still in its effects was the epoch-making discovery
of the protoplasmic cell as the common element of life in the plant and
animal world, made by the Germans Schleiden and Schwann (1838). It was
this that first bridged over what were held to be the fundamental
distinctions of animate nature, and made possible the conception of a
vital physical continuity which has since been accepted as an axiom of
biological science.

By Joule's great discovery (1840) that the same amount of work, whether
mechanical or electrical, and however expended, always produced exactly
the same amount of heat--that, in effect, heat and work were equivalent
and interchangeable--the way was opened to the conclusion that the
total energy of the material universe is constant in amount through all
its changes.

{24}

A theory to account for the black lines crossing the coloured band of
light, or spectrum, which is obtained by passing sunlight through a
glass prism, originally suggested by Sir George Stokes, and
subsequently reintroduced and verified by the German chemists, Bunsen
and Kirchhoff, led to the important discovery that the sun and the
stars are constituted of the very same elements as those of the earth
beneath our feet. Spectrum analysis, moreover, soon detected new
elements, _e.g._, helium, so-called because first observed as existing
in the sun.

But great and stimulating as these discoveries were, their effect upon
the thought of the age was not to be compared with that which was to be
exercised by a theory which, starting in the domain of biological
science, soon passed on to far more extended applications. The theory
took its rise from a suggestion made in two papers, by Charles Darwin
and Alfred Russel Wallace, which were read before the Linnean Society
on July 1st, 1858.

The Darwinian theory--for so it was soon named--undertook to explain
the formation of species by the principle of natural selection through
the survival of the fittest in the struggle for life.[1] {25} Darwin
started from the admitted achievements of artificial selection; from
the results attained by nurserymen and cattle breeders, who, by
selecting the kinds they wished to perpetuate, had been able to vary
and improve their stocks. He conceived that a like process had been
carried on by Nature through vast spaces of time, and that it was this
picking, choosing, continuing and abandoning of traits and qualities
which had resulted in the preservation of