On the Origin of Species by Means of Natural Selection (1859)
by Charles Darwin
CHAPTER XIV.
RECAPITULATION AND CONCLUSION.
Recapitulation of the
difficulties on the theory of Natural Selection—Recapitulation of
the general and special circumstances in its favour—Causes of the
general belief in the immutability of species—How far the theory of
natural selection may be extended—Effects of its adoption on the
study of Natural history—Concluding remarks.
S this whole volume is one long
argument, it may be convenient to the reader to have the leading
facts and inferences briefly recapitulated.
That many and grave objections may be advanced against the theory of
descent with modification through natural selection, I do not deny. I
have endeavoured to give to them their full force. Nothing at first can
appear more difficult to believe than that the more complex organs and
instincts should have been perfected, not by means superior to, though
analogous with, human reason, but by the accumulation of innumerable
slight variations, each good for the individual possessor. Nevertheless,
this difficulty, though appearing to our imagination insuperably great,
cannot be considered real if we admit the following propositions,
namely,—that gradations in the perfection of any organ or instinct,
which we may consider, either do now exist or could have existed, each
good of its kind,—that all organs and instincts are, in ever so
slight a degree, variable,—and, lastly, that there is a struggle
for existence leading to the preservation of each profitable deviation
of structure or instinct. The truth of these propositions cannot, I
think, be disputed.
It is, no doubt, extremely difficult even to conjecture by what
gradations many structures have been perfected, more especially amongst
broken and failing groups of organic beings; but we see so many strange
gradations in nature, as is proclaimed by the canon, "Natura non facit
saltum," that we ought to be extremely cautious in saying that any organ
or instinct, or any whole being, could not have arrived at its present
state by many graduated steps. There are, it must be admitted, cases of
special difficulty on the theory of natural selection; and one of the
most curious of these is the existence of two or three defined castes of
workers or sterile females in the same community of ants; but I have
attempted to show how this difficulty can be mastered.
With respect to the almost universal sterility of species when first
crossed, which forms so remarkable a contrast with the almost universal
fertility of varieties when crossed, I must refer the reader to the
recapitulation of the facts given at the end of the eighth chapter,
which seem to me conclusively to show that this sterility is no more a
special endowment than is the incapacity of two trees to be grafted
together, but that it is incidental on constitutional differences in the
reproductive systems of the intercrossed species. We see the truth of
this conclusion in the vast difference in the result, when the same two
species are crossed reciprocally; that is, when one species is first
used as the father and then as the mother.
The fertility of varieties when intercrossed and of their mongrel
offspring cannot be considered as universal; nor is their very general
fertility surprising when we remember that it is not likely that either
their constitutions or their reproductive systems should have been
profoundly modified. Moreover, most of the varieties which have been
experimentised on have been produced under domestication; and as
domestication apparently tends to eliminate sterility, we ought not to
expect it also to produce sterility.
The sterility of hybrids is a very different case from that of first
crosses, for their reproductive organs are more or less functionally
impotent; whereas in first crosses the organs on both sides are in a
perfect condition. As we continually see that organisms of all kinds are
rendered in some degree sterile from their constitutions having been
disturbed by slightly different and new conditions of life, we need not
feel surprise at hybrids being in some degree sterile, for their
constitutions can hardly fail to have been disturbed from being
compounded of two distinct organisations. This parallelism is supported
by another parallel, but directly opposite, class of facts; namely, that
the vigour and fertility of all organic beings are increased by slight
changes in their conditions of life, and that the offspring of slightly
modified forms or varieties acquire from being crossed increased vigour
and fertility. So that, on the one hand, considerable changes in the
conditions of life and crosses between greatly modified forms, lessen
fertility; and on the other hand, lesser changes in the conditions of
life and crosses between less modified forms, increase fertility.
Turning to geographical distribution, the difficulties encountered on
the theory of descent with modification are grave enough. All the
individuals of the same species, and all the species of the same genus,
or even higher group, must have descended from common parents; and
therefore, in however distant and isolated parts of the world they are
now found, they must in the course of successive generations have passed
from some one part to the others. We are often wholly unable even to
conjecture how this could have been effected. Yet, as we have reason to
believe that some species have retained the same specific form for very
long periods, enormously long as measured by years, too much stress
ought not to be laid on the occasional wide diffusion of the same
species; for during very long periods of time there will always be a
good chance for wide migration by many means. A broken or interrupted
range may often be accounted for by the extinction of the species in the
intermediate regions. It cannot be denied that we are as yet very
ignorant of the full extent of the various climatal and geographical
changes which have affected the earth during modern periods; and such
changes will obviously have greatly facilitated migration. As an
example, I have attempted to show how potent has been the influence of
the Glacial period on the distribution both of the same and of
representative species throughout the world. We are as yet profoundly
ignorant of the many occasional means of transport. With respect to
distinct species of the same genus inhabiting very distant and isolated
regions, as the process of modification has necessarily been slow, all
the means of migration will have been possible during a very long period;
and consequently the difficulty of the wide diffusion of species of the
same genus is in some degree lessened.
