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.
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.
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.
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.