Although, as we have seen, a succession of great writers and thinkers
had for more than half a century shown the necessity for some process
of evolution as the only rational or intelligible mode of origin
of existing species of animals and plants, as well as of the whole
physical universe, yet these views were by no means generally accepted
by the educated classes, while few bodies of students were less
influenced by them than zoologists and botanists, generally known as
naturalists.
Now, Darwin wrote especially for these classes, and no one knew better
than he did their great prejudice on this matter. Not only had such
men as Sir Charles Lyell and Sir John Herschel expressed themselves
strongly against all theories of the transmutation of species, but
the universal contempt and indignation of naturalists as well as
theologians against _The Vestiges of Creation_, published anonymously
a few years earlier, and giving a most temperate and even religious
exposition of the general arguments for the universality of evolution,
showed what any one might expect who advocated and attempted to
demonstrate a similar theory. This accounts for Darwin writing to Sir
Joseph Hooker, in 1844, of his being “almost convinced that species are
not (it is like confessing a murder) immutable,” and again, in 1845, to
the Rev. L. Blomefield, that he now saw the way in which new varieties
become exquisitely adapted to the external conditions of life and to
other surrounding beings, and he adds: “I am a bold man to lay myself
open to being thought a complete fool, and a most deliberate one.”
It is only by a consideration of the frame of mind of even advanced
thinkers at the time Darwin was preparing his work, and remembering
how small was the effect which had been produced by Buffon, Goethe,
Lamarck, the author of _Vestiges of Creation_, and the earlier writings
of Herbert Spencer, that we can adequately realize the marvellous work
that he accomplished. Let us now briefly consider the essential nature
of this new theory, which in a few brief years became the established
belief of the great majority of the students of nature, and which also
gave a new interest in nature to the whole thinking world.
The theory of natural selection is founded upon a few groups of
thoroughly ascertained and universally admitted facts, with the direct
and necessary results of those facts.
The first group of facts consists of the great powers of increase
of all organisms and the circumstance that, notwithstanding this
great yearly increase, the actual population of each species remains
stationary, there being no permanent increase. Now, these two facts
were recognized by Buffon, but though, of course, known to all
subsequent writers, were fully appreciated or thought out to their
logical results by none of them. Lamarck, so far as I can ascertain,
took no notice of them whatever. Darwin has given illustrations of
these facts in Chapter IV. of the _Origin of Species_, and I have added
others in the second chapter of my _Darwinism_. That the population
of each species remains stationary, with, of course, considerable
fluctuations, is both a matter of observation and of reasoning. The
powers of increase of all creatures are so great that if there is in
any country room and food for a larger number of any species they will
be produced in a year or two. It is impossible, therefore, to believe
that, in a state of nature, where all kinds of animals and plants have
lived together as they best could for thousands of years, there can be
any important difference in their numbers from year to year or from
century to century.
Now, it is as a consequence of these two indisputable facts that the
struggle for existence necessarily results. For if every year each pair
of animals or each plant produces only ten young animals or plants,
and this is very far below the average, and if the adult life of these
is taken at ten years, again below the average of the higher plants
and animals, then, unless some of the parents die, the whole of the
offspring must die off every year; or, in other words, only as many
young can survive as are necessary to replace the old ones that die.
Hence the deaths must always (on the average and in the long run) equal
the births. This terrible yearly destruction is an absolutely certain
fact, as well as an inevitable result of the two preceding facts, and
it is said to be due to the struggle for existence. This struggle
is manifold in its nature. Individuals of the same species struggle
together for food, for light, for moisture; they struggle also against
other species having the same wants; they struggle against every kind
of enemy, from parasitic worms and insects up to carnivorous animals;
and there is a continual struggle with the forces of nature—frosts,
rains, droughts, floods, and tempests.
These varied causes of destruction may be seen constantly at work
by any one who looks for them. They act from the moment of birth,
being more especially destructive to the young; and, as only one in
ten or fifty or a thousand (according to the rate of increase of
the particular species) can possibly come to the full breeding age,
we feel compelled to ask ourselves: What determines the nine or the
forty-nine or the nine hundred and ninety-nine, as the case may be,
which die, and the one which survives? Darwin calls this process of
extermination one of “natural selection”—that is, by this process
nature weeds out the weak, the unhealthy, the unadapted, the imperfect
in any way. Of course, what may be called chance or accident produces
many deaths of individuals otherwise well fitted to live, but if we
think of the process going on day by day and year by year till only
one in a hundred of those born in a given area are left alive, it is
impossible to suppose that the _one_ which has passed through all
the dangers and risks which have been fatal to, say, his ninety-nine
relations was not, in all the faculties and qualities essential to the
continuance of the race, decidedly better organized than the bulk of
those which succumbed. Herbert Spencer calls the process the “survival
of the fittest,” and though the term may not be strictly accurate in
the case of any one species in any one year, yet when we consider that
the struggle is going on _every_ year, during the whole duration of
each species, we cannot doubt that, on the whole, and in the long run,
those which survive are among the fittest. The struggle is so severe,
so incessant, that the smallest defect in any sense organ, any physical
weakness, any imperfection in constitution, will almost certainly, at
one time or another, be fatal.