As on the theory of natural selection an interminable number of
intermediate forms must have existed, linking together all the species
in each group by gradations as fine as our present varieties, it may be
asked, Why do we not see these linking forms all around us? Why are not
all organic beings blended together in an inextricable chaos? With
respect to existing forms, we should remember that we have no right to
expect (excepting in rare cases) to discover directly connecting links
between them, but only between each and some extinct and supplanted
form. Even on a wide area, which has during a long period remained
continuous, and of which the climate and other conditions of life change
insensibly in going from a district occupied by one species into another
district occupied by a closely allied species, we have no just right to
expect often to find intermediate varieties in the intermediate zone.
For we have reason to believe that only a few species are undergoing
change at any one period; and all changes are slowly effected. I have
also shown that the intermediate varieties which will at first probably
exist in the intermediate zones, will be liable to be supplanted by the
allied forms on either hand; and the latter, from existing in greater
numbers, will generally be modified and improved at a quicker rate than
the intermediate varieties, which exist in lesser numbers; so that the
intermediate varieties will, in the long run, be supplanted and
exterminated.
On this doctrine of the extermination of an infinitude of connecting
links, between the living and extinct inhabitants of the world, and at
each successive period between the extinct and still older species, why
is not every geological formation charged with such links? Why does not
every collection of fossil remains afford plain evidence of the
gradation and mutation of the forms of life? We meet with no such
evidence, and this is the most obvious and forcible of the many
objections which may be urged against my theory. Why, again, do whole
groups of allied species appear, though certainly they often falsely
appear, to have come in suddenly on the several geological stages? Why
do we not find great piles of strata beneath the Silurian system, stored
with the remains of the progenitors of the Silurian groups of fossils?
For certainly on my theory such strata must somewhere have been
deposited at these ancient and utterly unknown epochs in the world's
history.
I can answer these questions and grave objections only on the
supposition that the geological record is far more imperfect than most
geologists believe. It cannot be objected that there has not been time
sufficient for any amount of organic change; for the lapse of time has
been so great as to be utterly inappreciable by the human intellect. The
number of specimens in all our museums is absolutely as nothing compared
with the countless generations of countless species which certainly have
existed. We should not be able to recognise a species as the parent of
any one or more species if we were to examine them ever so closely,
unless we likewise possessed many of the intermediate links between
their past or parent and present states; and these many links we could
hardly ever expect to discover, owing to the imperfection of the
geological record. Numerous existing doubtful forms could be named which
are probably varieties; but who will pretend that in future ages so many
fossil links will be discovered, that naturalists will be able to
decide, on the common view, whether or not these doubtful forms are
varieties? As long as most of the links between any two species are
unknown, if any one link or intermediate variety be discovered, it will
simply be classed as another and distinct species. Only a small portion
of the world has been geologically explored. Only organic beings of
certain classes can be preserved in a fossil condition, at least in any
great number. Widely ranging species vary most, and varieties are often
at first local,—both causes rendering the discovery of intermediate
links less likely. Local varieties will not spread into other and
distant regions until they are considerably modified and improved; and
when they do spread, if discovered in a geological formation, they will
appear as if suddenly created there, and will be simply classed as new
species. Most formations have been intermittent in their accumulation;
and their duration, I am inclined to believe, has been shorter than the
average duration of specific forms. Successive formations are separated
from each other by enormous blank intervals of time; for fossiliferous
formations, thick enough to resist future degradation, can be
accumulated only where much sediment is deposited on the subsiding bed
of the sea. During the alternate periods of elevation and of stationary
level the record will be blank. During these latter periods there will
probably be more variability in the forms of life; during periods of
subsidence, more extinction.
With respect to the absence of fossiliferous formations beneath the
lowest Silurian strata, I can only recur to the hypothesis given in the
ninth chapter. That the geological record is imperfect all will admit;
but that it is imperfect to the degree which I require, few will be
inclined to admit. If we look to long enough intervals of time, geology
plainly declares that all species have changed; and they have changed in
the manner which my theory requires, for they have changed slowly and in
a graduated manner. We clearly see this in the fossil remains from
consecutive formations invariably being much more closely related to
each other, than are the fossils from formations distant from each other
in time.
Such is the sum of the several chief objections and difficulties which
may justly be urged against my theory; and I have now briefly
recapitulated the answers and explanations which can be given to them.
I have felt these difficulties far too heavily during many years to
doubt their weight. But it deserves especial notice that the more
important objections relate to questions on which we are confessedly
ignorant; nor do we know how ignorant we are. We do not know all the
possible transitional gradations between the simplest and the most
perfect organs; it cannot be pretended that we know all the varied means
of Distribution during the long lapse of years, or that we know how
imperfect the Geological Record is. Grave as these several difficulties
are, in my judgment they do not overthrow the theory of descent with
modification.
Now let us turn to the other side of the argument. Under domestication
we see much variability. This seems to be mainly due to the reproductive
system being eminently susceptible to changes in the conditions of life;
so that this system, when not rendered impotent, fails to reproduce
offspring exactly like the parent-form. Variability is governed by many
complex laws,—by correlation of growth, by use and disuse, and by
the direct action of the physical conditions of life. There is much
difficulty in ascertaining how much modification our domestic productions
have undergone; but we may safely infer that the amount has been large,
and that modifications can be inherited for long periods. As long as the
conditions of life remain the same, we have reason to believe that a
modification, which has already been inherited for many generations, may
continue to be inherited for an almost infinite number of generations.
On the other hand we have evidence that variability, when it has once
come into play, does not wholly cease; for new varieties are still
occasionally produced by our most anciently domesticated productions.
