embryonic age the species closely resemble each other. Therefore I
cannot doubt that the theory of descent with modification
embraces all the members of the same class. I believe that animals have
descended from at most only four or five progenitors, and plants from
an equal or lesser number.
Analogy would lead me one step further, namely, to the belief that all
animals and plants have descended from some one prototype. But analogy
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.
When the views entertained in this volume on the origin of species, or
when analogous views are generally 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 quite rejecting the consideration of the present
existence of intermediate gradations between any two forms, we shall be
361