... for human beings, rational and moral though they may be, are
still organisms; and man can in no wise alter or annul those
deep-lying facts which nature has throughout the ages been weaving
into the tissue of life.
The one animal of whose feelings I know anything definite and at
first hand, is myself. Of course, I believe in the feelings of
others; but when we come to very lowly organisms, we really do not
know whether they have feelings or not, or, if they do, to what
extent they feel.
The wonderful thing about this process is the power of the
fertilized ovum, produced by the union of two minute cells from
different parents, to develop into the likeness of these parents.
This likeness, however, though it extends to minute particulars, is
not absolute. The offspring is not exactly like either parent, nor
does it present a precise mean between the characters of the two
parents. There is always some amount of individual variability, the
effects of which, as we shall hereafter see, are of wide
importance. We are wont to say that these phenomena, the
transmission of parental characteristics, together with a margin of
difference, are due to heredity with variation. But this merely
names the facts. How the special reproductive cells have acquired
the secret of developing along special lines, and reproducing, with
a margin of variability, the likeness of the organisms which
produced them, is a matter concerning which we can at present only
make more or less plausible guesses.
The law of heredity may be regarded as that of persistence
exemplified in a series of organic generations. When, as in the
amœba and some other protozoa, reproduction is by simple fission,
two quite similar organisms being thus produced, there would seem
to be no reason why (modifications by surrounding circumstances
being disregarded) hereditary persistence should not continue
indefinitely.
Where, however, reproduction is effected by the detachment of a
single cell from a many-celled organism, hereditary persistence
will be complete only on the condition that this reproductive cell
is in some way in direct continuity with the cells of the parent
organism or the cell from which that parent organism itself
developed. And where, in the higher animals, two cells from two
somewhat different parents coalesce to give origin to a new
individual, the phenomena of hereditary persistence are still
further complicated by the blending of characters handed on in the
ovum and the sperm; still further complication being, perhaps,
produced by the emergence in the offspring of characters latent in
the parent, but derived from an earlier ancestor. And if characters
acquired by the parents in the course of their individual life be
handed on to the offspring, yet further complication will be thus
introduced. It is no matter for surprise, therefore, that,
notwithstanding the law of hereditary persistence, variations
should occur in the offspring of animals.
But even here, without discussing their origin, we must establish
the fact that variations do actually occur. Variations may be of
many kinds and in different directions. In colour, in size, in the
relative development of different parts, in complexity, in habits,
and in mental endowments, organisms or their organs may vary.
Observers of mammals, of birds, and of insects are well aware that
colour is a variable characteristic.
We must next revert to the fact to which attention was drawn in the
last chapter, that every species is tending, through natural
generation, to increase in numbers. Even in the case of the
slow-breeding elephant, the numbers tend to increase threefold in
each generation; for a single pair of elephants give birth to three
pairs of young. In many animals the tendency is to increase ten,
twenty, or thirtyfold in every generation; while among fishes,
amphibians, and great numbers of the lower organisms, the tendency
is to multiply by a hundredfold, a thousandfold, or even in some
cases ten thousandfold. But, as before noticed, this is only a
tendency. The law of increase is a law of one factor in life's
phenomena, the reproductive factor. In any area, the conditions of
which are not undergoing change, the numbers of the species which
constitute its fauna remain tolerably constant. They are not
actually increasing in geometrical progression. There is literally
no room for such increase. The large birth-rate of the constituent
species is accompanied by a proportionate death-rate, or else the
tendency is kept in check by the prevention of certain individuals
from mating and bearing young. Now, the high death-rate is, to a
large extent among the lower organisms and in a less degree among
higher animals, the result of indiscriminate destruction.
Those which are thus destroyed are nowise either better or worse
than those which escape.
active, free-swimming coral embryos are set free in immense
numbers. Presently they settle down for life. Some settle on a
muddy bottom, others in too great a depth of water. These are
destroyed. The few which take up a favourable position survive. But
they are no better than their less fortunate neighbours. The
destruction is indiscriminate. So, too, among fishes and the many
marine forms which produce a great number of fertilized eggs giving
rise to embryos that are from an early period free- swimming and
self-supporting. Such embryos are decimated by a destruction which
is quite indiscriminate.
Even making all due allowance, however, for this indiscriminate
destruction—which is to a large extent avoided by those higher
creatures which foster their young—there remain more individuals
than suffice to keep up the normal numbers of the species. Among
these there arises a struggle for existence, and hence what Darwin
named natural selection.