Man does not actually produce variability; he only unintentionally
exposes organic beings to new conditions of life, and then nature acts
on the organisation, and causes variability. But man can and does
select the variations given to him by nature, and thus accumulate them
in any desired manner. He thus adapts animals and plants for his own
benefit or pleasure. He may do this methodically, or he may do it
unconsciously by preserving the individuals most useful to him at the
time, without any thought of altering the breed. It is certain that he
can largely influence the character of a breed by selecting, in each
successive generation, individual differences so slight as to be quite
inappreciable by an uneducated eye. This process of selection has been
the great agency in the production of the most distinct and useful
domestic breeds. That many of the breeds produced by man have to a large
extent the character of natural species, is shown by the inextricable
doubts whether very many of them are varieties or aboriginal species.
There is no obvious reason why the principles which have acted so
efficiently under domestication should not have acted under nature. In
the preservation of favoured individuals and races, during the
constantly-recurrent Struggle for Existence, we see the most powerful
and ever-acting means of selection. The struggle for existence
inevitably follows from the high geometrical ratio of increase which is
common to all organic beings. This high rate of increase is proved by
calculation, by the effects of a succession of peculiar seasons, and by
the results of naturalisation, as explained in the third chapter. More
individuals are born than can possibly survive. A grain in the balance
will determine which individual shall live and which shall
die,—which variety or species shall increase in number, and which
shall decrease, or finally become extinct. As the individuals of the
same species come in all respects into the closest competition with each
other, the struggle will generally be most severe between them; it will
be almost equally severe between the varieties of the same species, and
next in severity between the species of the same genus. But the struggle
will often be very severe between beings most remote in the scale of
nature. The slightest advantage in one being, at any age or during any
season, over those with which it comes into competition, or better
adaptation in however slight a degree to the surrounding physical
conditions, will turn the balance.
With animals having separated sexes there will in most cases be a
struggle between the males for possession of the females. The most
vigorous individuals, or those which have most successfully struggled
with their conditions of life, will generally leave most progeny. But
success will often depend on having special weapons or means of defence,
or on the charms of the males; and the slightest advantage will lead to
victory.
As geology plainly proclaims that each land has undergone great physical
changes, we might have expected that organic beings would have varied
under nature, in the same way as they generally have varied under the
changed conditions of domestication. And if there be any variability
under nature, it would be an unaccountable fact if natural selection had
not come into play. It has often been asserted, but the assertion is
quite incapable of proof, that the amount of variation under nature is a
strictly limited quantity. Man, though acting on external characters
alone and often capriciously, can produce within a short period a great
result by adding up mere individual differences in his domestic
productions; and every one admits that there are at least individual
differences in species under nature. But, besides such differences, all
naturalists have admitted the existence of varieties, which they think
sufficiently distinct to be worthy of record in systematic works. No one
can draw any clear distinction between individual differences and slight
varieties; or between more plainly marked varieties and sub-species, and
species. Let it be observed how naturalists differ in the rank which
they assign to the many representative forms in Europe and North
America.
If then we have under nature variability and a powerful agent always
ready to act and select, why should we doubt that variations in any way
useful to beings, under their excessively complex relations of life,
would be preserved, accumulated, and inherited? Why, if man can by
patience select variations most useful to himself, should nature fail in
selecting variations useful, under changing conditions of life, to her
living products? What limit can be put to this power, acting during long
ages and rigidly scrutinising the whole constitution, structure, and
habits of each creature,—favouring the good and rejecting the bad?
I can see no limit to this power, in slowly and beautifully adapting
each form to the most complex relations of life. The theory of natural
selection, even if we looked no further than this, seems to me to be in
itself probable. I have already recapitulated, as fairly as I could, the
opposed difficulties and objections: now let us turn to the special
facts and arguments in favour of the theory.
On the view that species are only strongly marked and permanent
varieties, and that each species first existed as a variety, we can see
why it is that no line of demarcation can be drawn between species,
commonly supposed to have been produced by special acts of creation, and
varieties which are acknowledged to have been produced by secondary
laws. On this same view we can understand how it is that in each region
where many species of a genus have been produced, and where they now
flourish, these same species should present many varieties; for where
the manufactory of species has been active, we might expect, as a
general rule, to find it still in action; and this is the case if
varieties be incipient species. Moreover, the species of the larger
genera, which afford the greater number of varieties or incipient
species, retain to a certain degree the character of varieties; for they
differ from each other by a less amount of difference than do the
species of smaller genera. The closely allied species also of the larger
genera apparently have restricted ranges, and they are clustered in
little groups round other species-in which respects they resemble
varieties. These are strange relations on the view of each species
having been independently created, but are intelligible if all species
first existed as varieties.
As each species tends by its geometrical ratio of reproduction to
increase inordinately in number; and as the modified descendants of each
species will be enabled to increase by so much the more as they become
more diversified in habits and structure, so as to be enabled to seize
on many and widely different places in the economy of nature, there will
be a constant tendency in natural selection to preserve the most
divergent offspring of any one species. Hence during a long-continued
course of modification, the slight differences, characteristic of
varieties of the same species, tend to be augmented into the greater
differences characteristic of species of the same genus. New and
improved varieties will inevitably supplant and exterminate the older,
less improved and intermediate varieties; and thus species are rendered
to a large extent defined and distinct objects. Dominant species
belonging to the larger groups tend to give birth to new and dominant
forms; so that each large group tends to become still larger, and at
the same time more divergent in character. But as all groups cannot thus
succeed in increasing in size, for the world would not hold them, the
more dominant groups beat the less dominant. This tendency in the large
groups to go on increasing in size and diverging in character, together
with the almost inevitable contingency of much extinction, explains the
arrangement of all the forms of life, in groups subordinate to groups,
all within a few great classes, which we now see everywhere around us,
and which has prevailed throughout all time. This grand fact of the
grouping of all organic beings seems to me utterly inexplicable on the
theory of creation.
