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 number 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 certainly 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
long periods of time uniform, though assuredly the merest trifle would
give the victory to one organic being over another. Nevertheless, so
profound is our ignorance, and so high our presumption, that we marvel
when we hear of the extinction of an organic being; and, as we do not
see the cause, we invoke cataclysms to desolate the world, or invent
laws on the duration of the forms of life!
* * * * *
[Page 57.]
Nearly all our orchidaceous plants absolutely require the visits of
insects to remove their pollen-masses and thus to fertilize them. I
find from experiments that humble-bees are almost indispensable to
the fertilization of the heart’s-ease (_Viola tricolor_), for other
bees do not visit this flower. I have also found that the visits of
bees are necessary for the fertilization of some kinds of clover: for
instance, 20 heads of Dutch clover (_Trifolium repens_) yielded 2,290
seeds, but 20 other heads protected from bees produced not one. Again,
100 heads of red clover (_T. pratense_) produced 2,700 seeds, but the
same number of protected heads produced not a single seed. Humble-bees
alone visit red clover, as other bees can not reach the nectar. It
has been suggested that moths may fertilize the clovers; but I doubt
whether they could do so in the case of the red clover, from their
weight not being sufficient to depress the wing-petals. Hence we may
infer as highly probable that, if the whole genus of humble-bees became
extinct or very rare in England, the heart’s-ease and red clover would
become very rare, or wholly disappear. The number of humble-bees in
any district depends in a great measure on the number of field-mice,
which destroy their combs and nests; and Colonel Newman, who has long
attended to the habits of humble-bees, believes that “more than two
thirds of them are thus destroyed all over England.” Now, the number
of mice is largely dependent, as every one knows, on the number of
cats; and Colonel Newman says, “Near villages and small towns I have
found the nests of humble-bees more numerous than elsewhere, which I
attribute to the number of cats that destroy the mice.” Hence it is
quite credible that the presence of a feline animal in large numbers in
a district might determine, through the intervention first of mice and
then of bees, the frequency of certain flowers in that district!
NO SUCH THING AS CHANCE IN THE RESULT OF THE STRUGGLE.
[Page 58.]
When we look at the plants and bushes clothing an entangled bank, we
are tempted to attribute their proportional numbers and kinds to what
we call chance. But how false a view is this! Every one has heard
that, when an American forest is cut down, a very different vegetation
springs up; but it has been observed that ancient Indian ruins in
the Southern United States, which must formerly have been cleared of
trees, now display the same beautiful diversity and proportion of
kinds as 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; what war between insect
and insect--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 seedlings, or on the other plants which
first clothed the ground and thus checked the growth of the trees!
Throw up a handful of feathers, and all fall to the ground according
to definite laws; but how simple is the problem where each shall fall
compared to that of the action and reaction of the innumerable plants
and animals which have determined, in the course of centuries, the
proportional numbers and kinds of trees now growing on the old Indian
ruins!
* * * * *
[Page 61.]
It is good thus to try in imagination to give to any one species an
advantage over another. Probably in no single instance should we know
what to do. This ought to convince us of our ignorance on the mutual
relations of all organic beings--a conviction as necessary as it is
difficult to acquire. All that we can do is to keep steadily in mind
that each organic being is striving to increase in a geometrical ratio;
that each at some period of its life, during some season of the year,
during each generation or at intervals, has to struggle for life and
to suffer great destruction. 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.
V.
NATURAL SELECTION: OR, THE SURVIVAL OF THE FITTEST.
[Variation of
Animals and
Plants under
Domestication,
vol. i,
page 6.]
The preservation, during the battle for life, of varieties which
possess any advantage in structure, constitution, or instinct, I
have called Natural Selection; and Mr. Herbert Spencer has well
expressed the same idea by the Survival of the Fittest. The term
“natural selection” is in some respects a bad one, as it seems to
imply conscious choice; but this will be disregarded after a little
familiarity. No one objects to chemists speaking of “elective
affinity”; and certainly an acid has no more choice in combining with a
base than the conditions of life have in determining whether or not a
new form be selected or preserved. The term is so far a good one as it
brings into connection the production of domestic races by man’s power
of selection and the natural preservation of varieties and species
in a state of nature. For brevity sake I sometimes speak of natural
selection as an intelligent power; in the same way as astronomers speak
of the attraction of gravity as ruling the movements of the planets, or
as agriculturists speak of man making domestic races by his power of
selection. In the one case, as in the other, selection does nothing
without variability, and this depends in some manner on the action of
the surrounding circumstances in the organism. I have, also, often
personified the word Nature; for I have found it difficult to avoid
this ambiguity; but I mean by nature only the aggregate action and
product of many natural laws, and by laws only the ascertained sequence
of events.
AN INVENTED HYPOTHESIS.
