s 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 the botanist or the zoologist 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
surpasses imagination. At least a hundred thousand such kinds of insects
alone have been described and may be identified in collections, and the
number of separable kinds of living things is under estimated at half a
million. Seeing that most of these obvious kinds have their accidental
varieties, and that they often shade into others by imperceptible
degrees, it may well be imagined that the task of distinguishing between
what is permanent and what fleeting, what is a species and what a mere
variety, is sufficiently formidable.

But is it not possible to apply a test whereby a true species may be
known from a mere variet