Many and diverse were the discoveries made by Charles Darwin in the
course of a long and strenuous life, but none of them has had so
far-reaching an influence on the science and thought of his time as the
theory of selection. I do not believe that the theory of evolution would
have made its way so easily and so quickly after Darwin took up the
cudgels in favour of it, if he had not been able to support it by a
principle which was capable of solving, in a simple manner, the greatest
riddle that living nature presents to us,--I mean the purposiveness
of every living form relative to the conditions of its life and its
marvellously exact adaptation to these.
Everyone knows that Darwin was not alone in discovering the principle
of selection, and that the same idea occurred simultaneously and
independently to Alfred Russel Wallace. At the memorable meeting of the
Linnean Society on 1st July, 1858, two papers were read (communicated by
Lyell and Hooker) both setting forth the same idea of selection. One
was written by Charles Darwin in Kent, the other by Alfred Wallace
in Ternate, in the Malay Archipelago. It was a splendid proof of
the magnanimity of these two investigators, that they thus, in all
friendliness and without envy, united in laying their ideas before a
scientific tribunal: their names will always shine side by side as two
of the brightest stars in the scientific sky.
But it is with Charles Darwin that I am here chiefly concerned, since
this paper is intended to aid in the commemoration of the hundredth
anniversary of his birth.
The idea of selection set forth by the two naturalists was at the time
absolutely new, but it was also so simple that Huxley could say of it
later, "How extremely stupid not to have thought of that." As Darwin was
led to the general doctrine of descent, not through the labours of
his predecessors in the early years of the century, but by his own
observations, so it was in regard to the principle of selection. He was
struck by the innumerable cases of adaptation, as, for instance, that
of the woodpeckers and tree-frogs to climbing, or the hooks and
feather-like appendages of seeds, which aid in the distribution of
plants, and he said to himself that an explanation of adaptations was
the first thing to be sought for in attempting to formulate a theory of
evolution.
But since adaptations point to CHANGES which have been undergone by the
ancestral forms of existing species, it is necessary, first of all, to
inquire how far species in general are VARIABLE. Thus Darwin's attention
was directed in the first place to the phenomenon of variability, and
the use man has made of this, from very early times, in the breeding of
his domesticated animals and cultivated plants. He inquired carefully
how breeders set to work, when they wished to modify the structure and
appearance of a species to their own ends, and it was soon clear to him
that SELECTION FOR BREEDING PURPOSES played the chief part.
But how was it possible that such processes should occur in free
nature? Who is here the breeder, making the selection, choosing out one
individual to bring forth offspring and rejecting others? That was the
problem that for a long time remained a riddle to him.
Darwin himself relates how illumination suddenly came to him. He had
been reading, for his own pleasure, Malthus' book on Population, and, as
he had long known from numerous observations, that every species gives
rise to many more descendants than ever attain to maturity, and that,
therefore, the greater number of the descendants of a species perish
without reproducing, the idea came to him that the decision as to which
member of a species was to perish, and which was to attain to maturity
and reproduction might not be a matter of chance, but might be
determined by the constitution of the individuals themselves, according
as they were more or less fitted for survival. With this idea the
foundation of the theory of selection was laid.
In ARTIFICIAL SELECTION the breeder chooses out for pairing only such
individuals as possess the character desired by him in a somewhat higher
degree than the rest of the race. Some of the descendants inherit this
character, often in a still higher degree, and if this method be pursued
throughout several generations, the race is transformed in respect of
that particular character.