[Animals and
Plants, vol.
i, page 9.]
In scientific investigations it is permitted to invent any hypothesis,
and if it explains various large and independent classes of facts it
rises to the rank of a well-grounded theory. The undulations of the
ether and even its existence are hypothetical, yet every one now admits
the undulatory theory of light. The principle of natural selection
may be looked at as a mere hypothesis, but rendered in some degree
probable by what we positively know of the variability of organic
beings in a state of nature--by what we positively know of the struggle
for existence, and the consequent almost inevitable preservation of
favorable variations--and from the analogical formation of domestic
races. Now, this hypothesis may be tested--and this seems to me the
only fair and legitimate manner of considering the whole question--by
trying whether it explains several large and independent classes of
facts; such as the geological succession of organic beings, their
distribution in past and present times, and their mutual affinities
and homologies. If the principle of natural selection does explain
these and other large bodies of facts, it ought to be received. On the
ordinary view of each species having been independently created, we
gain no scientific explanation of any one of these facts. We can only
say that it has so pleased the Creator to command that the past and
present inhabitants of the world should appear in a certain order and
in certain areas; that he has impressed on them the most extraordinary
resemblances, and has classed them in groups subordinate to groups. But
by such statements we gain no new knowledge; we do not connect together
facts and laws; we explain nothing.
* * * * *
[Page 12.]
These facts have as yet received no explanation on the theory of
independent creation; they can not be grouped together under one point
of view, but each has to be considered as an ultimate fact. As the
first origin of life on this earth, as well as the continued life of
each individual, is at present quite beyond the scope of science, I
do not wish to lay much stress on the greater simplicity of the view
of a few forms or of only one form having been originally created,
instead of innumerable miraculous creations having been necessary at
innumerable periods; though this more simple view accords well with
Maupertuis’s philosophical axiom of “least action.”
HOW FAR THE THEORY MAY BE EXTENDED.
[Page 13.]
In considering how far the theory of natural selection may be
extended--that is, in determining from how many progenitors the
inhabitants of the world have descended--we may conclude that at
least all the members of the same class have descended from a single
ancestor. A number of organic beings are included in the same class,
because they present, independently of their habits of life, the same
fundamental type of structure, and because they graduate into each
other. Moreover, members of the same class can in most cases be shown
to be closely alike at an early embryonic age. These facts can be
explained on the belief of their descent from a common form; therefore
it may be safely admitted that all the members of the same class are
descended from one progenitor. But as the members of quite distinct
classes have something in common in structure and much in common in
constitution, analogy would lead us one step further, and to infer
as probable that all living creatures are descended from a single
prototype.
* * * * *
[Descent of
Man, part I.,
page 61.]
Thus a large yet undefined extension may safely be given to the direct
and indirect results of natural selection; but I now admit, after
reading the essay by Nägeli on plants, and the remarks by various
authors with respect to animals, more especially those recently made by
Professor Broca, that in the earlier editions of my “Origin of Species”
I perhaps attributed too much to the action of natural selection or
the survival of the fittest. I have altered the fifth edition of the
“Origin” so as to confine my remarks to adaptive changes of structure;
but I am convinced, from the light gained during even the last few
years, that very many structures which now appear to us useless will
hereafter be proved to be useful, and will therefore come within the
range of natural selection. Nevertheless, I did not formerly consider
sufficiently the existence of structures, which, as far as we can at
present judge, are neither beneficial nor injurious; and this I believe
to be one of the greatest oversights as yet detected in my work. I
may be permitted to say, as some excuse, that I had two distinct
objects in view: firstly, to show that species had not been separately
created; and, secondly, that natural selection had been the chief agent
of change, though largely aided by the inherited effects of habit,
and slightly by the direct action of the surrounding conditions. I
was not, however, able to annul the influence of my former belief,
then almost universal, that each species had been purposely created;
and this led to my tacit assumption that every detail of structure,
excepting rudiments, was of some special, though unrecognized, service.
Any one with this assumption in his mind would naturally extend too far
the action of natural selection, either during past or present times.
Some of those who admit the principle of evolution, but reject natural
selection, seem to forget, when criticising my book, that I had the
above two objects in view; hence if I have erred in giving to natural
selection great power, which I am very far from admitting, or in having
exaggerated its power, which is in itself probable, I have at least, as
I hope, done good service in aiding to overthrow the dogma of separate
creations.
IS THERE ANY LIMIT TO WHAT SELECTION CAN EFFECT?
[Animals and
Plants, vol.
ii, page 228.]
