this subject has been very ably treated by writers of eminence. Eggs
or very young animals seem generally to suffer the most, but this is
not invariably the case. While there is a vast destruction of the
seeds of plants, but it is the seedlings which are believed to suffer
the greatest, from germinating in ground already thickly stocked with
other plants, and from being destroyed in large numbers by various
enemies. The amount of food for each species of course determines the
extreme limit to which each can increase, but very often it is not
the obtaining of food, but the serving as prey to other animals which
fixes the average number of a species. Thus there seems to be little
doubt that the stock of partridges, grouse and hares on any large
estate depends mainly on the destruction of vermin. Were not a single
head of game shot during the next twenty years in England, says Darwin
in substance, and no vermin were at the same time destroyed, there
would in all probability be less game than at present exists, although
hundreds of thousands of game animals are now annually killed for
the market. In some cases, on the other hand, as in the case of the
elephant, none are destroyed by beasts of prey, for even the tiger
in India, bold and venturesome as he is known to be, rarely dares to
attack a young elephant protected by its mother. Climate, also, plays
an important part in determining the average number of a species, and
periodical seasons of extreme cold or drought are seemingly the most
effective checks of all. The action of climate appears at first sight
to be altogether independent of the Struggle for Existence; but in
so far as it chiefly acts in the reduction of food, it brings on the
most severe struggle between the individuals, whether of the same or
different species, which subsist on the same kind of fare. Even when
climate, extreme cold for example, acts directly, it will be the least
vigorous animals, or those which have been the poorest fed through
the advancing winter, that will suffer the greatest. This will be
most readily seen from what we shall now relate. When we travel from
south to north, or from a damp region to a dry, we invariably see some
species getting rarer and rarer by degrees, and finally disappearing.
Change of climate being conspicuous, we are inclined to ascribe the
entire effect to its direct action, but this is a false interpretation
of the phenomenon, for we fail to remember that each species, even
where it most prevails, is constantly suffering enormous destruction
at some period of its existence, from enemies or competitors for the
same station and food; and if these enemies or competitors be the
least favored by any slight change of climate, they will necessarily
increase in numbers, while the other species, each area being already
stocked with inhabitants, will correspondingly decrease. And when we
travel southward and see a species decreasing in numbers, we may feel
reasonably sure that the cause lies quite as much in other species
being favored as in this being hurt. So it is when we travel northward,
though in a less degree. When we go northward, or when we ascend a
mountain, we far oftener meet with stunted forms, due to the directly
injurious action of climate, than we do when we go southward or descend
a mountain. When, however, we reach the Arctic regions, or explore
snow-capped summits, or absolute deserts, we perceive the struggle for
life to be almost exclusively with the elements.
That climate operates mainly, but indirectly, in favoring other
species, may be clearly seen in the prodigious numbers of garden plants
that can thoroughly well endure our climate, but which can never become
naturalized, inasmuch as they cannot compete with native vegetation nor
resist destruction by native animals.
When a species, owing to highly favorable conditions, increases
inordinately in numbers in a small tract of country, epidemics,
especially in game animals, often occur, and here we have a limiting
check independent of the Struggle for Existence. But some of these
so-called epidemics appear to be due to parasitic worms, which have
from some cause, possibly in part through ease of diffusion among the
crowded animals, been disproportionately favored, and here comes in a
sort of struggle between the parasite and its more illustrious prey.
But, on the other hand, as is frequently the case, a large stock of
individuals of the same species, relatively to the number of its
enemies, is absolutely essential to its preservation. We thus see
how it is possible to raise with ease a plentiful supply of corn in
our fields, because the seeds are greatly in excess of the number
of birds which feed thereon. Nor can the birds, though blessed with
a superabundance of food at this one season, increase in number in
proportion to the supply of seed, as their numbers are checked during
the winter. Any one, however, who has made the experiment, knows how
troublesome it is to get seed from a few wheat or other such plants
sown broad-cast in a garden. Some singular facts in nature, such as
that of very rare plants being sometimes extremely abundant in the few
spots where they do occur, and that of some social plants being social,
or abounding in individuals, even on the extreme confines of their
range, are readily explainable by this view of the necessity of a
large stock of the same species for its preservation, for in such cases
we may believe that a plant could only exist where the conditions of
its life were so favorable that many could exist together and thus save
the species from extinction.
