primordial differences between species and species at their beginning have
not been effaced, nor largely obscured, by blending through variation.
Consequently, whenever two reputed species are found to blend in Nature
through a series of intermediate forms, community of origin is inferred,
and all the forms, however diverse, are held to belong to one species.
Moreover, since bisexuality is the rule in Nature (which is practically
carried out, in the long-run, far more generally than has been suspected),
and the heritable qualities of two distinct individuals are mingled in the
offspring, it is supposed that the general sterility of hybrid progeny
interposes an effectual barrier against the blending of the original
species by crossing.

From this generally-accepted view the well-known theory of Agassiz and the
recent one of Darwin diverge in exactly opposite directions.

That of Agassiz differs fundamentally from the ordinary view only in this,
that it discards the idea of a common descent as the real bond of union
among the individuals of a species, and also the idea of a local
origin--supposing, instead, that each species originated simultaneously,
generally speaking, over the whole geographical area it now occupies or has
occupied, and in perhaps as many individuals as it numbered at any
subsequent period.

Mr. Darwin, on the other hand, holds the orthodox view of the descent of
all the individuals of a species not only from a local birthplace, but from
a single ancestor or pair; and that each species has extended and
established itself, through natural agencies, wherever it could; so that
the actual geographical distribution of any species is by no means a
primordial arrangement, but a natural result. He goes farther, and this
volume is a protracted argument intended to prove that the species we