The foregoing discussion naturally leads to the question, What is the
limit to the possible amount of variation in any part or quality, and,
consequently, is there any limit to what selection can effect? Will a
race-horse ever be reared fleeter than Eclipse? Can our prize cattle
and sheep be still further improved? Will a gooseberry ever weigh more
than that produced by “London” in 1852? Will the beet-root in France
yield a greater percentage of sugar? Will future varieties of wheat and
other grain produce heavier crops than our present varieties? These
questions can not be positively answered; but it is certain that we
ought to be cautious in answering them by a negative. In some lines of
variation the limit has probably been reached. Youatt believes that
the reduction of bone in some of our sheep has already been carried so
far that it entails great delicacy of constitution.
* * * * *
[Page 229.]
No doubt there is a limit beyond which the organization can not
be modified compatibly with health or life. The extreme degree of
fleetness, for instance, of which a terrestrial animal is capable, may
have been acquired by our present race-horses; but, as Mr. Wallace
has well remarked, the question that interests us “is not whether
indefinite and unlimited change in any or all directions is possible,
but whether such differences as do occur in nature could have been
produced by the accumulation of varieties by selection.” And in the
case of our domestic productions, there can be no doubt that many parts
of the organization, to which man has attended, have been thus modified
to a greater degree than the corresponding parts in the natural species
of the same genera or even families. We see this in the form and size
of our light and heavy dogs or horses, in the beak and many other
characters of our pigeons, in the size and quality of many fruits, in
comparison with the species belonging to the same natural groups.
HAS ORGANIZATION ADVANCED?
[Origin of
Species,
page 308.]
The problem whether organization on the whole has advanced is in
many ways excessively intricate. The geological record, at all times
imperfect, does not extend far enough back to show with unmistakable
clearness that within the known history of the world organization has
largely advanced. Even at the present day, looking to members of the
same class, naturalists are not unanimous which forms ought to be
ranked as highest: thus, some look at the selaceans or sharks, from
their approach in some important points of structure to reptiles, as
the highest fish; others look at the teleosteans as the highest. The
ganoids stand intermediate between the selaceans and teleosteans;
the latter at the present day are largely preponderant in number;
but formerly selaceans and ganoids alone existed; and in this case,
according to the standard of highness chosen, so will it be said
that fishes have advanced or retrograded in organization. To attempt
to compare members of distinct types in the scale of highness seems
hopeless; who will decide whether a cuttle-fish be higher than a
bee--that insect which the great Von Baer believed to be “in fact
more highly organized than a fish, although upon another type”? In
the complex struggle for life it is quite credible that crustaceans,
not very high in their own class, might beat cephalopods, the highest
mollusks; and such crustaceans, though not highly developed, would
stand very high in the scale of invertebrate animals, if judged by
the most decisive of all trials--the law of battle. Besides these
inherent difficulties in deciding which forms are the most advanced
in organization, we ought not solely to compare the highest members
of a class at any two periods--though undoubtedly this is one and
perhaps the most important element in striking a balance--but we ought
to compare all the members, high and low, at the two periods. At an
ancient epoch the highest and lowest molluscoidal animals, namely,
cephalopods and brachiopods, swarmed in numbers; at the present
time both groups are greatly reduced, while others, intermediate in
organization, have largely increased; consequently some naturalists
maintain that mollusks were formerly more highly developed than at
present; but a stronger case can be made out on the opposite side, by
considering the vast reduction of brachiopods, and the fact that our
existing cephalopods, though few in number, are more highly organized
than their ancient representatives. We ought also to compare the
relative proportional numbers at any two periods of the high and low
classes throughout the world; if, for instance, at the present day
fifty thousand kinds of vertebrate animals exist, and if we knew that
at some former period only ten thousand kinds existed, we ought to look
at this increase in number in the highest class, which implies a great
displacement of lower forms, as a decided advance in the organization
of the world. We thus see how hopelessly difficult it is to compare
with perfect fairness, under such extremely complex relations, the
standard of organization of the imperfectly-known faunas of successive
periods.
* * * * *
[Origin of
Species,
page 121.]
There may truly be said to be a constant struggle going on between, on
the one hand, the tendency to reversion to a less perfect state, as
well as an innate tendency to new variations, and, on the other hand,
the power of steady selection to keep the breed true. In the long run
selection gains the day, and we do not expect to fail so completely
as to breed bird as coarse as a common tumbler-pigeon from a good
short-faced strain. But, as long as selection is rapidly going on, much
variability in the parts undergoing modification may always be expected.
A HIGHER WORKMANSHIP THAN MAN’S.
[Origin of
Species,
page 65.]
