We see the same
principle in the character of most of the blind animals inhabiting the
caves of America and of Europe. Other analogous facts could be given. It
will, I believe, be found universally true, that wherever in two regions,
let them be ever so distant, many closely allied or representative species
occur, there will likewise be found some identical species; and wherever
many closely-allied species occur, there will be found many forms which
some naturalists rank as distinct species, and others as mere varieties;
these doubtful forms showing us the steps in the process of modification.
The relation between the power and extent of migration in certain species,
either at the present or at some former period, and the existence at remote
points of the world of closely allied species, 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
difficult of proof. Among mammals, we see it strikingly displayed in Bats,
and in a lesser degree in the Felidae and Canidae. We see the same rule in
the distribution of butterflies and beetles. So it is with most of the
inhabitants of fresh water, for many of the genera in the most distinct
classes range over the world, and many of the species have enormous ranges.
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