As natural selection acts solely by accumulating slight, successive,
favourable variations, it can produce no great or sudden modification;
it can act only by very short and slow steps. Hence the canon of
"Natura non facit saltum," which every fresh addition to our knowledge
tends to make more strictly correct, is on this theory simply
intelligible. We can plainly see why nature is prodigal in variety,
though niggard in innovation. But why this should be a law of nature if
each species has been independently created, no man can explain.
Many other facts are, as it seems to me, explicable on this theory. How
strange it is that a bird, under the form of woodpecker, should have
been created to prey on insects on the ground; that upland geese, which
never or rarely swim, should have been created with webbed feet; that a
thrush should have been created to dive and feed on sub-aquatic insects;
and that a petrel should have been created with habits and structure
fitting it for the life of an auk or grebe! and so on in endless other
cases. But on the view of each species constantly trying to increase in
number, with natural selection always ready to adapt the slowly varying
descendants of each to any unoccupied or ill-occupied place in nature,
these facts cease to be strange, or perhaps might even have been
anticipated.
As natural selection acts by competition, it adapts the inhabitants of
each country only in relation to the degree of perfection of their
associates; so that we need feel no surprise at the inhabitants of any
one country, although on the ordinary view supposed to have been
specially created and adapted for that country, being beaten and
supplanted by the naturalised productions from another land. Nor ought
we to marvel if all the contrivances in nature be not, as far as we can
judge, absolutely perfect; and if some of them be abhorrent to our ideas
of fitness. We need not marvel at the sting of the bee causing the
bee's own death; at drones being produced in such vast numbers for one
single act, and being then slaughtered by their sterile sisters; at the
astonishing waste of pollen by our fir-trees; at the instinctive hatred
of the queen bee for her own fertile daughters; at ichneumonidę feeding
within the live bodies of caterpillars; and at other such cases. The
wonder indeed is, on the theory of natural selection, that more cases of
the want of absolute perfection have not been observed.
The complex and little known laws governing variation are the same, as
far as we can see, with the laws which have governed the production of
so-called specific forms. In both cases physical conditions seem to have
produced but little direct effect; yet when varieties enter any zone,
they occasionally assume some of the characters of the species proper to
that zone. In both varieties and species, use and disuse seem to have
produced some effect; for it is difficult to resist this conclusion when
we look, for instance, at the logger-headed duck, which has wings
incapable of flight, in nearly the same condition as in the domestic
duck; or when we look at the burrowing tucutucu, which is occasionally
blind, and then at certain moles, which are habitually blind and have
their eyes covered with skin; or when we look at the blind animals
inhabiting the dark caves of America and Europe. In both varieties and
species correlation of growth seems to have played a most important part,
so that when one part has been modified other parts are necessarily
modified. In both varieties and species reversions to long-lost
characters occur. How inexplicable on the theory of creation is the
occasional appearance of stripes on the shoulder and legs of the several
species of the horse-genus and in their hybrids! How simply is this fact
explained if we believe that these species have descended from a striped
progenitor, in the same manner as the several domestic breeds of pigeon
have descended from the blue and barred rock-pigeon!
On the ordinary view of each species having been independently created,
why should the specific characters, or those by which the species of the
same genus differ from each other, be more variable than the generic
characters in which they all agree? Why, for instance, should the colour
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 coloured flowers, than if all the species of the genus have
the same coloured flowers? If species are only well-marked varieties, of
which the characters have become in a high degree permanent, we can
understand this fact; for they have already varied since they branched
off from a common progenitor in certain characters, by which they have
come to be specifically distinct from each other; and therefore these
same characters would be more likely still to be variable than the
generic characters which have been inherited without change for an
enormous period. It is inexplicable on the theory of creation why a part
developed in a very unusual manner in any one species of a genus, and
therefore, as we may naturally infer, of great importance to the species,
should be eminently liable to variation; but, on my view, this part has
undergone, since the several species branched off from a common
progenitor, an unusual amount of variability and modification, and
therefore we might expect this part generally to be still variable. But
a part may be developed in the most unusual manner, like the wing of a
bat, and yet not be more variable than any other structure, if the part
be common to many subordinate forms, that is, if it has been inherited
for a very long period; for in this case it will have been rendered
constant by long-continued natural selection.
Glancing at instincts, marvellous as some are, they offer no greater
difficulty than does corporeal structure on the theory of the natural
selection of successive, slight, but profitable modifications. We can
thus understand why nature moves by graduated steps in endowing
different animals of the same class with their several instincts. I have
attempted to show how much light the principle of gradation throws on
the admirable architectural powers of the hive-bee. Habit no doubt
sometimes comes into play in modifying instincts; but it certainly is
not indispensable, as we see, in the case of neuter insects, which leave
no progeny to inherit the effects of long-continued habit. On the view
of all the species of the same genus having descended from a common
parent, and having inherited much in common, we can understand how it is
that allied species, when placed under considerably different conditions
of life, yet should follow nearly the same instincts; why the thrush of
South America, for instance, lines her nest with mud like our British
species. On the view of instincts having been slowly acquired through
natural selection we need not marvel at some instincts being apparently
not perfect and liable to mistakes, and at many instincts causing other
animals to suffer.
