Contributions to the theory of natural selection. A series of essays ... By Alfred Russel Wallace ...

ILLUSTRATIVE OF NATURAL SELECTION. 161
Few naturalists will doubt that all these may and probably have been derived from a common stock, and
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 nonintermixture 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