which kingdom they should be referred. As Professor Asa Gray
has remarked, "the spores and other reproductive bodies of many of the
lower algae may claim to have first a characteristically animal, and
then an unequivocally vegetable existence." Therefore, on the principle
of natural selection with divergence of character, it does not seem
incredible that, from some such low and intermediate form, both animals
and plants may have been developed; and, if we admit this, we must
likewise admit that all the organic beings which have ever lived on this
earth may be descended from some one primordial form. But this inference
is chiefly grounded on analogy, and it is immaterial whether or not it
be accepted. No doubt it is possible, as Mr. G.H. Lewes has urged, that
at the first commencement of life many different forms were evolved;
but if so, we may conclude that only a very few have left modified
descendants. For, as I have recently remarked in regard to the members
of each great kingdom, such as the Vertebrata, Articulata, etc., we have
distinct evidence in their embryological, homologous, and rudimentary
structures, that within each kingdom all the members are descended from
a single progenitor.

When the views advanced by me in this volume, and by Mr. Wallace or when
analogous views on the origin of species 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 a true 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 good 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 la