be found many forms which some naturalists
rank as distinct species, and some as varieties; these doubtful forms
showing us the steps in the process of modification.

This relation between the power and extent of migration of a species,
either at the present time or at some former period under different
physical conditions, and the existence at remote points of the world of
other species allied to it, is shown in another and more general way.
Mr. Gould remarked to me long ago, that in those genera of birds which
range over the world, many of the species have very wide ranges. I
can hardly doubt that this rule is generally true, though it would be
difficult to prove it. Amongst mammals, we see it strikingly displayed
in Bats, and in a lesser degree in the Felidae and Canidae. We see it,
if we compare the distribution of butterflies and beetles. So it is with
most fresh-water productions, in which so many genera range over the
world, and many individual species have enormous ranges. It is not meant
that in world-ranging genera all the species have a wide range, or even
that they have on an AVERAGE a wide range; but only that some of the
species range very widely; for the facility with which widely-ranging
species vary and give rise to new forms will largely determine their
average range. For instance, two varieties of the same species inhabit
America and Europe, and the species thus has an immense range; but, if
the variation had been a little greater, the two varieties would have
been ranked as distinct species, and the common range would have been
greatly reduced. Still less is it meant, that a species which apparently
has the capacity of crossing barriers and ranging widely, as in the case
of certain powerfully-winged birds, will necessarily range widely; for
we should never forget that to range widely implies not only the power
of crossing barriers, but the more important power of being victorious
in distant lands in the struggle for life with foreign associates. But
on the view of all the species of a genus having descended from a single
parent, though now distributed to the most remote points of the world,
we ought to find, and I believe as a general rule we do find, that some
at least of the species range very widely; for it is necessary that the
unmodified parent should range widely, undergoing modification during
its diffusion, and should place itself under diverse conditions
favourable for the conversion of its o