conception. We observe many of the means by which energy
is stored, and some of the complicated methods by which it
is captured, protected, and released. We shall see that highly
evolved organisms, such as the large reptiles and mammals
and man, present to the eye of the anatomist and physiologist
an inconceivable complexity of energy and form; but this we
may in part resolve by reading the pages of this volume back-
ward, Chinese fashion, from the mammal ¹ to the monad, in
which we reach a stage of relative simplicity. Thus the or-
ganism as an arena for energy and matter, as a complex of in-
tricate actions, becomes in a measure conceivable. The
heredity-germ, on the contrary, remains inconceivable in each
of its three powers, namely, in the Organism which it produces,
in the succession of germs to which it gives rise, and in its own
evolution in course of time.

Having now stated the main object of these lectures, I
invite the reader to study the following pages with care, be-
cause they review some of the past history and introduce some
of the new spirit and purpose of the search for causes in the
domain of energy. I begin with matters which are well known
to all biologists and proceed to matters which are somewhat
more difficult to understand and more novel in purpose.

In this review we need not devote any time or space to
fresh arguments for the truth of evolution. The demonstra-
tion of evolution as a universal law of living nature is the
great intellectual achievement of the nineteenth century.
Evolution has outgrown the rank of a theory, for it has won
a place in natural law beside Newton's law of gravitation,
and in one sense holds a still higher rank, because evolution is
the universal master, while gravitation is one among its many

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