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. Several experienced ornithologists consider our British red
grouse as only a strongly marked race of a Norwegian species, whereas
the greater number rank it as an undoubted species peculiar to Great
Britain. A wide distance between the homes of two doubtful forms leads
many naturalists to rank them as distinct species; but what distance, it
has been well asked, will suffice if that between America and Europe
is ample, will that between Europe and the Azores, or Madeira, or the
Canaries, or between the several islets of these small archipelagos, be
sufficient?

Mr. B.D. Walsh, a distinguished entomologist of the United States, has
described what he calls Phytophagic varieties and Phytophagic species.
Most vegetable-feeding insects live on one kind of plant or on one
group of plants; some feed indiscriminately on many kinds, but do not
in consequence vary. In several cases, however, insects found living on
different plants, have been observed by Mr. Walsh to present in their
larval or mature state, or in both states, slight, though constant
differences in colour, size, or in the nature of their secretions.
In some instances the males alone, in other instances, both males and
females, have been observed thus to differ in a slight degree. When the
differences are rather more strongly marked, and when both sexes and
all ages are affected, the forms are ranked by all entomologists as good
species. But no observer can determine for another, even if he can do so
for himself, which of these Phytophagic forms ought to be called species
and which varieties. Mr. Walsh ranks the forms which it may be supposed
would freely intercross, as varieties; and those which appear to have
lost this power, as species. As the differences depend on the insects
having