Complex and varied are the checks and relations between organic beings
which have to struggle together in the same country. In the case
of every species, many different checks, some very complicated and
unintelligible to man at present, acting at different periods of life,
and during different seasons or years, come into play, some one check
or some few being generally the most powerful, but all concurring in
determining the average number or even the existence of the species.
Widely-different checks sometimes act on the same species in different
districts. Looking at the plants and bushes that clothe an entangled
bank, we are tempted to ascribe their proportional numbers and kinds
to what we call chance. But this is a very false view to take of the
matter. Chance has no part in such things. They follow in obedience to
laws of which we know comparatively little. When an American forest
is cut down a very different vegetation springs up. Ancient Indian
ruins have been observed in the southern parts of the United States,
which must in former times have been cleared of trees, but which now
display the same beautiful diversity and proportion of kinds as are
now found in the surrounding virgin forest. What a struggle must have
gone on during long centuries between the several kinds of trees, each
annually scattering its seeds by the thousand, and what a war between
insect and insect, and between insects, snails and other animals with
birds and beasts of prey, all striving to increase, all feeding on each
other, or on the trees, their seeds and their seedlings, or on the
other plants which once clothed the soil, and thus checked the growth
of the trees! It is easier to account for the fall of an apple from a
tree, or the descent of a stone to the earth when hurled into the air,
than to account for the action and reaction of the innumerable plants
and animals that have determined in the course of untold centuries the
proportional numbers and kinds of trees that are now found growing on
these old Indian ruins. But the struggle will almost invariably be the
severest between individuals of the same species, for they frequent
the same districts, require the same food and are exposed to the same
dangers. In the case of varieties of the same species, the struggle
will generally be almost equally severe. If several varieties of wheat
be sown together, and the mixed seed be re-sown, some of the varieties
which best suit the soil or 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 others. Such extremely-close varieties
as the variously-colored sweet-peas must be separately harvested
each year, and the seed mixed in due proportion, or the weaker kinds
will steadily decrease in number and disappear. So, again, with the
varieties of sheep. Certain mountain-varieties will starve out other
mountain-varieties, so that they cannot be kept together. Similar
results have followed from keeping together different varieties of the
medicinal leech. In view of all that has been said, it is questionable
whether the varieties of any of our domestic plants and animals have
so exactly the same vigor, constitution and habits that the original
proportions of a mixed stock could be kept up for a half-dozen
generations if they were permitted to struggle together like beings in
a state of nature, if the seed or young were not annually assorted.
Species of the same genus having usually, though not invariably, much
similarity in habits and constitution, and always in structure, the
struggle will be more severe between species of the same genus, where
they come into competition with each other, than between species of
distinct genera. One species of swallow has caused in certain parts
of the United States the decrease of another species, just as the
missel-thrush in parts of Scotland has caused the decrease of the
song-thrush. The small Asiatic cockroach has everywhere in Russia
driven before it its great congener, and the imported European
hive-bee is rapidly exterminating in Australia the small, stingless
bee, indigenous to the country. Hundreds of such cases might be cited,
but we forbear. We can clearly see why the competition should be most
severe between allied forms, which fill nearly the same place in the
economy of nature; but it is perhaps not possible to individualize a
case and say with preciseness why such species has been victorious over
another in the battle of life. That the structure of every organic
being is related, in the most essential yet often hidden manner to
that of all the other organisms with which it comes into competition
for food or residence, or from which it has to escape, or on which
it preys, is a corollary of the highest importance deducible from
the foregoing remarks. Very obvious is this in the structure of the
teeth and talons of the tiger, and in that of the legs and claws
of the parasite which clings to the hair on the tiger’s body. But
in the beautifully-plumed seed of the dandelion and the flattened
and fringed legs of the water-beetle the relation seems at first
restricted to the elements of air and water, yet the advantage of
plumed seeds undoubtedly stands in the most intimate relation to the
land, being already densely clothed with other plants, so that the
seeds may be widely diffused and fall on unoccupied ground, while in
the water-beetle, the structure of its legs, so admirably adapted for
diving, allows it to compete with other aquatic insects, to hunt for
its own prey and to escape destruction by other predaceous animals. All
organic beings, it will thus be seen, are not only striving to increase
in numbers, but are called upon some time in their lives to struggle
for existence or to suffer serious if not utter destruction. When we
reflect on this struggle, we can console ourselves with the full belief
that this war of nature is not incessant, that no fear is felt, that
death is generally sudden, and that the vigorous, healthy and happy
survive and multiply.
