THE STRUGGLE FOR EXISTENCE
Its importance--The struggle among plants--Among
animals--Illustrative cases--Succession of trees in forests of
Denmark--The struggle for existence on the Pampas--Increase of
organisms in a geometrical ratio--Examples of great powers of
increase of animals--Rapid increase and wide spread of
plants--Great fertility not essential to rapid
increase--Struggle between closely allied species most
severe--The ethical aspect of the struggle for existence.
There is perhaps no phenomenon of nature that is at once so important,
so universal; and so little understood, as the struggle for existence
continually going on among all organised beings. To most persons nature
appears calm, orderly, and peaceful. They see the birds singing in the
trees, the insects hovering over the flowers, the squirrel climbing
among the tree-tops, and all living things in the possession of health
and vigour, and in the enjoyment of a sunny existence. But they do not
see, and hardly ever think of, the means by which this beauty and
harmony and enjoyment is brought about. They do not see the constant and
daily search after food, the failure to obtain which means weakness or
death; the constant effort to escape enemies; the ever-recurring
struggle against the forces of nature. This daily and hourly struggle,
this incessant warfare, is nevertheless the very means by which much of
the beauty and harmony and enjoyment in nature is produced, and also
affords one of the most important elements in bringing about the origin
of species. We must, therefore, devote some time to the consideration of
its various aspects and of the many curious phenomena to which it gives
rise.
It is a matter of common observation that if weeds are allowed to grow
unchecked in a garden they will soon destroy a number of the flowers.
It is not so commonly known that if a garden is left to become
altogether wild, the weeds that first take possession of it, often
covering the whole surface of the ground with two or three different
kinds, will themselves be supplanted by others, so that in a few years
many of the original flowers and of the earliest weeds may alike have
disappeared. This is one of the very simplest cases of the struggle for
existence, resulting in the successive displacement of one set of
species by another; but the exact causes of this displacement are by no
means of such a simple nature. All the plants concerned may be perfectly
hardy, all may grow freely from seed, yet when left alone for a number
of years, each set is in turn driven out by a succeeding set, till at
the end of a considerable period--a century or a few centuries
perhaps--hardly one of the plants which first monopolised the ground
would be found there.
Another phenomenon of an analogous kind is presented by the different
behaviour of introduced wild plants or animals into countries apparently
quite as well suited to them as those which they naturally inhabit.
Agassiz, in his work on Lake Superior, states that the roadside weeds of
the northeastern United States, to the number of 130 species, are all
European, the native weeds having disappeared westwards; and in New
Zealand there are no less than 250 species of naturalised European
plants, more than 100 species of which have spread widely over the
country, often displacing the native vegetation. On the other hand, of
the many hundreds of hardy plants which produce seed freely in our
gardens, very few ever run wild, and hardly any have become common. Even
attempts to naturalise suitable plants usually fail; for A. de Candolle
states that several botanists of Paris, Geneva, and especially of
Montpellier, have sown the seeds of many hundreds of species of hardy
exotic plants in what appeared to be the most favourable situations, but
that, in hardly a single case, has any one of them become
naturalised.[4] Even a plant like the potato--so widely cultivated, so
hardy, and so well adapted to spread by means of its many-eyed
tubers--has not established itself in a wild state in any part of
Europe. It would be thought that Australian plants would easily run
wild in New Zealand. But Sir Joseph Hooker informs us that the late Mr.
Bidwell habitually scattered Australian seeds during his extensive
travels in New Zealand, yet only two or three Australian plants appear
to have established themselves in that country, and these only in
cultivated or newly moved soil.
These few illustrations sufficiently show that all the plants of a
country are, as De Candolle says, at war with each other, each one
struggling to occupy ground at the expense of its neighbour. But,
besides this direct competition, there is one not less powerful arising
from the exposure of almost all plants to destruction by animals. The
buds are destroyed by birds, the leaves by caterpillars, the seeds by
weevils; some insects bore into the trunk, others burrow in the twigs
and leaves; slugs devour the young seedlings and the tender shoots,
wire-worms gnaw the roots. Herbivorous mammals devour many species
bodily, while some uproot and devour the buried tubers.