If species be only well-marked and permanent varieties, we can at once
see why their crossed offspring should follow the same complex laws in
their degrees and kinds of resemblance to their parents,—in being
absorbed into each other by successive crosses, and in other such
points,—as do the crossed offspring of acknowledged varieties. On
the other hand, these would be strange facts if species have been
independently created, and varieties have been produced by secondary
laws.
If we admit that the geological record is imperfect in an extreme degree,
then such facts as the record gives, support the theory of descent with
modification. New species have come on the stage slowly and at
successive intervals; and the amount of change, after equal intervals of
time, is widely different in different groups. The extinction of species
and of whole groups of species, which has played so conspicuous a part
in the history of the organic world, almost inevitably follows on the
principle of natural selection; for old forms will be supplanted by new
and improved forms. Neither single species nor groups of species
reappear when the chain of ordinary generation has once been broken. The
gradual diffusion of dominant forms, with the slow modification of their
descendants, causes the forms of life, after long intervals of time, to
appear as if they had changed simultaneously throughout the world. The
fact of the fossil remains of each formation being in some degree
intermediate in character between the fossils in the formations above
and below, is simply explained by their intermediate position in the
chain of descent. The grand fact that all extinct organic beings belong
to the same system with recent beings, falling either into the same or
into intermediate groups, follows from the living and the extinct being
the offspring of common parents. As the groups which have descended from
an ancient progenitor have generally diverged in character, the
progenitor with its early descendants will often be intermediate in
character in comparison with its later descendants; and thus we can see
why the more ancient a fossil is, the oftener it stands in some degree
intermediate between existing and allied groups. Recent forms are
generally looked at as being, in some vague sense, higher than ancient
and extinct forms; and they are in so far higher as the later and more
improved forms have conquered the older and less improved organic beings
in the struggle for life. Lastly, the law of the long endurance of
allied forms on the same continent,—of marsupials in Australia, of
edentata in America, and other such cases,—is intelligible, for
within a confined country, the recent and the extinct will naturally be
allied by descent.
Looking to geographical distribution, if we admit that there has been
during the long course of ages much migration from one part of the world
to another, owing to former climatal and geographical changes and to the
many occasional and unknown means of dispersal, then we can understand,
on the theory of descent with modification, most of the great leading
facts in Distribution. We can see why there should be so striking a
parallelism in the distribution of organic beings throughout space, and
in their geological succession throughout time; for in both cases the
beings have been connected by the bond of ordinary generation, and the
means of modification have been the same. We see the full meaning of the
wonderful fact, which must have struck every traveller, namely, that on
the same continent, under the most diverse conditions, under heat and
cold, on mountain and lowland, on deserts and marshes, most of the
inhabitants within each great class are plainly related; for they will
generally be descendants of the same progenitors and early colonists. On
this same principle of former migration, combined in most cases with
modification, we can understand, by the aid of the Glacial period, the
identity of some few plants, and the close alliance of many others, on
the most distant mountains, under the most different climates; and
likewise the close alliance of some of the inhabitants of the sea in the
northern and southern temperate zones, though separated by the whole
intertropical ocean. Although two areas may present the same physical
conditions of life, we need feel no surprise at their inhabitants being
widely different, if they have been for a long period completely
separated from each other; for as the relation of organism to organism
is the most important of all relations, and as the two areas will have
received colonists from some third source or from each other, at various
periods and in different proportions, the course of modification in the
two areas will inevitably be different.
On this view of migration, with subsequent modification, we can see why
oceanic islands should be inhabited by few species, but of these, that
many should be peculiar. We can clearly see why those animals which
cannot cross wide spaces of ocean, as frogs and terrestrial mammals,
should not inhabit oceanic islands; and why, on the other hand, new and
peculiar species of bats, which can traverse the ocean, should so often
be found on islands far distant from any continent. Such facts as the
presence of peculiar species of bats, and the absence of all other
mammals, on oceanic islands, are utterly inexplicable on the theory of
independent acts of creation.
The existence of closely allied or representative species in any two
areas, implies, on the theory of descent with modification, that the
same parents formerly inhabited both areas; and we almost invariably
find that wherever many closely allied species inhabit two areas, some
identical species common to both still exist. Wherever many closely
allied yet distinct species occur, many doubtful forms and varieties of
the same species likewise occur. It is a rule of high generality that
the inhabitants of each area are related to the inhabitants of the
nearest source whence immigrants might have been derived. We see this in
nearly all the plants and animals of the Galapagos archipelago, of Juan
Fernandez, and of the other American islands being related in the most
striking manner to the plants and animals of the neighbouring American
mainland; and those of the Cape de Verde archipelago and other African
islands to the African mainland. It must be admitted that these facts
receive no explanation on the theory of creation.
