the lesser things of life.
My thanks are due to Mr. John Murray for kindly consenting to the
reissue of the essay on _The Birth-time of the World_ from the
pages of _Science Progress_; to Messrs. Constable & Co. for leave
to reprint _Pleochroic Haloes_ from _Bedrock_, and also to make some
extracts from _Radioactivity and Geology_; and to the Council of
the Royal Dublin Society for permission to republish certain
papers from the Proceedings of the Society.
_Iveagh Geological Laboratory, Trinity College, Dublin._
July, 1915.
xvi
THE BIRTH-TIME OF THE WORLD [1]
LONG ago Lucretius wrote: "For lack of power to solve the
question troubles the mind with doubts, whether there was ever a
birth-time of the world and whether likewise there is to be any
end." "And if" (he says in answer) "there was no birth-time of
earth and heaven and they have been from everlasting, why before
the Theban war and the destruction of Troy have not other poets
as well sung other themes? Whither have so many deeds of men so
often passed away, why live they nowhere embodied in lasting
records of fame? The truth methinks is that the sum has but a
recent date, and the nature of the world is new and has but
lately had its commencement."[2]
Thus spake Lucretius nearly 2,000 years ago. Since then we have
attained another standpoint and found very different limitations.
To Lucretius the world commenced with man, and the answer he
would give to his questions was in accord with his philosophy: he
would date the birth-time of the world from the time when
[1] A lecture delivered before the Royal Dublin Society, February
6th, 1914. _Science Progress_, vol. ix., p. 37
[2] _De Rerum Natura_, translated by H. A. J. Munro (Cambridge,
1886).
1
poets first sang upon the earth. Modern Science has along with
the theory that the Earth dated its beginning with the advent of
man, swept utterly away this beautiful imagining. We can, indeed,
find no beginning of the world. We trace back events and come to
barriers which close our vista—barriers which, for all we know,
may for ever close it. They stand like the gates of ivory and of
horn; portals from which only dreams proceed; and Science cannot
as yet say of this or that dream if it proceeds from the gate of
horn or from that of ivory.
In short, of the Earth's origin we have no certain knowledge; nor
can we assign any date to it. Possibly its formation was an event
so gradual that the beginning was spread over immense periods. We
can only trace the history back to certain events which may with
considerable certainty be regarded as ushering in our geological
era.
Notwithstanding our limitations, the date of the birth-time of
our geological era is the most important date in Science. For in
taking into our minds the spacious history of the universe, the
world's age must play the part of time-unit upon which all our
conceptions depend. If we date the geological history of the
Earth by thousands of years, as did our forerunners, we must
shape our ideas of planetary time accordingly; and the duration
of our solar system, and of the heavens, becomes comparable with
that of the dynasties of ancient nations. If by millions of
years, the sun and stars are proportionately venerable. If by
hundreds or thousands of millions of
2
years the human mind must consent to correspondingly vast epochs
for the duration of material changes. The geological age plays
the same part in our views of the duration of the universe as the
Earth's orbital radius does in our views of the immensity of
space. Lucretius knew nothing of our time-unit: his unit was the
life of a man. So also he knew nothing of our space-unit, and he
marvels that so small a body as the sun can shed so much, heat
and light upon the Earth.
A study of the rocks shows us that the world was not always what
it now is and long has been. We live in an epoch of denudation.
The rains and frosts disintegrate the hills; and the rivers roll
to the sea the finely divided particles into which they have been
resolved; as well as the salts which have been leached from them.
The sediments collect near the coasts of the continents; the
dissolved matter mingles with the general ocean. The geologist
has measured and mapped these deposits and traced them back into
the past, layer by layer. He finds them ever the same;
sandstones, slates, limestones, etc. But one thing is not the
same. _Life_ grows ever less diversified