In animals, it is the eggs or the very young that suffer most from their
various enemies; in plants, the tender seedlings when they first appear
above the ground. To illustrate this latter point Mr. Darwin cleared and
dug a piece of ground three feet long and two feet wide, and then marked
all the seedlings of weeds and other plants which came up, noting what
became of them. The total number was 357, and out of these no less than
295 were destroyed by slugs and insects. The direct strife of plant with
plant is almost equally fatal when the stronger are allowed to smother
the weaker. When turf is mown or closely browsed by animals, a number of
strong and weak plants live together, because none are allowed to grow
much beyond the rest; but Mr. Darwin found that when the plants which
compose such turf are allowed to grow up freely, the stronger kill the
weaker. In a plot of turf three feet by four, twenty distinct species of
plants were found to be growing, and no less than nine of these perished
altogether when the other species were allowed to grow up to their full
size.[5]
But besides having to protect themselves against competing plants and
against destructive animals, there is a yet deadlier enemy in the
forces of inorganic nature. Each species can sustain a certain amount of
heat and cold, each requires a certain amount of moisture at the right
season, each wants a proper amount of light or of direct sunshine, each
needs certain elements in the soil; the failure of a due proportion in
these inorganic conditions causes weakness, and thus leads to speedy
death. The struggle for existence in plants is, therefore, threefold in
character and infinite in complexity, and the result is seen in their
curiously irregular distribution over the face of the earth. Not only
has each country its distinct plants, but every valley, every hillside,
almost every hedgerow, has a different set of plants from its adjacent
valley, hillside, or hedgerow--if not always different in the actual
species yet very different in comparative abundance, some which are rare
in the one being common in the other. Hence it happens that slight
changes of conditions often produce great changes in the flora of a
country. Thus in 1740 and the two following years the larva of a moth
(Phalaena graminis) committed such destruction in many of the meadows of
Sweden that the grass was greatly diminished in quantity, and many
plants which were before choked by the grass sprang up, and the ground
became variegated with a multitude of different species of flowers. The
introduction of goats into the island of St. Helena led to the entire
destruction of the native forests, consisting of about a hundred
distinct species of trees and shrubs, the young plants being devoured by
the goats as fast as they grew up. The camel is a still greater enemy to
woody vegetation than the goat, and Mr. Marsh believes that forests
would soon cover considerable tracts of the Arabian and African deserts
if the goat and the camel were removed from them.[6] Even in many parts
of our own country the existence of trees is dependent on the absence of
cattle. Mr. Darwin observed, on some extensive heaths near Farnham, in
Surrey, a few clumps of old Scotch firs, but no young trees over
hundreds of acres. Some portions of the heath had, however, been
enclosed a few years before, and these enclosures were crowded with
young fir-trees growing too close together for all to live; and these
were not sown or planted, nothing having been done to the ground beyond
enclosing it so as to keep out cattle. On ascertaining this, Mr. Darwin
was so much surprised that he searched among the heather in the
unenclosed parts, and there he found multitudes of little trees and
seedlings which had been perpetually browsed down by the cattle. In one
square yard, at a point about a hundred yards from one of the old clumps
of firs, he counted thirty-two little trees, and one of them had
twenty-six rings of growth, showing that it had for many years tried to
raise its head above the stems of the heather and had failed. Yet this
heath was very extensive and very barren, and, as Mr. Darwin remarks, no
one would ever have imagined that cattle would have so closely and so
effectually searched it for food.
In the case of animals, the competition and struggle are more obvious.
The vegetation of a given district can only support a certain number of
animals, and the different kinds of plant-eaters will compete together
for it. They will also have insects for their competitors, and these
insects will be kept down by birds, which will thus assist the mammalia.
But there will also be carnivora destroying the herbivora; while small
rodents, like the lemming and some of the field-mice, often destroy so
much vegetation as materially to affect the food of all the other groups
of animals. Droughts, floods, severe winters, storms and hurricanes will
injure these in various degrees, but no one species can be diminished in
numbers without the effect being felt in various complex ways by all the
rest. A few illustrations of this reciprocal action must be given.
_Illustrative Cases of the Struggle for Life_.