Seeing what a potent influence the principle of Selection has in the
hands of man, in regard to variation, can it be applied in nature?
We can see that it can act most effectually. But in our domestic
productions the variability is not directly produced by man, for he
can neither originate varieties nor prevent their occurrence. All he
can do is to preserve and accumulate such as do occur. Unintentionally
he exposes organic beings to new and changing conditions of life, for
under domestication, plant and animal organizations become in some
degree plastic, and variability ensues. Similar changes, however,
do occur in nature. When it is borne in mind how infinitely complex
and close-fitting are the mutual relations of all organic beings
to each other, and to their environment, and consequently what
infinitely-varied diversities of structure may be of advantage to each
being under altered conditions, can it then be thought improbable,
seeing that variations useful to man have undoubtedly occurred, that
other variations useful in some way to each being in the great and
complex battle of life should sometimes occur in the course of tens
of thousands of generations? If such do occur, can we doubt, when it
is remembered that many more individuals are born than can possibly
survive, that individuals possessing any advantage, no matter how
slight, over their fellows would have the best chance of surviving and
of procreating their kind? Any variation, on the other hand, we may
feel sure if in the least degree injurious would be rigidly destroyed.
This preservation of useful and favorable variations, and the
destruction of those that are injurious, is called Natural Selection,
or the Survival of the Fittest. Variations neither advantageous nor
deleterious would not be affected by Natural Selection, and would be
left either a fluctuating element, as seen in certain polymorphic
species, or would alternately become fixed, owing to the nature both of
the organism and its conditions.
We shall best understand the probable cause of Natural Selection by
taking a country undergoing some physical change, as of climate for
example. The proportional number of its inhabitants would almost
immediately undergo a change, and some of its species might become
extinct. From the complex and very intimate manner in which the
inhabitants of each country are bound together, we may conclude that
any change in the numerical proportion of some of its inhabitants,
independently of the change of climate itself, would seriously affect
the others. Were the country open on its borders, new forms would
certainly immigrate, and this, too, would often seriously disturb the
relations of some of its former inhabitants. In the case, however,
of an island, or a country hemmed in by barriers, into which new and
better-adapted forms could not readily enter, we would then meet with
places in the economy of nature which would assuredly be better filled
up, if some of the original occupants were in some manner modified, for
had the area been open to immigration, these same places would have
been seized by intruders. Thus, slight modifications, which any way
favored the individuals of a species, would by better adapting them
to changed conditions tend to become preserved, and Natural Selection
would there have free scope for the work of improvement. Changes in
the conditions of life cause or excite a tendency to vary. In the
foregoing case the conditions are supposed to have changed, and this
would manifestly be favorable, by giving a better chance of profitable
variations occurring, to Natural Selection, for unless such do occur,
Natural Selection can do nothing. As man, by adding up in any given
direction individual differences, can certainly produce a great result
with his domestic animals and plants, so could Natural Selection,
but far more easily from having an incomparably longer time for its
action. No great physical change, as of climate, nor any unusual degree
of isolation to check immigration, is actually necessary, it would
seem, to produce new and unoccupied places for Natural Selection to
fill up by modifying and improving some of the varying inhabitants,
for as all the inhabitants of a country are struggling together
with nicely-balanced forces, extremely-slight modifications in the
structure or habits of one species would often give it an advantage
over others; and still further modifications, so long as the species
continued under the same conditions of life and profited by similar
means of subsistence and defence, would often still further augment the
advantage. No country can be mentioned whose native inhabitants are now
so perfectly adapted to each other and to their environment that none
could be better adapted and improved, for in all countries the natives
have been so far conquered by naturalized productions as to have
allowed them to take firm possession of the land. And as foreigners
have thus in every country beaten some of the natives, it may be safely
concluded that the latter might have been modified with profit so as to
have better resisted the intruders.
