way, the generally-received definition of species, and in sweeping away the
ground of their objective existence in Nature. The orthodox conception of
species is that of lineal descent: all the descendants of a common parent,
and no other, constitute a species; they have a certain identity because of
their descent, by which they are supposed to be recognizable. So
naturalists had a distinct idea of what they meant by the term species, and
a practical rule, which was hardly the less useful because difficult to
apply in many cases, and because its application was indirect: that is, the
community of origin had to be inferred from the likeness; such degree of
similarity, and such only, being held to be con-specific as could be shown
or reasonably inferred to be compatible with a common origin. And the usual
concurrence of the whole body of naturalists (having the same data before
them) as to what forms are species attests the value of the rule, and also
indicates some real foundation for it in Nature. But if species were created
in numberless individuals over broad spaces of territory, these individuals
are connected only in idea, and species differ from varieties on the one
hand, and from genera, tribes, etc., on the other, only in degree; and no
obvious natural reason remains for fixing upon this or that degree as
specific, at least no natural standard, by which the opinions of different
naturalists may be correlated. Species upon this view are enduring, but
subjective and ideal. Any three or more of the human races, for example, are
species or not species, according to the bent of the naturalist's mind.
Darwin's theory brings us the other way to the same result. In his view,
not only all the individuals of a species are descendants of a common
parent, but of all the related species also. Affinity, relationship, all
the terms which naturalists use figuratively to express an underived,
unexplained resemblance among species, have a literal meaning upon Darwin's
system, which they little suspected, namely, that of inheritance. Varieties
are the latest offshoots of the genealogical tree in "an unlineal" order;
species, those of an earlier date, but of no definite distinction; genera,