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island. The local forms
→of the same species
are moderately constant and distinct in each separate island; but when all
the several islands are
→the differences become
so
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
→though not by a definite,
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.
years ago, when comparing, and seeing others compare, the birds from the
islands of the Galapagos
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
→certainly would
be ranked as distinct species by many entomologists. Even
has a few animals, now generally regarded as varieties, but which have been ranked as species by some zoologists. Several
experienced ornithologists
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