Sir Charles Lyell observes that if, by the attacks of seals or other
marine foes, salmon are reduced in numbers, the consequence will be that
otters, living far inland, will be deprived of food and will then
destroy many young birds or quadrupeds, so that the increase of a marine
animal may cause the destruction of many land animals hundreds of miles
away. Mr. Darwin carefully observed the effects produced by planting a
few hundred acres of Scotch fir, in Staffordshire, on part of a very
extensive heath which had never been cultivated. After the planted
portion was about twenty-five years old he observed that the change in
the native vegetation was greater than is often seen in passing from
one quite different soil to another. Besides a great change in the
proportional numbers of the native heath-plants, twelve species which
could not be found on the heath flourished in the plantations. The
effect on the insect life must have been still greater, for six
insectivorous birds which were very common in the plantations were not
to be seen on the heath, which was, however, frequented by two or three
different species of insectivorous birds. It would have required
continued study for several years to determine all the differences in
the organic life of the two areas, but the facts stated by Mr. Darwin
are sufficient to show how great a change may be effected by the
introduction of a single kind of tree and the keeping out of cattle.
The next case I will give in Mr. Darwin's own words: "In several parts
of the world insects determine the existence of cattle. Perhaps Paraguay
offers the most curious instance of this; for here neither cattle nor
horses nor dogs have ever run wild, though they swarm southward and
northward in a feral state; and Azara and Rengger have shown that this
is caused by the greater numbers, in Paraguay, of a certain fly which
lays its eggs in the navels of these animals when first born. The
increase of these flies, numerous as they are, must be habitually
checked by some means, probably by other parasitic insects. Hence, if
certain insectivorous birds were to decrease in Paraguay, the parasitic
insects would probably increase; and this would lessen the number of the
navel-frequenting flies--then cattle and horses would become feral, and
this would greatly alter (as indeed I have observed in parts of South
America) the vegetation: this again would largely affect the insects,
and this, as we have just seen in Staffordshire, the insectivorous
birds, and so onward in ever-increasing circles of complexity. Not that
under nature the relations will ever be as simple as this. Battle within
battle must be continually recurring with varying success; and yet in
the long run the forces are so nicely balanced, that the face of nature
remains for a long time uniform, though assuredly the merest trifle
would give the victory to one organic being over another."[7]
Such cases as the above may perhaps be thought exceptional, but there
is good reason to believe that they are by no means rare, but are
illustrations of what is going on in every part of the world, only it is
very difficult for us to trace out the complex reactions that are
everywhere occurring. The general impression of the ordinary observer
seems to be that wild animals and plants live peaceful lives and have
few troubles, each being exactly suited to its place and surroundings,
and therefore having no difficulty in maintaining itself. Before showing
that this view is, everywhere and always, demonstrably untrue, we will
consider one other case of the complex relations of distinct organisms
adduced by Mr. Darwin, and often quoted for its striking and almost
eccentric character. It is now well known that many flowers require to
be fertilised by insects in order to produce seed, and this
fertilisation can, in some cases, only be effected by one particular
species of insect to which the flower has become specially adapted. Two
of our common plants, the wild heart's-ease (Viola tricolor) and the red
clover (Trifolium pratense), are thus fertilised by humble-bees almost
exclusively, and if these insects are prevented from visiting the
flowers, they produce either no seed at all or exceedingly few. Now it
is known that field-mice destroy the combs and nests of humble-bees, and
Colonel Newman, who has paid great attention to these insects, believes
that more than two-thirds of all the humble-bees' nests in England are
thus destroyed. But the number of mice depends a good deal on the number
of cats; and the same observer says that near villages and towns he has
found the nests of humble-bees more numerous than elsewhere, which he
attributes to the number of cats that destroy the mice. Hence it
follows, that the abundance of red clover and wild heart's-ease in a
district will depend on a good supply of cats to kill the mice, which
would otherwise destroy and keep down the humble-bees and prevent them
from fertilising the flowers. A chain of connection has thus been found
between such totally distinct organisms as flesh-eating mammalia and
sweet-smelling flowers, the abundance or scarcity of the one closely
corresponding to that of the other!
