On the Origin of Species by Means of Natural Selection
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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.
As 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 large 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 ichneumonidae 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.
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