It is not meant that all, but that some of the species have very wide
ranges in the genera which range very widely. Nor is it meant that the
species in such genera have, on an average, a very wide range; for this
will largely depend on how far the process of modification has gone; for
instance, two varieties of the same species inhabit America and Europe, and
thus the species has an immense range; but, if variation were to be carried
a little further, the two varieties would be ranked as distinct species,
and their range would be greatly reduced. Still less is it meant, that
species which have 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 according to the view that all the species of a genus,
though distributed to the most remote points of the world, are descended
from a single progenitor, we ought to find, and I believe as a general rule
we do find, that some at least of the species range very widely.
We should bear in mind that many genera in all classes are of ancient
origin, and the species in this case will have had ample time for dispersal
and subsequent modification.
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