nd
colouration, except that the latter always wants a very conspicuous red
spot on the under surface, which is found not only in P. Jason, but in
all the allied species. It is only by breeding the two insects that it
can be determined whether this is a case of a co-existing variety or of
dimorphism. In the former case, however, the difference being constant
and so very conspicuous and easily defined, I see not how we could
escape considering it as a distinct species. A true case of co-existing
forms would, I consider, be produced, if a slight variety had become
fixed as a local form, and afterwards been brought into contact with the
parent species, with little or no intermixture of the two; and such
instances do very probably occur.

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. Few naturalists will doubt that all these may and
probably have been derived from a common stock, and