
from this island to Mexico and to other parts of America, and in 1587,
sixty-five years after the conquest of Mexico, the Spaniards exported
64,350 hides from that country and 35,444 from St. Domingo, an
indication of the vast numbers of these animals which must then have
existed there, since those captured and killed could have been only a
small portion of the whole. In the pampas of Buenos Ayres there were, at
the end of the last century, about twelve million cows and three million
horses, besides great numbers in all other parts of America where open
pastures offered suitable conditions. Asses, about fifty years after
their introduction, ran wild and multiplied so amazingly in Quito, that
the Spanish traveller Ulloa describes them as being a nuisance. They
grazed together in great herds, defending themselves with their mouths,
and if a horse strayed among them they all fell upon him and did not
cease biting and kicking till they left him dead. Hogs were turned out
in St. Domingo by Columbus in 1493, and the Spaniards took them to other
places where they settled, the result being, that in about half a
century these animals were found in great numbers over a large part of
America, from 25 deg. north to 40 deg. south latitude. More recently, in New
Zealand, pigs have multiplied so greatly in a wild state as to be a
serious nuisance and injury to agriculture. To give some idea of their
numbers, it is stated that in the province of Nelson there were killed
in twenty months 25,000 wild pigs.[10] Now, in the case of all these
animals, we know that in their native countries, and even in America at
the present time, they do not increase at all in numbers; therefore the
whole normal increase must be kept down, year by year, by natural or
artificial means of destruction.

_Rapid Increase and Wide Spread of Plants_.

In the case of plants, the power of increase is even greater and its
effects more distinctly visible. Hundreds of square miles of the plains
of La Plata are now covered with two or three species of European
thistle, often to the exclusion of almost every other plant; but in the
native countries of these thistles they occupy, except in cultivated or
waste ground, a very subordinate part in the vegetation. Some American
plants, like the cotton-weed (Asclepias cuiussayica), have now become
common weeds over a large portion of the tropics. White clover
(Trifolium repens) spreads over all the temperate regions of the world,
and in New Zealand is exterminating many native species, including even
the native flax (Phormium tenax), a large plant with iris-like leaves 5
or 6 feet high. Mr. W.L. Travers has paid much attention to the effects
of introduced plants in New Zealand, and notes the following species as
being especially remarkable. The common knotgrass (Polygonum aviculare)
grows most luxuriantly, single plants covering a space 4 or 5 feet in
diameter, and sending their roots 3 or 4 feet deep. A large sub-aquatic
dock (Rumex obtusifolius) abounds in every river-bed, even far up among
the mountains. The common sow-thistle (Sonchus oleraceus) grows all over
the country up to an elevation of 6000 feet. The water-cress (Nasturtium
officinale) grows with amazing vigour in many of the rivers, forming
stems 12 feet long and 3/4 inch in diameter, and completely choking them
up. It cost L300 a year to keep the Avon at Christchurch free from it.
The sorrel (Rumex acetosella) covers hundreds of acres with a sheet of
red. It forms a dense mat, exterminating other plants, and preventing
cultivation. It can, however, be itself exterminated by sowing the
ground with red clover, which will also vanquish the Polygonum
aviculare. The most noxious weed in New Zealand appears, however, to be
the Hypochaeris radicata, a coarse yellow-flowered composite not
uncommon in our meadows and waste places. This has been introduced with
grass seeds from England, and is very destructive. It is stated that
excellent pasture was in three years destroyed by this weed, which
absolutely displaced every other plant on the ground. It grows in every
kind of soil, and is said even to drive out the white clover, which is
usually so powerful in taking possession of the soil.

In Australia another composite plant, called there the Cape-weed
(Cryptostemma calendulaceum), did much damage, and was noticed by Baron
Von Hugel in 1833 as "an unexterminable weed"; but, after forty years'
occupation, it was found to give way to the dense herbage formed by
lucerne and choice grasses.

In Ceylon we are told by Mr. Thwaites, in his _Enumeration of Ceylon
Plants_, that a plant introduced into the island less than fifty years
ago is helping to alter the character of the vegetation up to an
elevation of 3000 feet. This is the Lantana mixta, a verbenaceous plant
introduced from the West Indies, which appears to have found in Ceylon
a soil and climate exactly suited to it. It now covers thousands of
acres with its dense masses of foliage, taking complete possession of
land where cultivation has been neglected or abandoned, preventing the
growth of any other plants, and even destroying small trees, the tops of
which its subscandent stems are able to reach. The fruit of this plant
is so acceptable to frugivorous birds of all kinds that, through their
instrumentality, it is spreading rapidly, to the complete exclusion of
the indigenous vegetation where it becomes established.

