ation of effects which we have seen
to be the cause of progress in general, so far as we have yet traced it.

When, leaving the development of single plants and animals, we pass to
that of the Earth's flora and fauna, the course of our argument again
becomes clear and simple. Though, as was admitted in the first part of
this article, the fragmentary facts Palaeontology has accumulated, do not
clearly warrant us in saying that, in the lapse of geologic time, there
have been evolved more heterogeneous organisms, and more heterogeneous
assemblages of organisms, yet we shall now see that there _must_ ever
have been a tendency towards these results. We shall find that the
production of many effects by one cause, which, as already shown, has
been all along increasing the physical heterogeneity of the Earth, has
further involved an increasing heterogeneity in its flora and fauna,
individually and collectively. An illustration will make this clear.

Suppose that by a series of upheavals, occurring, as they are now known
to do, at long intervals, the East Indian Archipelago were to be, step
by step, raised into a continent, and a chain of mountains formed along
the axis of elevation. By the first of these upheavals, the plants and
animals inhabiting Borneo, Sumatra, New Guinea, and the rest, would be
subjected to slightly modified sets of conditions. The climate in
general would be altered in temperature, in humidity, and in its
periodical variations; while the local differences would be multiplied.
These modifications would affect, perhaps inappreciably, the entire
flora and fauna of the region. The change of level would produce
additional modifications: varying in different species, and also in
different members of the same species, according to their distance from
the axis of elevation. Plants, growing only on the sea-shore in special
localities, might become extinct. Others, living only in swamps of a
certain humidity, would, if they survived at all, probably undergo
visible changes of appearance. While still greater alterations would
occur in the plants gradually spreading over the lands newly raised
above the sea. The animals and insects living on these modified plants,
would themselves be in some degree modified by change of food, as well
as by change of climate; and the modification would be more marked
where, from the dwindling or disappearance of one kind of plant, an
allied kind was eaten. In the lapse of the many gen