The following account of the struggle between trees in the forests of
Denmark, from the researches of M. Hansten-Blangsted, strikingly
illustrates our subject.[8] The chief combatants are the beech and the
birch, the former being everywhere successful in its invasions. Forests
composed wholly of birch are now only found in sterile, sandy tracts;
everywhere else the trees are mixed, and wherever the soil is favourable
the beech rapidly drives out the birch. The latter loses its branches at
the touch of the beech, and devotes all its strength to the upper part
where it towers above the beech. It may live long in this way, but it
succumbs ultimately in the fight--of old age if of nothing else, for the
life of the birch in Denmark is shorter than that of the beech. The
writer believes that light (or rather shade) is the cause of the
superiority of the latter, for it has a greater development of its
branches than the birch, which is more open and thus allows the rays of
the sun to pass through to the soil below, while the tufted, bushy top
of the beech preserves a deep shade at its base. Hardly any young plants
can grow under the beech except its own shoots; and while the beech can
nourish under the shade of the birch, the latter dies immediately under
the beech. The birch has only been saved from total extermination by the
facts that it had possession of the Danish forests long before the beech
ever reached the country, and that certain districts are unfavourable to
the growth of the latter. But wherever the soil has been enriched by the
decomposition of the leaves of the birch the battle begins. The birch
still flourishes on the borders of lakes and other marshy places, where
its enemy cannot exist. In the same way, in the forests of Zeeland, the
fir forests are disappearing before the beech. Left to themselves, the
firs are soon displaced by the beech. The struggle between the latter
and the oak is longer and more stubborn, for the branches and foliage of
the oak are thicker, and offer much resistance to the passage of light.
The oak, also, has greater longevity; but, sooner or later, it too
succumbs, because it cannot develop in the shadow of the beech. The
earliest forests of Denmark were mainly composed of aspens, with which
the birch was apparently associated; gradually the soil was raised, and
the climate grew milder; then the fir came and formed large forests.
This tree ruled for centuries, and then ceded the first place to the
holm-oak, which is now giving way to the beech. Aspen, birch, fir, oak,
and beech appear to be the steps in the struggle for the survival of the
fittest among the forest-trees of Denmark.
It may be added that in the time of the Romans the beech was the
principal forest-tree of Denmark as it is now, while in the much earlier
bronze age, represented by the later remains found in the peat bogs,
there were no beech-trees, or very few, the oak being the prevailing
tree, while in the still earlier stone period the fir was the most
abundant. The beech is a tree essentially of the temperate zone, having
its northern limit considerably southward of the oak, fir, birch, or
aspen, and its entrance into Denmark was no doubt due to the
amelioration of the climate after the glacial epoch had entirely passed
away. We thus see how changes of climate, which are continually
occurring owing either to cosmical or geographical causes, may initiate
a struggle among plants which may continue for thousands of years, and
which must profoundly modify the relations of the animal world, since
the very existence of innumerable insects, and even of many birds and
mammals, is dependent more or less completely on certain species of
plants.
_The Struggle for Existence on the Pampas_.
Another illustration of the struggle for existence, in which both plants
and animals are implicated, is afforded by the pampas of the southern
part of South America. The absence of trees from these vast plains has
been imputed by Mr. Darwin to the supposed inability of the tropical and
sub-tropical forms of South America to thrive on them, and there being
no other source from which they could obtain a supply; and that
explanation was adopted by such eminent botanists as Mr. Ball and
Professor Asa Gray. This explanation has always seemed to me
unsatisfactory, because there are ample forests both in the temperate
regions of the Andes and on the whole west coast down to Terra del
Fuego; and it is inconsistent with what we know of the rapid variation
and adaptation of species to new conditions. What seems a more
satisfactory explanation has been given by Mr. Edwin Clark, a civil
engineer, who resided nearly two years in the country and paid much
attention to its natural history. He says: "The peculiar characteristics
of these vast level plains which descend from the Andes to the great
river basin in unbroken monotony, are the absence of rivers or
water-storage, and the periodical occurrence of droughts, or 'siccos,'
in the summer months. These conditions determine the singular character
both of its flora and fauna.
"The soil is naturally fertile and favourable for the growth of trees,
and they grow luxuriantly wherever they are protected. The eucalyptus is
covering large tracts wherever it is enclosed, and willows, poplars, and
the fig surround every estancia when fenced in.
"The open plains are covered with droves of horses and cattle, and
overrun by numberless wild rodents, the original tenants of the pampas.
