gest that the proximate cause of each advance in
embryonic complication is the action of incident forces upon the
complication previously existing. Indeed, we may find _a priori_ reason
to think that the evolution proceeds after this manner. For since no
germ, animal or vegetal, contains the slightest rudiment or indication
of the future organism--since the microscope has shown us that the first
process set up in every fertilized germ, is a process of repeated
spontaneous fissions ending in the production of a mass of cells, not
one of which exhibits any special character; there seems no alternative
but to suppose that the partial organization at any moment existing in a
growing embryo, is transformed by the agencies acting upon it into the
succeeding phase of organization, and this into the next, until, through
ever-increasing complexities, the ultimate form is reached. Not indeed
that we can thus really explain the production of any plant or animal.
We are still in the dark respecting those mysterious properties in
virtue of which the germ, when subject to fit influences, undergoes the
special changes that begin the series of transformations. All we aim to
show, is, that given a germ possessing those particular proclivities
distinguishing the species to which it belongs, and the evolution of an
organism from it, probably depends on that multiplication 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 Paleontology 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, ste