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 long fed on distinct plants, it cannot be expected that
intermediate links connecting the several forms should now be found. The
naturalist thus loses his best guide in determining whether to rank
doubtful forms as varieties or species. This likewise necessarily occurs
with closely allied organisms, which inhabit distinct continents or