therefore it seems
desirable that there should be a unity in our method of treating them;
either call them all _varieties_ or all _species_. Varieties, however,
continually get overlooked; in lists of species they are often
altogether unrecorded; and thus we are in danger of neglecting the
interesting phenomena of variation and distribution which they present.
I think it advisable, therefore, to name all such forms; and those who
will not accept them as species may consider them as subspecies or
races.

6. _Species._--Species are merely those strongly marked races or local
forms which when in contact do not intermix, and when inhabiting
distinct areas are generally believed to have had a separate origin, and
to be incapable of producing a fertile hybrid offspring. But as the test
of hybridity cannot be applied in one case in ten thousand, and even if
it could be applied would prove nothing, since it is founded on an
assumption of the very question to be decided--and as the test of
separate origin is in every case inapplicable--and as, further, the test
of non-intermixture is useless, except in those rare cases where the
most closely allied species are found inhabiting the same area, it will
be evident that we have no means whatever of distinguishing so-called
"true species" from the several modes of variation here pointed out, and
into which they so often pass by an insensible gradation. It is quite
true that, in the great majority of cases, what we term "species" are
so well marked and definite that there is no difference of opinion about
them; but as the test of a true theory is, that it accounts for, or at
the very least is not inconsistent with, the whole of the phenomena and
apparent anomalies of the problem to be solved, it is reasonable to ask
that those who deny the origin of species by variation and selection
should grapple with the facts in detail, and show how the doctrine of
the distinct origin and permanence of species will explain and harmonize
them. It has been recently asserted by Dr. J. E. Gray (in the
Proceedings of the Zoological Society for 1863, page 134), that the
difficulty of limiting species is in proportion to our ignorance, and
that just as groups or countries are more accurately known and studied
in greater detail the limits of species become settled. This statement
has, like many other general assertions, its portion of both truth and
error. There is no doubt that many uncertain species, fo