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 {405} 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 offspring, firstly into new
varieties and ultimately into new species.

In considering the wide distribution of certain genera, we should bear in
mind that some are extremely ancient, and must have branched off from a
common parent at a remote epoch; so that in such cases there will have been
ample time for great climatal and geographical changes and for accidents of
transport; and consequently for the migration of some of the species into
all {406} quarters of the world, where they may have become slightly
modified in relation to th