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 inevit
ably
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