can create nothing, it can only reject. It is true that it cannot
create either the living substance or the variations of it; both must be
given. But in rejecting one thing it preserves another, intensifies
it, combines it, and in this way CREATES what is new. EVERYTHING in
organisms depends on adaptation; that is to say, everything must be
admitted through the narrow door of selection, otherwise it can take
no part in the building up of the whole. But, it is asked, what of the
direct effect of external conditions, temperature, nutrition, climate
and the like? Undoubtedly these can give rise to variations, but they
too must pass through the door of selection, and if they cannot do this
they are rejected, eliminated from the constitution of the species.

It may, perhaps, be objected that such external influences are often of
a compelling power, and that every animal MUST submit to them, and that
thus selection has no choice and can neither select nor reject. There
may be such cases; let us assume for instance that the effect of the
cold of the Arctic regions was to make all the mammals become black; the
result would be that they would all be eliminated by selection, and
that no mammals would be able to live there at all. But in most cases a
certain percentage of animals resists these strong influences, and
thus selection secures a foothold on which to work, eliminating the
unfavourable variation, and establishing a useful colouring, consistent
with what is required for the maintenance of the species.

Everything depends upon adaptation! We have spoken much of adaptation
in colouring, in connection with the examples brought into prominence by
Darwin, because these are conspicuous, easily verified, and at the same
time convincing for the theory of selection. But is it only desert and
polar animals whose colouring is determined through adaptation? Or the
leaf-butterflies, and the mimetic species, or the terrifying markings,
and "warning-colours" and a thousand other kinds of sympathetic
colouring? It is, indeed, never the colouring alone which makes up
the adaptation; the structure of the animal plays a part, often a very
essential part, in the protective disguise, and thus MANY variations may
cooperate towards ONE common end. And it is to be noted that it is by
no means only external parts that are changed; internal parts are ALWAYS
modified at the same time--for instance, the delicate elements of the
nervous system on whi