The fact, as we have seen, that all past and present organic beings
constitute one grand natural system, with group subordinate to group,
and with extinct groups often falling in between recent groups, is
intelligible on the theory of natural selection with its contingencies
of extinction and divergence of character. On these same principles we
see how it is, that the mutual affinities of the species and genera
within each class are so complex and circuitous. We see why certain
characters are far more serviceable than others for
classification;—why adaptive characters, though of paramount
importance to the being, are of hardly any importance in classification;
why characters derived from rudimentary parts, though of no service to
the being, are often of high classificatory value; and why embryological
characters are the most valuable of all. The real affinities of all
organic beings are due to inheritance or community of descent. The
natural system is a genealogical arrangement, in which we have to
discover the lines of descent by the most permanent characters, however
slight their vital importance may be.
The framework of bones being the same in the hand of a man, wing of a
bat, fin of the porpoise, and leg of the horse,—the same number of
vertebrę forming the neck of the giraffe and of the elephant,—and
innumerable other such facts, at once explain themselves on the theory
of descent with slow and slight successive modifications. The similarity
of pattern in the wing and leg of a bat, though used for such different
purpose,—in the jaws and legs of a crab,—in the petals,
stamens, and pistils of a flower, is likewise intelligible on the view
of the gradual modification of parts or organs, which were alike in the
early progenitor of each class. On the principle of successive
variations not always supervening at an early age, and being inherited
at a corresponding not early period of life, we can clearly see why the
embryos of mammals, birds, reptiles, and fishes should be so closely
alike, and should be so unlike the adult forms. We may cease marvelling
at the embryo of an air-breathing mammal or bird having branchial slits
and arteries running in loops, like those in a fish which has to breathe
the air dissolved in water, by the aid of well-developed branchiæ.
Disuse, aided sometimes by natural selection, will often tend to reduce
an organ, when it has become useless by changed habits or under changed
conditions of life; and we can clearly understand on this view the
meaning of rudimentary organs. But disuse and selection will generally
act on each creature, when it has come to maturity and has to play its
full part in the struggle for existence, and will thus have little power
of acting on an organ during early life; hence the organ will not be
much reduced or rendered rudimentary at this early age. The calf, for
instance, has inherited teeth, which never cut through the gums of the
upper jaw, from an early progenitor having well-developed teeth; and we
may believe, that the teeth in the mature animal were reduced, during
successive generations, by disuse or by the tongue and palate having
been fitted by natural selection to browse without their aid; whereas in
the calf, the teeth have been left untouched by selection or disuse, and
on the principle of inheritance at corresponding ages have been
inherited from a remote period to the present day. On the view of each
organic being and each separate organ having been specially created, how
utterly inexplicable it is that parts, like the teeth in the embryonic
calf or like the shrivelled wings under the soldered wing-covers of some
beetles, should thus so frequently bear the plain stamp of inutility!
Nature may be said to have taken pains to reveal, by rudimentary organs
and by homologous structures, her scheme of modification, which it seems
that we wilfully will not understand.
I have now recapitulated the chief facts and considerations which have
thoroughly convinced me that species have changed, and are still slowly
changing by the preservation and accumulation of successive slight
favourable variations. Why, it may be asked, have all the most eminent
living naturalists and geologists rejected this view of the mutability
of species? It cannot be asserted that organic beings in a state of
nature are subject to no variation; it cannot be proved that the amount
of variation in the course of long ages is a limited quantity; no clear
distinction has been, or can be, drawn between species and well-marked
varieties. It cannot be maintained that species when intercrossed are
invariably sterile, and varieties invariably fertile; or that sterility
is a special endowment and sign of creation. The belief that species
were immutable productions was almost unavoidable as long as the history
of the world was thought to be of short duration; and now that we have
acquired some idea of the lapse of time, we are too apt to assume,
without proof, that the geological record is so perfect that it would
have afforded us plain evidence of the mutation of species, if they had
undergone mutation.
But the chief cause of our natural unwillingness to admit that one
species has given birth to other and distinct species, is that we are
always slow in admitting any great change of which we do not see the
intermediate steps. The difficulty is the same as that felt by so many
geologists, when Lyell first insisted that long lines of inland cliffs
had been formed, and great valleys excavated, by the slow action of the
coast-waves. The mind cannot possibly grasp the full meaning of the term
of a hundred million years; it cannot add up and perceive the full
effects of many slight variations, accumulated during an almost infinite
number of generations.
Although I am fully convinced of the truth of the views given in this
volume under the form of an abstract, I by no means expect to convince
experienced naturalists whose minds are stocked with a multitude of
facts all viewed, during a long course of years, from a point of view
directly opposite to mine. It is so easy to hide our ignorance under
such expressions as the "plan of creation," "unity of design," &c., and
to think that we give an explanation when we only restate a fact. Any
one whose disposition leads him to attach more weight to unexplained
difficulties than to the explanation of a certain number of facts will
certainly reject my theory. A few naturalists, endowed with much
flexibility of mind, and who have already begun to doubt on the
immutability of species, may be influenced by this volume; but I look
with confidence to the future, to young and rising naturalists, who will
be able to view both sides of the question with impartiality. Whoever is
led to believe that species are mutable will do good service by
conscientiously expressing his conviction; for only thus can the load of
prejudice by which this subject is overwhelmed be removed.
Several eminent naturalists have of late published their belief that a
multitude of reputed species in each genus are not real species; but
that other species are real, that is, have been independently created.
