llustrations will serve to show that natural selection always
works primarily for the life-interests of the species--and, indeed, only
works for those of the individual at all in so far as the latter happen
to coincide with the former. Or, otherwise stated, the object of natural
selection is always that of producing and maintaining specific types in
the highest degree of efficiency, no matter what may become of the
constituent individuals. Which is a striking republication by Science of
a general truth previously stated by Poetry:--

So careful of the type she seems,
So careless of the single life.

Tennyson thus noted the fact, and a few years later Darwin supplied the
explanation.

But of course in many, if not in the majority of cases, anything that
adds to the life-sustaining power of the single life thereby ministers
also to the life-sustaining power of the type; and thus we can
understand why all mechanisms and instincts which minister to the single
life have been developed--namely, because the life of the species is
made up of the lives of all its constituent individuals. It is only
where the interests of the one clash with those of the other that
natural selection works against the individual. So long as the interests
are coincident, it works in favour of both.

Natural selection, then, is a theory which seeks to explain by natural
causes the occurrence of every kind of adaptation which is to be met
with in organic nature, on the assumption that adaptations of every kind
have primary reference to the preservation of species, and therefore
also, as a general rule, to the preservation of their constituent
individuals. And from this it follows that where it is for the benefit
of a species to change its type, natural selection will effect that
change, thus leading to a specific transmutation, or the evolution of a
new species. In such cases the old species may or may not become
extinct. If the transmutation affects the species as a whole, or
throughout its entire range, of course _that_ particular type becomes
extinct, although it does so by becoming changed into a still more
suitable type in the course of successive generations. If, on the other
hand, the transmutation affects only a part of the original species, or
not throughout its entire range, then the other parts of that species
may survive for any number of ages as they originally were. In the one
case there is a ladder-like transmutation of spec