ormation even of the earth's crust, the existence of water in the
liquid state being necessary for life in any of its forms. And human
life itself, though the extent of its past duration is seen to be
greater the more deeply we study the records, is yet a relatively recent
thing. The utmost, it appears, that we can assign to our past would be
perhaps six million years, taking our species back to mid-Miocene times.
Doubtless this is a mighty age as compared with the few thousand years
allotted to us in bygone chronologies; but, looked at _sub specie
aeternitatis_, and with an eye which is prepared to look forward also,
and especially with relation to what we know and can predict regarding
the sun, these past six million years may reasonably be held to comprise
only the infantine period of man's life.

It is very true that on such estimates as those of Lord Kelvin, and
according to what astronomers and geologists believed not more than
twelve or even eight years ago, regarding the secular cooling of earth
and sun--that, according to these, the time is by no means "unending
long," and we may foresee, not so remotely, the end of the solar heat
and light of which we are the beneficiaries. But the discovery of radium
and the phenomena of radio-activity have profoundly modified these
estimates, justifying, indeed, the acumen of Lord Kelvin, who always
left the way open for reconsideration should a new source of heat and
energy in general be discovered. We know now that, to consider the earth
first, its crust is not self-cooling, or at any rate not self-cooling
only, for it is certainly self-heating. There is an almost embarrassing
amount of radium in the earth's crust, so far as we have examined it; a
quantity, that is to say, so great that if the same proportion were
maintained at deeper levels as at those which we can investigate, the
earth would have to be far hotter than it is. Similar reasoning applies
to the sun. Definite, immediate proof of the presence of radium there is
not forthcoming yet, but that presence is far more than probable,
especially since the existence of solar uranium, the known ancestor of
radium, has been demonstrated. The reckonings of Helmholtz and others,
based upon the supposition that the solar energy is entirely derived
from its gravitational contraction, must be superseded. It would require
but a very small proportion of radium in the solar constitution to
account for all the energy which the cen