"Species originated by means of natural selection, or through the
preservation of the favoured races in the struggle for life." To render
this thesis intelligible, it is necessary to interpret its terms. In the
first place, what is a species? The question is a simple one, but the right
answer to it is hard to find, even if we appeal to those who should know
most about it. It is all those animals or plants which have descended from
a single pair of parents; it is the smallest distinctly definable group of
living organisms; it is an eternal and immutable entity; it is a mere
abstraction of the human intellect having no existence in nature. Such are
a few of the significations attached to this simple word which may be
culled from authoritative sources; and if, leaving terms and theoretical
subtleties aside, we turn to facts and endeavour to gather a meaning for
ourselves, by studying the things to which, in practice, the name of
species is applied, it profits us little. For practice varies as much as
theory. Let two botanists or two zoologists examine and describe the
productions of a country, and one will pretty certainly disagree with the
other as to the number, limits, and definitions of the species into which
he groups the very same things. In these islands, we are in the habit of
regarding mankind as of one species, but a fortnight's steam will land us
in a country where divines and savants, for once in agreement, vie with one
another in loudness of assertion, if not in cogency of proof, that men are
of different species; and, more particularly, that the species negro is so
distinct from our own that the Ten Commandments have actually no reference
to him. Even in the calm region of entomology, where, if anywhere in this
sinful world, passion and prejudice should fail to stir the mind, one
learned coleopterist will fill ten attractive volumes with descriptions of
species of beetles, nine-tenths of which are immediately declared by his
brother beetle-mongers to be no species at all.

The truth is that the number of distinguishable living creatures almost