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 generations arising before the next upheaval, the sensible or insensible alterations thus produced in each species would become organised—­there would be a more or less complete adaptation to the new conditions.  The next upheaval would superinduce further organic changes, implying wider divergences from the primary forms; and so repeatedly.