r, in which life is
moving whilst developing the original impulse. True, we shall not
witness the detailed accomplishment of a plan. Nature is more and better
than a plan in course of realization. A plan is a term assigned to a
labor: it closes the future whose form it indicates. Before the
evolution of life, on the contrary, the portals of the future remain
wide open. It is a creation that goes on for ever in virtue of an
initial movement. This movement constitutes the unity of the organized
world--a prolific unity, of an infinite richness, superior to any that
the intellect could dream of, for the intellect is only one of its
aspects or products.

But it is easier to define the method than to apply it. The complete
interpretation of the evolution movement in the past, as we conceive it,
would be possible only if the history of the development of the
organized world were entirely known. Such is far from being the case.
The genealogies proposed for the different species are generally
questionable. They vary with their authors, with the theoretic views
inspiring them, and raise discussions to which the present state of
science does not admit of a final settlement. But a comparison of the
different solutions shows that the controversy bears less on the main
lines of the movement than on matters of detail; and so, by following
the main lines as closely as possible, we shall be sure of not going
astray. Moreover, they alone are important to us; for we do not aim,
like the naturalist, at finding the order of succession of different
species, but only at defining the principal directions of their
evolution. And not all of these directions have the same interest for
us: what concerns us particularly is the path that leads to man. We
shall therefore not lose sight of the fact, in following one direction
and another, that our main business is to determine the relation of man
to the animal kingdom, and the place of the animal kingdom itself in the
organized world as a whole.

* * * * *

To begin with the second point, let us say that no definite
characteristic distinguishes the plant from the animal. Attempts to
define the two kingdoms strictly have always come to naught. There is
not a single property of vegetable life that is not found, in some
degree, in certain animals; not a single characteristic feature of the
animal that has not been seen in certain species or at certain moments
in the veg