During the long periods of drought, which are so great a scourge to the
country, these animals are starved by thousands, destroying, in their
efforts to live, every vestige of vegetation. In one of these 'siccos,'
at the time of my visit, no less than 50,000 head of oxen and sheep and
horses perished from starvation and thirst, after tearing deep out of
the soil every trace of vegetation, including the wiry roots of the
pampas-grass. Under such circumstances the existence of an unprotected
tree is impossible. The only plants that hold their own, in addition to
the indestructible thistles, grasses, and clover, are a little
herbaceous oxalis, producing viviparous buds of extraordinary vitality,
a few poisonous species, such as the hemlock, and a few tough, thorny
dwarf-acacias and wiry rushes, which even a starving rat refuses.
"Although the cattle are a modern introduction, the numberless
indigenous rodents must always have effectually prevented the
introduction of any other species of plants; large tracts are still
honeycombed by the ubiquitous biscacho, a gigantic rabbit; and numerous
other rodents still exist, including rats and mice, pampas-hares, and
the great nutria and carpincho (capybara) on the river banks."[9]
Mr. Clark further remarks on the desperate struggle for existence which
characterises the bordering fertile zones, where rivers and marshy
plains permit a more luxuriant and varied vegetable and animal life.
After describing how the river sometimes rose 30 feet in eight hours,
doing immense destruction, and the abundance of the larger carnivora and
large reptiles on its banks, he goes on: "But it was among the flora
that the principle of natural selection was most prominently displayed.
In such a district--overrun with rodents and escaped cattle, subject to
floods that carried away whole islands of botany, and especially to
droughts that dried up the lakes and almost the river itself--no
ordinary plant could live, even on this rich and watered alluvial
debris. The only plants that escaped the cattle were such as were either
poisonous, or thorny, or resinous, or indestructibly tough. Hence we had
only a great development of solanums, talas, acacias, euphorbias, and
laurels. The buttercup is replaced by the little poisonous yellow oxalis
with its viviparous buds; the passion-flowers, asclepiads, bignonias,
convolvuluses, and climbing leguminous plants escape both floods and
cattle by climbing the highest trees and towering overhead in a flood of
bloom. The ground plants are the portulacas, turneras, and cenotheras,
bitter and ephemeral, on the bare rock, and almost independent of any
other moisture than the heavy dews. The pontederias, alismas, and
plantago, with grasses and sedges, derive protection from the deep and
brilliant pools; and though at first sight the 'monte' doubtless
impresses the traveller as a scene of the wildest confusion and ruin,
yet, on closer examination, we found it far more remarkable as a
manifestation of harmony and law, and a striking example of the
marvellous power which plants, like animals, possess, of adapting
themselves to the local peculiarities of their habitat, whether in the
fertile shades of the luxuriant 'monte' or on the arid, parched-up
plains of the treeless pampas."
A curious example of the struggle between plants has been communicated
to me by Mr. John Ennis, a resident in New Zealand. The English
water-cress grows so luxuriantly in that country as to completely choke
up the rivers, sometimes leading to disastrous floods, and necessitating
great outlay to keep the stream open. But a natural remedy has now been
found in planting willows on the banks. The roots of these trees
penetrate the bed of the stream in every direction, and the water-cress,
unable to obtain the requisite amount of nourishment, gradually
disappears.
_Increase of Organisms in a Geometrical Ratio_.
The facts which have now been adduced, sufficiently prove that there is
a continual competition, and struggle, and war going on in nature, and
that each species of animal and plant affects many others in complex and
often unexpected ways. We will now proceed to show the fundamental cause
of this struggle, and to prove that it is ever acting over the whole
field of nature, and that no single species of animal or plant can
possibly escape from it. This results from the fact of the rapid
increase, in a geometrical ratio, of all the species of animals and
plants. In the lower orders this increase is especially rapid, a single
flesh-fly (Musca carnaria) producing 20,000 larvae, and these growing so
quickly that they reach their full size in five days; hence the great
Swedish naturalist, Linnaeus, asserted that a dead horse would be
devoured by three of these flies as quickly as by a lion. Each of these
larvae remains in the pupa state about five or six days, so that each
parent fly may be increased ten thousand-fold in a fortnight. Supposing
they went on increasing at this rate during only three months of summer,
there would result one hundred millions of millions of millions for each
fly at the commencement of summer,--a number greater probably than
exists at any one time in the whole world. And this is only one species,
while there are thousands of other species increasing also at an
enormous rate; so that, if they were unchecked, the whole atmosphere
would be dense with flies, and all animal food and much of animal life
would be destroyed by them. To prevent this tremendous increase there
must be incessant war against these insects, by insectivorous birds and
reptiles as well as by other insects, in the larva as well as in the
perfect state, by the action of the elements in the form of rain, hail,
or drought, and by other unknown causes; yet we see nothing of this
ever-present war, though by its means alone, perhaps, we are saved from
famine and pestilence.
