e eighty-odd chemical elements on the earth to-day: are
they all the outcome of an inorganic evolution, element giving rise to
element, going back and back to some primeval stuff from which they
were all originally derived infinitely long ago? Is there an evolution
in the inorganic world which may be going on, parallel to that of the
evolution of living things; or is organic evolution a continuation of
inorganic evolution? We have seen what evidence there is of this
inorganic evolution in the case of the stars. We cannot go deeply into
the matter here, nor has the time come for any direct statement that can
be based on the findings of modern investigation. Taking it altogether
the evidence is steadily accumulating, and there are authorities who
maintain that already the evidence of inorganic evolution is convincing
enough. The heavier atoms would appear to behave as though they were
evolved from the lighter. The more complex forms, it is supposed, have
_evolved_ from the simpler forms. Moseley's discovery, to which
reference has been made, points to the conclusion that the elements are
built up one from another.

Sec. 8

Other New Views

We may here refer to another new conception to which the discovery of
radio-activity has given rise. Lord Kelvin, who estimated the age of the
earth at twenty million years, reached this estimate by considering the
earth as a body which is gradually cooling down, "losing its primitive
heat, like a loaf taken from the oven, at a rate which could be
calculated, and that the heat radiated by the sun was due to
contraction." Uranium and radio-activity were not known to Kelvin, and
their discovery has upset both his arguments. Radio-active substances,
which are perpetually giving out heat, introduce an entirely new factor.
We cannot now assume that the earth is necessarily cooling down; it may
even, for all we know, be getting hotter. At the 1921 meeting of the
British Association, Professor Rayleigh stated that further knowledge
had extended the probable period during which there had been life on
this globe to about one thousand million years, and the total age of
the earth to some small multiple of that. The earth, he considers, is
not cooling, but "contains an internal source of heat from the
disintegration of uranium in the outer crust." On the whole the estimate
obtained would seem to be in agreement with the geological estimates.
The question, of course, cannot, in the present sta