This seems to me a strange conclusion to arrive at. They admit that a
multitude of forms, which till lately they themselves thought were
special creations, and which are still thus looked at by the majority of
naturalists, and which consequently have every external characteristic
feature of true species,—they admit that these have been produced
by variation, but they refuse to extend the same view to other and very
slightly different forms. Nevertheless they do not pretend that they can
define, or even conjecture, which are the created forms of life, and
which are those produced by secondary laws. They admit variation as a
vera causa in one case, they arbitrarily reject it in another, without
assigning any distinction in the two cases. The day will come when this
will be given as a curious illustration of the blindness of preconceived
opinion. These authors seem no more startled at a miraculous act of
creation than at an ordinary birth. But do they really believe that at
innumerable periods in the earth's history certain elemental atoms have
been commanded suddenly to flash into living tissues? Do they believe
that at each supposed act of creation one individual or many were
produced? Were all the infinitely numerous kinds of animals and plants
created as eggs or seed, or as full grown? and in the case of mammals,
were they created bearing the false marks of nourishment from the
mother's womb? Although naturalists very properly demand a full
explanation of every difficulty from those who believe in the mutability
of species, on their own side they ignore the whole subject of the first
appearance of species in what they consider reverent silence.
It may be asked how far I extend the doctrine of the modification of
species. The question is difficult to answer, because the more distinct
the forms are which we may consider, by so much the arguments fall away
in force. But some arguments of the greatest weight extend very far. All
the members of whole classes can be connected together by chains of
affinities, and all can be classified on the same principle, in groups
subordinate to groups. Fossil remains sometimes tend to fill up very
wide intervals between existing orders. Organs in a rudimentary
condition plainly show that an early progenitor had the organ in a fully
developed state; and this in some instances necessarily implies an
enormous amount of modification in the descendants. Throughout whole
classes various structures are formed on the same pattern, and at an
embryonic age the species closely resemble each other. Therefore I
cannot doubt that the theory of descent with modification embraces all
the members of the same class. I believe that animals have descended
from at most only four or five progenitors, and plants from an equal or
lesser number.
Analogy would lead me one step further, namely, to the belief that all
animals and plants have descended from some one prototype. But analogy
may be a deceitful guide. Nevertheless all living things have much in
common, in their chemical composition, their germinal vesicles, their
cellular structure, and their laws of growth and reproduction. We see
this even in so trifling a circumstance as that the same poison often
similarly affects plants and animals; or that the poison secreted by the
gall-fly produces monstrous growths on the wild rose or oak-tree.
Therefore I should infer from analogy that probably all the organic
beings which have ever lived on this earth have descended from some one
primordial form, into which life was first breathed.
When the views entertained in this volume on the origin of species, or
when analogous views are generally admitted, we can dimly foresee that
there will be a considerable revolution in natural history. Systematists
will be able to pursue their labours as at present; but they will not be
incessantly haunted by the shadowy doubt whether this or that form be in
essence a species. This I feel sure, and I speak after experience, will
be no slight relief. The endless disputes whether or not some fifty
species of British brambles are true species will cease. Systematists
will have only to decide (not that this will be easy) whether any form
be sufficiently constant and distinct from other forms, to be capable of
definition; and if definable, whether the differences be sufficiently
important to deserve a specific name. This latter point will become a
far more essential consideration than it is at present; for differences,
however slight, between any two forms, if not blended by intermediate
gradations, are looked at by most naturalists as sufficient to raise
both forms to the rank of species. Hereafter we shall be compelled to
acknowledge that the only distinction between species and well-marked
varieties is, that the latter are known, or believed, to be connected at
the present day by intermediate gradations, whereas species were
formerly thus connected. Hence, without quite rejecting the
consideration of the present existence of intermediate gradations
between any two forms, we shall be led to weigh more carefully and to
value higher the actual amount of difference between them. It is quite
possible that forms now generally acknowledged to be merely varieties
may hereafter be thought worthy of specific names, as with the primrose
and cowslip; and in this case scientific and common language will come
into accordance. In short, we shall have to treat species in the same
manner as those naturalists treat genera, who admit that genera are
merely artificial combinations made for convenience. This may not be a
cheering prospect; but we shall at least be freed from the vain search
for the undiscovered and undiscoverable essence of the term species.
The other and more general departments of natural history will rise
greatly in interest. The terms used by naturalists of affinity,
relationship, community of type, paternity, morphology, adaptive
characters, rudimentary and aborted organs, &c., will cease to be
metaphorical, and will have a plain signification. When we no longer
look at an organic being as a savage looks at a ship, as at something
wholly beyond his comprehension; when we regard every production of
nature as one which has had a history; when we contemplate every complex
structure and instinct as the summing up of many contrivances, each
useful to the possessor, nearly in the same way as when we look at any
great mechanical invention as the summing up of the labour, the
experience, the reason, and even the blunders of numerous workmen; when
we thus view each organic being, how far more interesting, I speak from
experience, will the study of natural history become!
A grand and almost untrodden field of inquiry will be opened, on the
causes and laws of variation, on correlation of growth, on the effects
of use and disuse, on the direct action of external conditions, and so
forth. The study of domestic productions will rise immensely in value.
A new variety raised by man will be a far more important and interesting
subject for study than one more species added to the infinitude of
already recorded species. Our classifications will come to be, as far as
they can be so made, genealogies; and will then truly give what may be
called the plan of creation. The rules for classifying will no doubt
become simpler when we have a definite object in view. We possess no
pedigrees or armorial bearings; and we have to discover and trace the
many diverging lines of descent in our natural genealogies, by
characters of any kind which have long been inherited. Rudimentary
organs will speak infallibly with respect to the nature of long-lost
structures. Species and groups of species, which are called aberrant,
and which may fancifully be called living fossils, will aid us in
forming a picture of the ancient forms of life. Embryology will reveal
to us the structure, in some degree obscured, of the prototypes of each
great class.
