various animals, especially on the Lepidoptera, inhabiting the islands of
the great Malayan Archipelago, shows that they may be classed under four
heads, namely, as variable forms, as local forms, as geographical races or
sub-species, and as true representative species. The first or variable
forms vary much within the limits of the same island. The local forms are
moderately constant and distinct in each separate island; but when all from
the several islands are compared together, the differences are seen to be
so slight and graduated that it is impossible to define or describe them,
though at the same time the extreme forms are sufficiently distinct. The
geographical races or sub-species are local forms completely fixed and
isolated; but as they do not differ from each other by strongly marked and
important characters, "There is no possible test but individual opinion to
determine which of them shall be considered as species and which as
varieties." Lastly, representative species fill the same place in the
natural economy of each island as do the local forms and sub-species; but
as they are distinguished from each other by a greater amount of difference
than that between the local forms and sub-species, they are almost
universally ranked by naturalists as true species. Nevertheless, no
certain criterion can possibly be given by which variable forms, local
forms, sub species and representative species can be recognised.

Many years ago, when comparing, and seeing others compare, the birds from
the closely neighbouring islands of the Galapagos Archipelago, one with
another, and with those from the American mainland, I was much struck how
entirely vague and arbitrary is the distinction between species and
varieties. On the islets of the little Madeira group there are many
insects which are characterized as varieties in Mr. Wollaston's admirable
work, but which would certainly be ranked as distinct species by many
entomologists. Even Ireland has a few animals, now generally regarded as
varieties, but which have been ranked as species by some zoologists.