A man by his methodical and unconscious means of selection can produce
and has produced great results. What may not Natural Selection effect?
Man can only operate on external and visible characters, but nature
cares nothing for appearances, except in so far as they are beneficial
to any being. She can act on every internal organ, on every shade of
constitutional difference and, in fine, on the entire machinery of
life. Man selects exclusively for his own advantage, but nature solely
for that of the being she tends, and under her judicious selection
the slightest difference of structure or constitution may well turn
the nicely-balanced scale in the Struggle for Existence, and thus
be preserved. As fleeting as are the wishes and efforts of man, and
as short as is his earthly career, so poor, therefore, must be the
results which he accomplishes when compared with those accumulated by
nature during whole geological periods. Is it a wonder, then, that her
productions should be far _truer_ in character than man’s, and that
they should be infinitely better adapted to the most complex conditions
of life and should bear the stamp of far higher workmanship?
Metaphorically speaking, Natural Selection may be said to be daily and
hourly scrutinizing, throughout the world, the slightest variations,
rejecting the bad, preserving and adding up the good, and silently
and insensibly working, whenever and wherever opportunities occur, at
the betterment of each organic being in relation to its organic and
inorganic conditions of life. So slow is her work that we see nothing
of the changes in progress, and only when the hand of time has marked
the lapse of ages do we perceive that changes have been produced; but
then so imperfect is our view into long-past geological periods, that
we see only that the forms of life are now different from what they
formerly were. That any great amount of modification in any point
should be effected, a variety once formed must again, perhaps after
a long interval of time, present individual differences of the same
favorable character, and these must again be preserved, and so onward
step by step. As individual differences of all kinds perpetually
recur, this can hardly be considered as an unwarrantable assumption.
Judged by the extent the hypothesis accords with and explains the
general phenomena of nature, notwithstanding the ordinary belief that
the amount of possible variation is a strictly-limited quantity, we
are justified, it seems to us, in assuming that all this has actually
taken place. But in looking at many small points of difference between
species, which in our ignorance seem quite unimportant, we must not
lose sight of the facts that climate, food and modes of life may have
produced some direct effect, and also of the truth that, owing to
the Law of Correlation, when one part varies, and the variations are
accumulated through the Survival of the Fittest, other modifications
often of the most unlooked-for nature will ensue.
As under domestication these variations are known to appear at a
particular period of life, and tend to reappear in the offspring at the
same period, so, in a state of nature, it is reasonable to infer that
Natural Selection will be enabled to act on and modify organic beings
at any age, by the accumulation of variations useful at that age, and
by their inheritance at a corresponding age. Thus, if it be profitable
to a plant to have its seeds more and more widely disseminated by the
wind, there can be no greater difficulty in conceiving this to be
effected through Natural Selection than in conceiving the increasing
and improving of the down in the pods on his cotton-trees by a wise
selection upon the part of a cotton-planter. Natural Selection may
modify and adapt the larva of an insect to a score of contingencies,
wholly different from those which affect the mature insect, and these
modifications through Correlation may work changes in the structure of
the adult. On the other hand, modifications of the adult may affect the
structure of the larva, but in all such cases Natural Selection will
insure that these changes shall not be injurious, for, if they were so,
the extinction of the species would be the inevitable result. Thousands
of instances might be given to show the influence which Natural
Selection, or Sexual Selection, which is only a less vigorous phase
of the former, has had all through the ages in the adaptation of life
to the places in nature which it was intended to occupy in pursuance
of the plan formulated by the Great Originator and Designer of the
Universe.