When we can feel assured that all the individuals of the same species,
and all the closely allied species of most genera, have within a not
very remote period descended from one parent, and have migrated from
some one birthplace; and when we better know the many means of migration,
then, by the light which geology now throws, and will continue to throw,
on former changes of climate and of the level of the land, we shall
surely be enabled to trace in an admirable manner the former migrations
of the inhabitants of the whole world. Even at present, by comparing the
differences of the inhabitants of the sea on the opposite sides of a
continent, and the nature of the various inhabitants of that continent
in relation to their apparent means of immigration, some light can be
thrown on ancient geography.
The noble science of Geology loses glory from the extreme imperfection
of the record. The crust of the earth with its embedded remains must not
be looked at as a well-filled museum, but as a poor collection made at
hazard and at rare intervals. The accumulation of each great
fossiliferous formation will be recognised as having depended on an
unusual concurrence of circumstances, and the blank intervals between
the successive stages as having been of vast duration. But we shall be
able to gauge with some security the duration of these intervals by a
comparison of the preceding and succeeding organic forms. We must be
cautious in attempting to correlate as strictly contemporaneous two
formations, which include few identical species, by the general
succession of their forms of life. As species are produced and
exterminated by slowly acting and still existing causes, and not by
miraculous acts of creation and by catastrophes; and as the most
important of all causes of organic change is one which is almost
independent of altered and perhaps suddenly altered physical conditions,
namely, the mutual relation of organism to organism,—the
improvement of one being entailing the improvement or the extermination
of others; it follows, that the amount of organic change in the fossils
of consecutive formations probably serves as a fair measure of the lapse
of actual time. A number of species, however, keeping in a body might
remain for a long period unchanged, whilst within this same period,
several of these species, by migrating into new countries and coming
into competition with foreign associates, might become modified; so that
we must not overrate the accuracy of organic change as a measure of time.
During early periods of the earth's history, when the forms of life were
probably fewer and simpler, the rate of change was probably slower; and
at the first dawn of life, when very few forms of the simplest structure
existed, the rate of change may have been slow in an extreme degree. The
whole history of the world, as at present known, although of a length
quite incomprehensible by us, will hereafter be recognised as a mere
fragment of time, compared with the ages which have elapsed since the
first creature, the progenitor of innumerable extinct and living
descendants, was created.
In the distant future I see open fields for far more important
researches. Psychology will be based on a new foundation, that of the
necessary acquirement of each mental power and capacity by gradation.
Light will be thrown on the origin of man and his history.
Authors of the highest eminence seem to be fully satisfied with the view
that each species has been independently created. To my mind it accords
better with what we know of the laws impressed on matter by the Creator,
that the production and extinction of the past and present inhabitants
of the world should have been due to secondary causes, like those
determining the birth and death of the individual. When I view all
beings not as special creations, but as the lineal descendants of some
few beings which lived long before the first bed of the Silurian system
was deposited, they seem to me to become ennobled. Judging from the past,
we may safely infer that not one living species will transmit its
unaltered likeness to a distant futurity. And of the species now living
very few will transmit progeny of any kind to a far distant futurity;
for the manner in which all organic beings are grouped, shows that the
greater number of species of each genus, and all the species of many
genera, have left no descendants, but have become utterly extinct. We
can so far take a prophetic glance into futurity as to foretel that it
will be the common and widely-spread species, belonging to the larger
and dominant groups, which will ultimately prevail and procreate new and
dominant species. As all the living forms of life are the lineal
descendants of those which lived long before the Silurian epoch, we may
feel certain that the ordinary succession by generation has never once
been broken, and that no cataclysm has desolated the whole world. Hence
we may look with some confidence to a secure future of equally
inappreciable length. And as natural selection works solely by and for
the good of each being, all corporeal and mental endowments will tend to
progress towards perfection.
It is interesting to contemplate an entangled bank, clothed with many
plants of many kinds, with birds singing on the bushes, with various
insects flitting about, and with worms crawling through the damp earth,
and to reflect that these elaborately constructed forms, so different
from each other, and dependent on each other in so complex a manner,
have all been produced by laws acting around us. These laws, taken in
the largest sense, being Growth with Reproduction; Inheritance which is
almost implied by reproduction; Variability from the indirect and
direct action of the external conditions of life, and from use and
disuse; a Ratio of Increase so high as to lead to a Struggle for Life,
and as a consequence to Natural Selection, entailing Divergence of
Character and the Extinction of less-improved forms. Thus, from the war
of nature, from famine and death, the most exalted object which we are
capable of conceiving, namely, the production of the higher animals,
directly follows. There is grandeur in this view of life, with its
several powers, having been originally breathed into a few forms or into
one; and that, whilst this planet has gone cycling on according to the
fixed law of gravity, from so simple a beginning endless forms most
beautiful and most wonderful have been, and are being, evolved.
[ Charles Darwin,
On
the Origin Of Species: A Facsimile of the First Edition,
Cambridge, Massachusetts: Harvard University Press, 1964, pp. 459-490. ]
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