which Darwin's
doctrine is directly opposed, and of which Agassiz is, among
naturalists, the most important advocate. The famous work of Agassiz,
'Essay on Classification,' which is in direct opposition to Darwin's,
and appeared about the same time, has carried out logically to the
utmost the absurd anthropomorphic doctrine of a Creator." (p. 17)

The monistic theory is called "mechanical and causal," because it
supposes that all the phenomena of the universe, organic and inorganic,
vegetable and animal, vital and mental, are due to mechanical or
necessarily operating causes (causae efficientes); just as the dualistic
theory is called "teleological or vitalistic," because it refers natural
organisms to causes working for the accomplishment of a given end (causae
finales). (p. 67)

The grand difficulty in the way of the mechanical or monistic theory was
the occurrence of innumerable organisms, apparently at least, indicative
of design. To get over this difficulty, Haeckel says, some who could not
believe in a creative and controlling mind adopted the idea of a
metaphysical ghost called vitality. The grand service rendered by Darwin
to science is, that his theory enables us to account for the
appearances of design in nature without assuming final causes, or, a
mind working for a foreseen and intended end. "All that had appeared
before Darwin," he says, "failed to secure success, and to meet with
general acceptance of the doctrine of the mechanical production of
vegetable and animal organisms. This was accomplished by Darwin's
theory." (p. 20)

The precise difficulty which Mr. Darwin's doctrine has, according to
Haeckel, enabled men of science to surmount, is thus clearly stated on
p. 633. It is, "that organs for a definite end should be produced by
undesigning or mechanical causes." This difficulty is overcome by the
doctrine of evolution. "Through the theory of descent, we are for the
first time able to establish the monistic doctrine of the unity of
nature, that a mechanic-causal explanation of the most complicated
organisms, _e. g._ the formation and constitution of the organs of
sense, have no more difficulty for the common understanding, than the
mechanical explanation of any physical process, as, for example,
earthquakes, the direction of the winds, or the currents of the sea. We
thus arrive at the conviction of the last importance, that all natural
bodies with which we are acquainted are equally endowed with li