ng the outcome of purely inductive
reasoning, that it would be hard to overrate the influence of
metaphysical, and even of theological, considerations upon the
development of all three. The peculiar merit of our epoch is that it
has shown how these hypotheses connect a vast number of seemingly
independent partial generalisations; that it has given them that
precision of expression which is necessary for their exact
verification; and that it has practically proved their value as
guides to the discovery of new truth. All three doctrines are
intimately connected, and each is applicable to the whole physical
cosmos. But, as might have been expected from the nature of the case,
the first two grew, mainly, out of the consideration of
physico-chemical phenomena; while the third, in great measure, owes
its rehabilitation, if not its origin, to the study of biological
phenomena.

[Sidenote: (1) Molecular constitution of matter.]

In the early decades of this century, a number of important truths
applicable, in part, to matter in general, and, in part, to particular
forms of matter, had been ascertained by the physicists and chemists.

The laws of motion of visible and tangible, or _molar_, matter had
been worked out to a great degree of refinement and embodied in the
branches of science known as Mechanics, Hydrostatics, and Pneumatics.
These laws had been shown to hold good, so far as they could be
checked by observation and experiment, throughout the universe, on the
assumption that all such masses of matter possessed inertia and were
susceptible of acquiring motion, in two ways, firstly by impact, or
impulse from without; and, secondly, by the operation of certain
hypothetical causes of motion termed 'forces,' which were usually
supposed to be resident in the particles of the masses themselves, and
to operate at a distance, in such a way as to tend to draw any two
such masses together, or to separate them more widely.

[Sidenote: The two theories as to matter.]

With respect to the ultimate constitution of these masses, the same
two antagonistic opinions which had existed since the time of
Democritus and of Aristotle were still face to face. According to the
one, matter was discontinuous and consisted of minute indivisible
particles or atoms, separated by a universal vacuum; according to the
other, it was continuous, and the finest distinguishable, or
imaginable, particles were scattered through the attenuated general
s