imate sphere is
that the process of study in both has been from the larger to the
smaller elements. The microscope has played at least as decisive a part
as the telescope, and it dates from about the same time, at the
beginning of the seventeenth century. Since then it has penetrated
farther and farther into the infinitesimal elements of life and matter,
and in each case there seems to be no assignable limit to our analysis.
The cell is broken up into physiological units to which almost every
investigator gives a new name. We are now confronted by the fascinating
theory of Arrhenius of an infinite universe filled with vital spores,
wafted about by radio-activity, and beginning their upward course of
evolution wherever they find a kindly soil on which to rest. To such a
vision the hopes and fears of mortal existence, catastrophes of nature
or of society, even the decay of man, seem transient and trivial, and
the infinities embrace.

A third point, perhaps the most important in the comparison, is the way
by which the order of science has entered into our notions of life,
through a great theory, the theory of evolution or the doctrine of
descent. In this we find a solid basis for the co-ordination of facts:
it was the rise of this theory in the hands of one thinker of
unconquerable patience and love of truth which has put the study of
biology in the pre-eminent position which it now holds. But it is
necessary to consider the evolution theory as something both older and
wider than Darwin's presentation of it. Darwin's work was to suggest a
_vera causa_ for a process which earlier philosophers had imagined
almost from the beginning of abstract thought. He observed and collected
a multitude of facts which made his explanations of the change of
species--within their limits--as convincing as they are plausible. But
the idea that species change, by slow and regular steps, was an old one,
and his particular explanations, natural and sexual selection, are seen
on further reflection to have only a limited scope.

This is no place, of course, to discuss the details of the greatest and
most vexed question in the whole science of life. But it belongs to our
argument to consider it from one or two general points of view. Its
analogies with, and its differences from, the great generalizations of
mathematical physics, are both highly instructive. The first crude
hypothesis of the gradual evolution of various vegetable and animal
forms