method of science, and having at their disposal immensely improved
appliances, have devoted themselves to the enlargement of the
boundaries of natural knowledge in greater number than during any
previous half-century of the world's history.

[Sidenote: The three great achievements. Doctrines of (1) molecular
constitution of matter, (2) conservation of energy, (3) evolution.]

I have said that our epoch can produce achievements in physical
science of greater moment than any other has to show, advisedly; and
I think that there are three great products of our time which justify
the assertion. One of these is that doctrine concerning the
constitution of matter which, for want of a better name, I will call
'molecular;' the second is the doctrine of conservation of energy; the
third is the doctrine of evolution. Each of these was foreshadowed,
more or less distinctly, in former periods of the history of science;
and, so far is either from being 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