This continual weeding out of the less fit, in every generation, and
with exceptional severity in recurring adverse seasons, will produce
two distinct effects, which require to be clearly distinguished. The
first is the preservation of each species in the highest state of
adaptation to the conditions of its existence; and, therefore, so
long as these conditions remained unchanged, the effect of natural
selection is to keep each well-adapted species also unchanged. The
second effect is produced whenever the conditions vary, when, taking
advantage of the variations continually occurring in all well-adapted
and therefore populous species, the same process will slowly but surely
bring about complete adaptation to the new conditions. And here another
fact—the normal variability of all populous or dominant species, which
is seldom realized except by those who have largely and minutely
compared the individuals of many species in a state of nature—comes
into play. There are some writers who admit all the preceding facts
and reasoning, so far as the action of natural selection in weeding
out the unfit and thus keeping every species in the highest state of
efficiency is concerned, but who deny that it can modify them in such a
way as to adapt them to new conditions, because they allege that “the
right variations will not always occur at the right time.” This seems
a strong and real objection to many of their readers, but to those
who have studied the variability of species in nature, it is a mere
verbal difficulty dependent on ignorance of the actual facts. A brief
statement of the facts must therefore be given.
Of late years, and chiefly since Darwin’s works were written, the
variability of animals and plants in a state of nature has been
carefully studied, by actual comparison and measurement of scores,
hundreds, and even thousands of individuals of many common, that is,
abundant and widely distributed species; and it is found that in almost
every case they vary greatly, and, what is still more important, that
every organ and every appendage varies independently and to a large
amount. Some of the best known of these facts of variation are adduced
in my _Darwinism_, and are illustrated by numerous diagrams, and much
more extensive series have since been examined, always with the same
general result. By large variability is meant a variation of from ten
to twenty-five per cent. on each side of the mean size, this amount
of variation occurring in at least five or ten per cent. of the whole
number of individuals, and in every organ or part as yet examined,
external or internal.
Now, as the weeding-out process is so severe, only from one in ten to
one in a hundred of those born surviving to produce young, the above
proportion of variations affords ample scope for the selection of
any variation needed in order to modify the species so as to bring it
into harmony with new or changing conditions. And this will be the
more easy and certain if we consider how slowly land-surfaces and
climates undergo permanent changes; and these are certainly the kind of
changes that initiate and compel alterations, first, perhaps, in the
distribution, and afterwards in the structure and habits of species.
It follows, therefore, as an absolutely necessary conclusion from the
facts, if natural selection can and does keep each continually varying
species in close adaptation to an unchanging environment, that it
preserves the fixity of its mean or average condition, and almost every
objector admits this. Then, given a slowly changing environment, the
same power must inevitably bring about whatever corresponding change is
needed for the well-being and permanent survival of the various species
which are subjected to those changed conditions.
I shall not add here a further consideration of the objections and
difficulties alleged by critics of the theory. All of these have,
I believe, been fully answered either by Darwin or myself, many of
the most recent having been discussed in review articles. Suffice
it to say here that this theory of natural selection—meaning the
elimination of the least fit, and therefore the ultimate “survival of
the fittest”—has furnished a rational and precise explanation of the
means of adaptation of all existing organisms to their conditions,
and therefore of their transformation from the series of distinct but
allied species which occupied the earth at some preceding epoch. In
this sense it has actually demonstrated the “origin of species,” and,
by carrying back this process step by step into earlier and earlier
geological times, we are able mentally to follow out the evolution of
all forms of life from one or a few primordial forms. Natural selection
has thus supplied that motive power of change and adaptation that was
wanting in all earlier attempts at explanation, and this has led to its
very general acceptance both by naturalists and by the great majority
of thinkers and men of science.
The brief sketch now given of the progress of human thought on the
questions of the fact and the mode of the evolution of the material
universe indicates how great has been the progress during the
nineteenth as compared with all preceding centuries.
Although the philosophical writers of classical times obtained a few
glimpses of the action of law in nature regulating its successive
changes, nothing satisfactory could be effected till the actual facts
had been better ascertained by the whole body of workers who, during
the last five centuries, have penetrated ever more and more deeply into
nature’s mysteries and laws. By their labors we became possessed of
such a body of carefully observed facts that, towards the end of the
eighteenth century, such thinkers as Laplace and Hutton were enabled
to give us the first rudiments of theories of evolution as applied to
the solar system and the earth’s crust, both of which have been greatly
developed and rendered more secure during the century just passed away.
In like manner Buffon and Goethe may be said to have started the idea
of organic evolution, more systematically treated a little later by
Lamarck, but still without any discovery of laws adequate to produce
the results we see everywhere in nature. The subject then languished,
till, after twenty years of observation and research, Charles Darwin
produced a work which at once satisfied many thinkers that the
long-desired clew had been discovered. Its acceptance by almost the
whole scientific world soon followed: it threw new light on almost
every branch of research, and it will probably take its place, in the
opinion of future generations, as the crowning achievement of the
nineteenth century.