If it be said "So much for the _fact_ of evolution, but what of the
_factors_?" the answer is not easy. For not only is the problem the
greatest of all scientific problems, but the inquiry is still very
young. The scientific study of evolution practically dates from the
publication of _The Origin of Species_ in 1859.
Heritable novelties or variations often crop up in living creatures, and
these form the raw material of evolution. These variations are the
outcome of expression of changes in the germ-cells that develop into
organisms. But why should there be changes in the constitution of the
germ-cells? Perhaps because the living material is very complex and
inherently liable to change; perhaps because it is the vehicle of a
multitude of hereditary items among which there are very likely to be
reshufflings or rearrangements; perhaps because the germ-cells have very
changeful surroundings (the blood, the body-cavity fluid, the
sea-water); perhaps because deeply saturating outside influences, such
as change of climate and habitat, penetrate through the body to its
germ-cells and provoke them to vary. But we must be patient with the
wearisome reiteration of "perhaps." Moreover, every many-celled organism
reproduced in the usual way, arises from an egg-cell fertilised by a
sperm-cell, and the changes involved in and preparatory to this
fertilisation may make new permutations and combinations of the living
items and hereditary qualities not only possible but necessary. It is
something like shuffling a pack of cards, but the cards are living. As
to the changes wrought on the body during its lifetime by peculiarities
in nurture, habits, and surroundings, these dents or modifications are
often very important for the individual, but it does not follow that
they are directly important for the race, since it is not certain that
they are transmissible.
Given a crop of variations or new departures or mutations, whatever the
inborn novelties may be called, we have then to inquire how these are
sifted. The sifting, which means the elimination of the relatively less
fit variations and the selection of the relatively more fit, effected in
many different ways in the course of the struggle for existence. The
organism plays its new card in the game of life, and the consequences
may determine survival. The relatively less fit to given conditions
will tend to be eliminated, while the relatively more fit will tend to
survive. If the variations are hereditary and reappear, perhaps
increased in amount, generation after generation, and if the process of
sifting continue consistently, the result will be the evolution of the
species. The sifting process may be helped by various forms of
"isolation" which lessen the range of free intercrossing between members
of a species, e.g. by geographical barriers. Interbreeding of similar
forms tends to make a stable stock; out-breeding among dissimilars tends
to promote variability. But for an outline like this it is enough to
suggest the general method of organic evolution: Throughout the ages
organisms have been making tentatives--new departures of varying
magnitude--and these tentatives have been tested. The method is that of
testing all things and holding fast that which is good.