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° north to 40° 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 £300 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]
Even so thoughtful a writer as Professor Huxley adopts similar views. In
a recent article on "The Struggle for Existence" he speaks of the
myriads of generations of herbivorous animals which "have been tormented
and devoured by carnivores"; of the carnivores and herbivores alike
"subject to all the miseries incidental to old age, disease, and
over-multiplication"; and of the "more or less enduring suffering,"
which is the meed of both vanquished and victor. And he concludes that,
since thousands of times a minute, were our ears sharp enough, we should
hear sighs and groans of pain like those heard by Dante at the gate of
hell, the world cannot be governed by what we call benevolence.[14]
Now there is, I think, good reason to believe that all this is greatly
exaggerated; that the supposed "torments" and "miseries" of animals have
little real existence, but are the reflection of the imagined sensations
of cultivated men and women in similar circumstances; and that the
amount of actual suffering caused by the struggle for existence among
animals is altogether insignificant. Let us, therefore, endeavour to
ascertain what are the real facts on which these tremendous accusations
are founded.
In the first place, we must remember that animals are entirely spared
the pain we suffer in the anticipation of death--a pain far greater, in
most cases, than the reality. This leads, probably, to an almost
perpetual enjoyment of their lives; since their constant watchfulness
against danger, and even their actual flight from an enemy, will be the
enjoyable exercise of the powers and faculties they possess, unmixed
with any serious dread. There is, in the next place, much evidence to
show that violent deaths, if not too prolonged, are painless and easy;
even in the case of man, whose nervous system is in all probability much
more susceptible to pain than that of most animals. In all cases in
which persons have escaped after being seized by a lion or tiger, they
declare that they suffered little or no pain, physical or mental. A
well-known instance is that of Livingstone, who thus describes his
sensations when seized by a lion: "Starting and looking half round, I
saw the lion just in the act of springing on me. I was upon a little
height; he caught my shoulder as he sprang, and we both came to the
ground below together. Growling horribly close to my ear, he shook me as
a terrier-dog does a rat. The shock produced a stupor similar to that
which seems to be felt by a mouse after the first shake of the cat. It
causes a sort of dreaminess, _in which there was no sense of pain or
feeling of terror_, though I was quite conscious of all that was
happening. It was like what patients partially under the influence of
chloroform describe, who see all the operation, but feel not the knife.
This singular condition was not the result of any mental process. The
shake annihilated fear, and allowed no sense of horror in looking round
at the beast."