NATURAL SELECTION depends on the same three factors as ARTIFICIAL
SELECTION: on VARIABILITY, INHERITANCE, and SELECTION FOR BREEDING, but
this last is here carried out not by a breeder but by what Darwin called
the "struggle for existence." This last factor is one of the special
features of the Darwinian conception of nature. That there are
carnivorous animals which take heavy toll in every generation of the
progeny of the animals on which they prey, and that there are herbivores
which decimate the plants in every generation had long been known, but
it is only since Darwin's time that sufficient attention has been paid
to the facts that, in addition to this regular destruction, there exists
between the members of a species a keen competition for space and food,
which limits multiplication, and that numerous individuals of each
species perish because of unfavourable climatic conditions. The
"struggle for existence," which Darwin regarded as taking the place
of the human breeder in free nature, is not a direct struggle between
carnivores and their prey, but is the assumed competition for survival
between individuals OF THE SAME species, of which, on an average, only
those survive to reproduce which have the greatest power of resistance,
while the others, less favourably constituted, perish early. This
struggle is so keen, that, within a limited area, where the conditions
of life have long remained unchanged, of every species, whatever be the
degree of fertility, only two, ON AN AVERAGE, of the descendants of each
pair survive; the others succumb either to enemies, or to disadvantages
of climate, or to accident. A high degree of fertility is thus not an
indication of the special success of a species, but of the numerous
dangers that have attended its evolution. Of the six young brought forth
by a pair of elephants in the course of their lives only two survive in
a given area; similarly, of the millions of eggs which two thread-worms
leave behind them only two survive. It is thus possible to estimate the
dangers which threaten a species by its ratio of elimination, or, since
this cannot be done directly, by its fertility.
Although a great number of the descendants of each generation fall
victims to accident, among those that remain it is still the greater
or lesser fitness of the organism that determines the "selection
for breeding purposes," and it would be incomprehensible if, in this
competition, it were not ultimately, that is, on an average, the best
equipped which survive, in the sense of living long enough to reproduce.
Thus the principle of natural selection is THE SELECTION OF THE BEST FOR
REPRODUCTION, whether the "best" refers to the whole constitution,
to one or more parts of the organism, or to one or more stages of
development. Every organ, every part, every character of an animal,
fertility and intelligence included, must be improved in this manner,
and be gradually brought up in the course of generations to its highest
attainable state of perfection. And not only may improvement of parts
be brought about in this way, but new parts and organs may arise,
since, through the slow and minute steps of individual or "fluctuating"
variations, a part may be added here or dropped out there, and thus
something new is produced.
The principle of selection solved the riddle as to how what was
purposive could conceivably be brought about without the intervention
of a directing power, the riddle which animate nature presents to our
intelligence at every turn, and in face of which the mind of a Kant
could find no way out, for he regarded a solution of it as not to be
hoped for. For, even if we were to assume an evolutionary force that is
continually transforming the most primitive and the simplest forms of
life into ever higher forms, and the homogeneity of primitive times into
the infinite variety of the present, we should still be unable to infer
from this alone how each of the numberless forms adapted to particular
conditions of life should have appeared PRECISELY AT THE RIGHT MOMENT
IN THE HISTORY OF THE EARTH to which their adaptations were appropriate,
and precisely at the proper place in which all the conditions of life to
which they were adapted occurred: the humming-birds at the same time as
the flowers; the trichina at the same time as the pig; the bark-coloured
moth at the same time as the oak, and the wasp-like moth at the same
time as the wasp which protects it. Without processes of selection we
should be obliged to assume a "pre-established harmony" after the famous
Leibnitzian model, by means of which the clock of the evolution of
organisms is so regulated as to strike in exact synchronism with that of
the history of the earth! All forms of life are strictly adapted to the
conditions of their life, and can persist under these conditions alone.
There must therefore be an intrinsic connection between the conditions
and the structural adaptations of the organism, and, SINCE THE
CONDITIONS OF LIFE CANNOT BE DETERMINED BY THE ANIMAL ITSELF, THE
ADAPTATIONS MUST BE CALLED FORTH BY THE CONDITIONS.
The selection theory teaches us how this is conceivable, since it
enables us to understand that there is a continual production of what is
non-purposive as well as of what is purposive, but the purposive alone
survives, while the non-purposive perishes in the very act of arising.
This is the old wisdom taught long ago by Empedocles.