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

160 THE MALAYAN PAPILIONIDZE AS
5. Race or subspecies.-These are local forms completely fixed and isolated; and there is no possible test
but individual opinion to determine which of them shall
be considered as species and which varieties. If stability of form and " the constant transmission of some
characteristic peculiarity of organization" is the test of
a species (and I can find no other test that is more
certain than individual opinion) then every one of
these fixed races, confined as they almost always are
to distinct and limited areas, must be regarded as a
species; and as such I have in most cases treated them.
The various modifications of Papilio Ulysses, P. Peranthus, P. Codrus, P. Eurypilus, P. Helenus, &c., are
excellent examples; for while some present great and
well-marked, others offer slight and inconspicuous differences, yet in all cases these differences seem equally
fixed and permanent. If, therefore, we call some of
these forms species, and others varieties, we introduce a
purely arbitrary distinction, and shall never be able to
decide where to draw the line. The races of Papilio
Ulysses, for example, vary in amount of modification
from the scarcely differing New Guinea form to those
of Woodlark Island and New Caledonia, but all seem
equally constant; and as most of these had already
been named and described as species, I have added the
New Guinea form under the name of P. Autolycus.
We thus get a little group of Ulyssine Papilios, the
whole comprised within a very limited area, each one
confined to a separate portion of that area, and, though
differing in various amounts, each apparently constant.