As man can produce, and certainly has produced, a great result by his
methodical and unconscious means of selection, what may not natural
selection affect? Man can act only on external and visible characters:
Nature, if I may be allowed to personify the natural preservation or
survival of the fittest, cares nothing for appearances, except in so
far as they are useful to any being. She can act on every internal
organ, on every shade of constitutional difference, on the whole
machinery of life. Man selects only for his own good: Nature only for
that of the being which she tends. Every selected character is fully
exercised by her, as is implied by the fact of their selection. Man
keeps the natives of many climates in the same country; he seldom
exercises each selected character in some peculiar and fitting manner;
he feeds a long and a short beaked pigeon on the same food; he does
not exercise a long-backed or long-legged quadruped in any peculiar
manner; he exposes sheep with long and short wool to the same climate.
He does not allow the most vigorous males to struggle for the females.
He does not rigidly destroy all inferior animals, but protects during
each varying season, as far as lies in his power, all his productions.
He often begins his selection by some half-monstrous form; or at
least by some modification prominent enough to catch the eye or to
be plainly useful to him. Under nature, the slightest differences of
structure or constitution may well turn the nicely-balanced scale in
the struggle for life, and so be preserved. How fleeting are the wishes
and efforts of man! how short his time! and consequently how poor will
be his results, compared with those accumulated by Nature during whole
geological periods! Can we wonder, then, that Nature’s productions
should be far “truer” in character than man’s productions; that they
should be infinitely better adapted to the most complex conditions of
life, and should plainly bear the stamp of far higher workmanship?
It may metaphorically be said that natural selection is daily and
hourly scrutinizing, throughout the world, the slightest variations:
rejecting those that are bad, preserving and adding up all that
are good; silently and insensibly working, _whenever and wherever
opportunity offers_, at the improvement of each organic being in
relation to its organic and inorganic conditions of life. We see
nothing of these slow changes in progress, until the hand of Time
has marked the lapse of ages, and then so imperfect is our view into
long-past geological ages that we see only that the forms of life are
now different from what they formerly were.
* * * * *
[Page 66.]
Although natural selection can act only through and for the good
of each being, yet characters and structures, which we are apt to
consider as of very trifling importance, may thus be acted on. When
we see leaf-eating insects green and bark-feeders mottled-gray, the
Alpine ptarmigan white in winter, the red-grouse the color of heather,
we must believe that these tints are of service to these birds and
insects in preserving them from danger. Grouse, if not destroyed at
some period of their lives, would increase in countless numbers; they
are known to suffer largely from birds of prey; and hawks are guided
by eye-sight to their prey--so much so, that on parts of the Continent
persons are warned not to keep white pigeons, as being the most liable
to destruction. Hence natural selection might be effective in giving
the proper color to each kind of grouse, and in keeping that color,
when once acquired, true and constant. Nor ought we to think that the
occasional destruction of an animal of any particular color would
produce little effect: we should remember how essential it is in a
flock of white sheep to destroy a lamb with the faintest trace of black.
WHY HABITS AND STRUCTURE ARE NOT IN AGREEMENT.
[Origin of
Species,
page 142.]
He who believes that each being has been created as we now see it must
occasionally have felt surprise when he has met with an animal having
habits and structure not in agreement. What can be plainer than that
the webbed feet of ducks and geese are formed for swimming? Yet there
are upland geese with webbed feet which rarely go near the water; and
no one except Audubon has seen the frigate-bird, which has all its four
toes webbed, alight on the surface of the ocean. On the other hand,
grebes and coots are eminently aquatic, although their toes are only
bordered by membrane. What seems plainer than that the long toes, not
furnished with membrane, of the _Grallatores_, are formed for walking
over swamps and floating plants?--the water-hen and land-rail are
members of this order, yet the first is nearly as aquatic as the coot,
and the second nearly as terrestrial as the quail or partridge. In such
cases, and many others could be given, habits have changed without a
corresponding change of structure. The webbed feet of the upland goose
may be said to have become almost rudimentary in function, though not
in structure. In the frigate-bird, the deeply-scooped membrane between
the toes shows that structure has begun to change.
He who believes in separate and innumerable acts of creation may
say that in these cases it has pleased the Creator to cause a being
of one type to take the place of one belonging to another type; but
this seems to me only restating the fact in dignified language. He
who believes in the struggle for existence and in the principle
of natural selection, will acknowledge that every organic being is
constantly endeavoring to increase in numbers; and that if any one
being varies ever so little, either in habits or structure, and thus
gains an advantage over some other inhabitant of the same country, it
will seize on the place of that inhabitant, however different that may
be from its own place. Hence it will cause him no surprise that there
should be geese and frigate-birds with webbed feet, living on the dry
land and rarely alighting on the water; that there should be long-toed
corn-crakes, living in meadows instead of in swamps; that there should
be woodpeckers where hardly a tree grows; that there should be diving
thrushes and diving _Hymenoptera_, and petrels with the habits of auks.
NO MODIFICATION IN ONE SPECIES DESIGNED FOR THE GOOD OF ANOTHER.
[Origin of
Species,
page 162.]