_Great Fertility not essential to Rapid Increase_.

The not uncommon circumstance of slow-breeding animals being very
numerous, shows that it is usually the amount of destruction which an
animal or plant is exposed to, not its rapid multiplication, that
determines its numbers in any country. The passenger-pigeon (Ectopistes
migratorius) is, or rather was, excessively abundant in a certain area
in North America, and its enormous migrating flocks darkening the sky
for hours have often been described; yet this bird lays only two eggs.
The fulmar petrel exists in myriads at St. Kilda and other haunts of the
species, yet it lays only one egg. On the other hand the great shrike,
the tree-creeper, the nut-hatch, the nut-cracker, the hoopoe, and many
other birds, lay from four to six or seven eggs, and yet are never
abundant. So in plants, the abundance of a species bears little or no
relation to its seed-producing power. Some of the grasses and sedges,
the wild hyacinth, and many buttercups occur in immense profusion over
extensive areas, although each plant produces comparatively few seeds;
while several species of bell-flowers, gentians, pinks, and mulleins,
and even some of the composite, which produce an abundance of minute
seeds, many of which are easily scattered by the wind, are yet rare
species that never spread beyond a very limited area.

The above-mentioned passenger-pigeon affords such an excellent example
of an enormous bird-population kept up by a comparatively slow rate of
increase, and in spite of its complete helplessness and the great
destruction which it suffers from its numerous enemies, that the
following account of one of its breeding-places and migrations by the
celebrated American naturalist, Alexander Wilson, will be read with
interest:--

"Not far from Shelbyville, in the State of Kentucky, about five years
ago, there was one of these breeding-places, which stretched through the
woods in nearly a north and south direction, was several miles in
breadth, and was said to be upwards of 40 miles in extent. In this tract
almost every tree was furnished with nests wherever the branches could
accommodate them. The pigeons made their first appearance there about
the 10th of April, and left it altogether with their young before the
25th of May. As soon as the young were fully grown and before they left
the nests, numerous parties of the inhabitants from all parts of the
adjacent country came with waggons, axes, beds, cooking utensils, many
of them accompanied by the greater part of their families, and encamped
for several days at this immense nursery. Several of them informed me
that the noise was so great as to terrify their horses, and that it was
difficult for one person to hear another without bawling in his ear. The
ground was strewed with broken limbs of trees, eggs, and young squab
pigeons, which had been precipitated from above, and on which herds of
hogs were fattening. Hawks, buzzards, and eagles were sailing about in
great numbers, and seizing the squabs from the nests at pleasure; while,
from 20 feet upwards to the top of the trees, the view through the woods
presented a perpetual tumult of crowding and fluttering multitudes of
pigeons, their wings roaring like thunder, mingled with the frequent
crash of falling timber; for now the axemen were at work cutting down
those trees that seemed most crowded with nests, and contrived to fell
them in such a manner, that in their descent they might bring down
several others; by which means the falling of one large tree sometimes
produced 200 squabs little inferior in size to the old birds, and almost
one heap of fat. On some single trees upwards of a hundred nests were
found, each containing one squab only; a circumstance in the history of
the bird not generally known to naturalists.[11] It was dangerous to
walk under these flying and fluttering millions, from the frequent fall
of large branches, broken down by the weight of the multitudes above,
and which in their descent often destroyed numbers of the birds
themselves; while the clothes of those engaged in traversing the woods
were completely covered with the excrements of the pigeons.

