may
be a deceitful guide. Nevertheless all living things have much in common,
in their chemical composition, their germinal vesicles, their cellular
structure, and their laws of growth and reproduction. We see this even in
so trifling a circumstance as that the same poison often similarly affects
plants and animals; or that the poison secreted by the gall-fly produces
monstrous growths on the wild rose or oak-tree. Therefore I should infer
from analogy that probably all the organic beings which have ever lived on
this earth have descended from some one primordial form, into which life
was first breathed by the Creator.

When the views advanced by me in this volume, and by Mr. Wallace in the
Linnean Journal, or when analogous views on the origin of species are
generally {485} admitted, we can dimly foresee that there will be a
considerable revolution in natural history. Systematists will be able to
pursue their labours as at present; but they will not be incessantly
haunted by the shadowy doubt whether this or that form be in essence a
species. This I feel sure, and I speak after experience, will be no slight
relief. The endless disputes whether or not some fifty species of British
brambles are true species will cease. Systematists will have only to decide
(not that this will be easy) whether any form be sufficiently constant and
distinct from other forms, to be capable of definition; and if definable,
whether the differences be sufficiently important to deserve a specific
name. This latter point will become a far more essential consideration than
it is at present; for differences, however slight, between any two forms,
if not blended by intermediate gradations, are looked at by most
naturalists as sufficient to raise both forms to the rank of species.
Hereafter we shall be compelled to acknowledge that the only distinction
between species and well-marked varieties is, that the latter are known, or
believed, to be connected at the present day by intermediate gradations,
whereas species were formerly thus connected. Hence, without rejecting the
consideration of the present existence of intermediate gradations between
any two forms, we shall be led to weigh more carefully and to value higher
the actual amount of difference between them. It is quite possible that
forms now generally acknowledged to be merely varieties may hereafter be
thought worthy of specific names, as with the primrose and cowslip; and in
this case s