This absence of pain is not peculiar to those seized by wild beasts, but
is equally produced by any accident which causes a general shock to the
system. Mr. Whymper describes an accident to himself during one of his
preliminary explorations of the Matterhorn, when he fell several hundred
feet, bounding from rock to rock, till fortunately embedded in a
snow-drift near the edge of a tremendous precipice. He declares that
while falling and feeling blow after blow, he neither lost consciousness
nor suffered pain, merely thinking, calmly, that a few more blows would
finish him. We have therefore a right to conclude, that when death
follows soon after any great shock it is as easy and painless a death as
possible; and this is certainly what happens when an animal is seized by
a beast of prey. For the enemy is one which hunts for food, not for
pleasure or excitement; and it is doubtful whether any carnivorous
animal in a state of nature begins to seek after prey till driven to do
so by hunger. When an animal is caught, therefore, it is very soon
devoured, and thus the first shock is followed by an almost painless
death. Neither do those which die of cold or hunger suffer much. Cold is
generally severest at night and has a tendency to produce sleep and
painless extinction. Hunger, on the other hand, is hardly felt during
periods of excitement, and when food is scarce the excitement of seeking
for it is at its greatest. It is probable, also, that when hunger
presses, most animals will devour anything to stay their hunger, and
will die of gradual exhaustion and weakness not necessarily painful, if
they do not fall an earlier prey to some enemy or to cold.[15]
Now let us consider what are the enjoyments of the lives of most
animals. As a rule they come into existence at a time of year when food
is most plentiful and the climate most suitable, that is in the spring
of the temperate zone and at the commencement of the dry season in the
tropics. They grow vigorously, being supplied with abundance of food;
and when they reach maturity their lives are a continual round of
healthy excitement and exercise, alternating with complete repose. The
daily search for the daily food employs all their faculties and
exercises every organ of their bodies, while this exercise leads to the
satisfaction of all their physical needs. In our own case, we can give
no more perfect definition of happiness, than this exercise and this
satisfaction; and we must therefore conclude that animals, as a rule,
enjoy all the happiness of which they are capable. And this normal state
of happiness is not alloyed, as with us, by long periods--whole lives
often--of poverty or ill-health, and of the unsatisfied longing for
pleasures which others enjoy but to which we cannot attain. Illness, and
what answers to poverty in animals--continued hunger--are quickly
followed by unanticipated and almost painless extinction. Where we err
is, in giving to animals feelings and emotions which they do not
possess. To us the very sight of blood and of torn or mangled limbs is
painful, while the idea of the suffering implied by it is heartrending.
We have a horror of all violent and sudden death, because we think of
the life full of promise cut short, of hopes and expectations
unfulfilled, and of the grief of mourning relatives. But all this is
quite out of place in the case of animals, for whom a violent and a
sudden death is in every way the best. Thus the poet's picture of
"Nature red in tooth and claw
With ravine"
is a picture the evil of which is read into it by our imaginations, the
reality being made up of full and happy lives, usually terminated by the
quickest and least painful of deaths.
On the whole, then, we conclude that the popular idea of the struggle
for existence entailing misery and pain on the animal world is the very
reverse of the truth. What it really brings about, is, the maximum of
life and of the enjoyment of life with the minimum of suffering and
pain. Given the necessity of death and reproduction--and without these
there could have been no progressive development of the organic
world,--and it is difficult even to imagine a system by which a greater
balance of happiness could have been secured. And this view was
evidently that of Darwin himself, who thus concludes his chapter on the
struggle for existence: "When we reflect on this struggle, we may
console ourselves with the full belief that the war of nature is not
incessant, that no fear is felt, that death is generally prompt, and
that the vigorous, the healthy, and the happy survive and multiply."
FOOTNOTES:
[Footnote 4: _Géographic Botanique_, p. 798.]
[Footnote 5: _The Origin of Species_, p. 53.]
[Footnote 6: _The Earth as Modified by Human Action_, p. 51.]
[Footnote 7: _The Origin of Species_, p. 56.]
[Footnote 8: See _Nature_, vol. xxxi. p. 63.]
[Footnote 9: _A Visit to South America_, 1878; also _Nature_, vol. xxxi.
pp. 263-339.]
[Footnote 10: Still more remarkable is the increase of rabbits both in
New Zealand and Australia. No less than seven millions of rabbit-skins
have been exported from the former country in a single year, their value
being £67,000. In both countries, sheep-runs have been greatly
deteriorated in value by the abundance of rabbits, which destroy the
herbage; and in some cases they have had to be abandoned altogether.]
[Footnote 11: Later observers have proved that two eggs are laid and
usually two young produced, but it may be that in most cases only one of
these comes to maturity.]
[Footnote 12: _Origin of Species_, p. 59. Professor A. Newton, however,
informs me that these species do not interfere with one another in the
way here stated.]
[Footnote 13: Winwood Reade's _Martyrdom of Man,_ p. 520.]
[Footnote 14: _Nineteenth Century,_ February 1888, pp. 162, 163.]
[Footnote 15: The Kestrel, which usually feeds on mice, birds, and
frogs, sometimes stays its hunger with earthworms, as do some of the
American buzzards. The Honey-buzzard sometimes eats not only earthworms
and slugs, but even corn; and the Buteo borealis of North America, whose
usual food is small mammals and birds, sometimes eats crayfish.]