"These circumstances were related to me by many of the most respectable
part of the community in that quarter, and were confirmed in part by
what I myself witnessed. I passed for several miles through this same
breeding-place, where every tree was spotted with nests, the remains of
those above described. In many instances I counted upwards of ninety
nests on a single tree; but the pigeons had abandoned this place for
another, 60 or 80 miles off, towards Green River, where they were said
at that time to be equally numerous. From the great numbers that were
constantly passing over our heads to or from that quarter, I had no
doubt of the truth of this statement. The mast had been chiefly consumed
in Kentucky; and the pigeons, every morning a little before sunrise, set
out for the Indiana territory, the nearest part of which was about sixty
miles distant. Many of these returned before ten o'clock, and the great
body generally appeared on their return a little after noon. I had left
the public road to visit the remains of the breeding-place near
Shelbyville, and was traversing the woods with my gun, on my way to
Frankfort, when about ten o'clock the pigeons which I had observed
flying the greater part of the morning northerly, began to return in
such immense numbers as I never before had witnessed. Coming to an
opening by the side of a creek, where I had a more uninterrupted view, I
was astonished at their appearance: they were flying with great
steadiness and rapidity, at a height beyond gunshot, in several strata
deep, and so close together that, could shot have reached them, one
discharge could not have failed to bring down several individuals. From
right to left, as far as the eye could reach, the breadth of this vast
procession extended, seeming everywhere equally crowded. Curious to
determine how long this appearance would continue, I took out my watch
to note the time, and sat down to observe them. It was then half-past
one; I sat for more than an hour, but instead of a diminution of this
prodigious procession, it seemed rather to increase, both in numbers and
rapidity; and anxious to reach Frankfort before night, I rose and went
on. About four o'clock in the afternoon I crossed Kentucky River, at the
town of Frankfort, at which time the living torrent above my head seemed
as numerous and as extensive as ever. Long after this I observed them in
large bodies that continued to pass for six or eight minutes, and these
again were followed by other detached bodies, all moving in the same
south-east direction, till after six o'clock in the evening. The great
breadth of front which this mighty multitude preserved would seem to
intimate a corresponding breadth of their breeding-place, which, by
several gentlemen who had lately passed through part of it, was stated
to me at several miles."

From these various observations, Wilson calculated that the number of
birds contained in the mass of pigeons which he saw on this occasion was
at least two thousand millions, while this was only one of many similar
aggregations known to exist in various parts of the United States. The
picture here given of these defenceless birds, and their still more
defenceless young, exposed to the attacks of numerous rapacious enemies,
brings vividly before us one of the phases of the unceasing struggle for
existence ever going on; but when we consider the slow rate of increase
of these birds, and the enormous population they are nevertheless able
to maintain, we must be convinced that in the case of the majority of
birds which multiply far more rapidly, and yet are never able to attain
such numbers, the struggle against their numerous enemies and against
the adverse forces of nature must be even more severe or more
continuous.

_Struggle for Life between, closely allied Animals and Plants often the
most severe._

The struggle we have hitherto been considering has been mainly that
between an animal or plant and its direct enemies, whether these enemies
are other animals which devour it, or the forces of nature which destroy
it. But there is another kind of struggle often going on at the same
time between closely related species, which almost always terminates in
the destruction of one of them. As an example of what is meant, Darwin
states that the recent increase of the missel-thrush in parts of
Scotland has caused the decrease of the song-thrush.[12] The black rat
(Mus rattus) was the common rat of Europe till, in the beginning of the
eighteenth century, the large brown rat (Mus decumanus) appeared on the
Lower Volga, and thence spread more or less rapidly till it overran all
Europe, and generally drove out the black rat, which in most parts is
now comparatively rare or quite extinct. This invading rat has now been
carried by commerce all over the world, and in New Zealand has
completely extirpated a native rat, which the Maoris allege they brought
with them from their home in the Pacific; and in the same country a
native fly is being supplanted by the European house-fly. In Russia the
small Asiatic cockroach has driven away a larger native species; and in
Australia the imported hive-bee is exterminating the small stingless
native bee.

The reason why this kind of struggle goes on is apparent if we consider
that the allied species fill nearly the same place in the economy of
nature. They require nearly the same kind of food, are exposed to the
same enemies and the same dangers. Hence, if one has ever so slight an
advantage over the other in procuring food or in avoiding danger, in its
rapidity of multiplication or its tenacity of life, it will increase
more rapidly, and by that very fact will cause the other to decrease and
often become altogether extinct. In some cases, no doubt, there is
actual war between the two, the stronger killing the weaker; but this is
by no means necessary, and there may be cases in which the weaker
species, physically, may prevail, by its power of more rapid
multiplication, its better withstanding vicissitudes of climates, or its
greater cunning in escaping the attacks of the common enemies. The same
principle is seen at work in the fact that certain mountain varieties of
sheep will starve out other mountain varieties, so that the two cannot
be kept together. In plants the same thing occurs. If several distinct
varieties of wheat are sown together, and the mixed seed resown, some of
the varieties which best suit the soil and climate, or are naturally the
most fertile, will beat the others and so yield more seed, and will
consequently in a few years supplant the other varieties.

