l--from the low to the high, from the few and simple to the many
and complex--has been the law of organic nature. And of course it is
needless to say that this is precisely the law to which the process of
descent with adaptive modification would of necessity give rise.

[1] Some of the lower vertebrata (Elasmobranch and Ganoid fishes) occur,
indeed, in early strata (upper Silurian); but still far from the
earliest in which some of the invertebrata are found. The general
statement in the text applies chiefly to the more highly organised forms
of the vertebrate series.

IV.

THE ARGUMENT FROM GEOGRAPHICAL DISTRIBUTION.

The argument from geology is the argument from the distribution of
species in time. I will, therefore, next take the argument from the
distribution of species in space--that is, the present geographical
distribution of plants and animals. It is easy to see that this must be
a most important argument, if we reflect that as the theory of descent
with adaptive modification implies slow and gradual change of one
species into another, and a still more slow and gradual change of one
genus, family, or order into another genus, family, or order, we should
expect on this theory that the organic types living on any given
geographical area should be found to resemble or to differ from organic
types living elsewhere, according as the area is connected or
disconnected with other geographical areas. And this we find to be the
case, as abundant evidence proves. For, to quote from Mr. Darwin,
"barriers of any kind, or obstacles to free migration, are related in a
close and important manner to the differences between the productions of
various regions. We see this in the great difference in nearly all the
terrestrial productions of the New and Old Worlds, excepting in the
northern parts, where the land almost joins.... We see the same fact in
the great difference between the inhabitants of Australia, Africa, and
South America under the same latitude, for these countries are almost as
much isolated from one another as possible. On each continent, also, we
see the same fact; for on the opposite sides of lofty and continuous
mountain ranges, of great deserts, and even of large rivers, we find
different productions; though as mountain chains, deserts, &c., are not
so impassable, or likely to have endured so long as the ocean-separated
continents, the differences are very inferior in degree to those
characteristic o