Despite the imperfection of the geological record, which has been urged
as a serious objection to the theory of descent with modification,
sensible, intelligent, educated men no longer doubt that species have
all changed, and that they have changed in the way required, for they
have changed slowly and in a graduated manner. This is clearly seen in
the fossil remains from consecutive formations being invariably much
more closely allied to each other than are those from widely-separated
formations. It is true geological research does not yield those
infinitely fine gradations between past and present species which the
theory of Natural Selection requires, but when it is remembered that
only a small portion of the world has been geologically explored; that
only organic beings of certain classes, at least in any great number,
can be preserved in a fossil condition; that many species when once
formed never undergo any further change, but become extinct without
leaving any modified descendants; that dominant and widely-ranging
species vary the most and the most frequently, and that varieties
are often at first only local, it is not at all surprising that the
discovery of intermediate links to any considerable extent should not
have been made. Local varieties, as is well known, will not diffuse
themselves into other and distant localities until they have become
very much modified and improved, and when they have thus diffused
themselves, and are discovered in a geological formation, they will
appear as if suddenly created there, and will simply be ranked as new
species. Besides, formations have often been intermittent in their
accumulation, and their duration has probably been shorter than the
average duration of specific forms. And as successive formations in
most cases are separated from each other by blank intervals of time
of considerable length, and as fossiliferous formations thick enough
to withstand future degradation can as a general rule be accumulated
only where much sediment is laid down in the subsiding bed of the
ocean, it follows that during the alternate periods of elevation and
of stationary level the record will generally be blank or devoid of
fossil remains. During these latter periods there will doubtless be
more variability in the forms of life, and during the periods of
subsidence a greater amount of extinction. Now, as geology plainly
declares that each land has undergone great physical changes, we have
a right to expect that organic beings have varied under nature in the
same manner as they have varied under domestication, and such have
scientific study and research found to be the case. And if there has
been any variability under nature, such a fact would seem unaccountable
unless Natural Selection, or the Survival of the Fittest, did not come
into play. Upon the view that variations have occurred in nature and
have been preserved and accumulated by Natural Selection, and not in
the ordinary view of independent creation, we can understand why the
specific characters, or those by which the species of the same genus
differ from each other, should be more variable than the generic
characters in which they all agree. Inexplicable as is the occasional
appearance of stripes on the shoulders and legs of the different
equine species and their hybrids on the theory of creation, yet how
simply is the fact explained if we believe that they are all descended
from a striped progenitor just as the different domestic breeds of
pigeons are descended from the blue and barred rock-pigeons. Why, for
example, should the color of a flower be more likely to vary in any
one species of a genus, if the other species, supposed to have been
created independently, have differently-colored flowers, than if all
the species of the genus have the same colored flowers? On the theory
that species are only well-marked varieties, of which the characters
have become in a high degree permanent, the fact is intelligible, for
they have already varied in certain characters since they branched off
from a common progenitor, and by these characters they have come to be
specifically distinct from each other. Therefore, these same characters
would be more likely again to vary than the generic characters which
have been inherited without change for an enormous period of time.
Upon the theory of Natural Selection, or the Survival of the Fittest,
with its contingencies of extinction and divergence of character,
we can see how it is that all past and present organic beings can
be arranged within a few classes, in groups subordinate to groups,
and with the extinct groups often falling in between the recent
groups. We can see how it is that the mutual affinities of the forms
within each class are so complex and diversified, and only adaptive
characters, though of superior importance to the beings, are of
scarcely any significance in classification, while those derived from
rudimentary parts, though of no recognized service, are often of high
classificatory value, and only embryological characters are frequently
the most valuable of all. The real affinities of all organisms, in
contradistinction to their adaptive likenesses, are due to inheritance
or community of descent. Hence, a natural system of classification is
a genealogical arrangement, with the acquired grades of difference,
denoted by varieties, species, genera, families, etc., and their lines
of descent have to be discovered by the most permanent characters,
whatever they may be and how little of vital importance they may
possess.