As an effect of this principle, we seldom find closely allied species
of animals or plants living together, but often in distinct though
adjacent districts where the conditions of life are somewhat different.
Thus we may find cowslips (Primula veris) growing in a meadow, and
primroses (P. vulgaris) in an adjoining wood, each in abundance, but not
often intermingled. And for the same reason the old turf of a pasture or
heath consists of a great variety of plants matted together, so much so
that in a patch little more than a yard square Mr. Darwin found twenty
distinct species, belonging to eighteen distinct genera and to eight
natural orders, thus showing their extreme diversity of organisation.
For the same reason a number of distinct grasses and clovers are sown in
order to make a good lawn instead of any one species; and the quantity
of hay produced has been found to be greater from a variety of very
distinct grasses than from any one species of grass.

It may be thought that forests are an exception to this rule, since in
the north-temperate and arctic regions we find extensive forests of
pines or of oaks. But these are, after all, exceptional, and
characterise those regions only where the climate is little favourable
to forest vegetation. In the tropical and all the warm temperate parts
of the earth, where there is a sufficient supply of moisture, the
forests present the same variety of species as does the turf of our old
pastures; and in the equatorial virgin forests there is so great a
variety of forms, and they are so thoroughly intermingled, that the
traveller often finds it difficult to discover a second specimen of any
particular species which he has noticed. Even the forests of the
temperate zones, in all favourable situations, exhibit a considerable
variety of trees of distinct genera and families, and it is only when we
approach the outskirts of forest vegetation, where either drought or
winds or the severity of the winter is adverse to the existence of most
trees, that we find extensive tracts monopolised by one or two species.
Even Canada has more than sixty different forest trees and the Eastern
United States a hundred and fifty; Europe is rather poor, containing
about eighty trees only; while the forests of Eastern Asia, Japan, and
Manchuria are exceedingly rich, about a hundred and seventy species
being already known. And in all these countries the trees grow
intermingled, so that in every extensive forest we have a considerable
variety, as may be seen in the few remnants of our primitive woods in
some parts of Epping Forest and the New Forest.

Among animals the same law prevails, though, owing to their constant
movements and power of concealment, it is not so readily observed. As
illustrations we may refer to the wolf, ranging over Europe and Northern
Asia, while the jackal inhabits Southern Asia and Northern Africa; the
tree-porcupines, of which there are two closely allied species, one
inhabiting the eastern, the other the western half of North America; the
common hare (Lepus timidus) in Central and Southern Europe, while all
Northern Europe is inhabited by the variable hare (Lepus variabilis);
the common jay (Garrulus glandarius) inhabiting all Europe, while
another species (Garrulus Brandti) is found all across Asia from the
Urals to Japan; and many species of birds in the Eastern United States
are replaced by closely allied species in the west. Of course there are
also numbers of closely related species in the same country, but it will
almost always be found that they frequent different stations and have
somewhat different habits, and so do not come into direct competition
with each other; just as closely allied plants may inhabit the same
districts, when one prefers meadows the other woods, one a chalky soil
the other sand, one a damp situation the other a dry one. With plants,
fixed as they are to the earth, we easily note these peculiarities of
station; but with wild animals, which we see only on rare occasions, it
requires close and long-continued observation to detect the
peculiarities in their mode of life which may prevent all direct
competition between closely allied species dwelling in the same area.

_The Ethical Aspect of the Struggle for Existence_.

Our exposition of the phenomena presented by the struggle for existence
may be fitly concluded by a few remarks on its ethical aspect. Now that
the war of nature is better known, it has been dwelt upon by many
writers as presenting so vast an amount of cruelty and pain as to be
revolting to our instincts of humanity, while it has proved a
stumbling-block in the way of those who would fain believe in an
all-wise and benevolent ruler of the universe. Thus, a brilliant writer
says: "Pain, grief, disease, and death, are these the inventions of a
loving God? That no animal shall rise to excellence except by being
fatal to the life of others, is this the law of a kind Creator? It is
useless to say that pain has its benevolence, that massacre has its
mercy. Why is it so ordained that bad should be the raw material of
good? Pain is not the less pain because it is useful; murder is not less
murder because it is conducive to development. Here is blood upon the
hand still, and all the perfumes of Arabia will not sweeten it."[13]