
- t / = = c . =a 
Beet) qj. AGH OFTHE HARTH.. 5 55 
— 
far as it goes, warrants the belief that changes in the organic world 
proceed with extreme slowness. Yet in the stratified rocks of the 
terrestrial crust we have abundant proof that the whole fauna and 
flora of the earth’s surface have passed through numerous cycles of 
revolution,—species, genera, families, orders, appearing and disappear- 
ing many times in succession. On any supposition it must be 
admitted that these vicissitudes in the organic world can only have 
been effected with the lapse of vast periods of time, though no 
reliable standard seems to be available whereby these periods are to 
be measured. The argument from geological evidence is strongly in 
favour of an interval of probably not much less than 100 million 
years since the earliest forms of life appeared upon the earth, and the 
oldest stratified rocks began to be laid down. 
2. The argument from physics as to the age of our planet 
is based by Sir William Thomson upon three kinds of evidence :— 
(1) the internal heat and rate of cooling of the earth; (2) the tidal 
retardation of the earth’s rotation; and (8) the origin and age of 
the sun’s heat. | 
(1.) Applying Fourier’s theory of thermal conductivity, he pointed 
out some years ago (1862) that in the known rate of increase of 
temperature downward beneath the surface, and the rate of loss of 
heat from the earth, we have a limit to the antiquity of the planet. 
He showed, from the data available at the time, that the superficial 
consolidation of the globe could not have occurred less than 20 
million years ago, or the underground heat would have been greater 
than it is; nor more than 400 million years ago, otherwise the 
underground temperature would have shown no sensible increase 
downwards. He admitted that very wide limits were necessary. In 
more recently discussing the subject, he inclines rather towards the 
lower than the higher antiquity, but concludes that the limit, from 
a consideration of all the evidence, must be placed within some 
such period of past time as 100 millions of years. * 
(2.) The reasoning from tidal retardation proceeds on the 
admitted fact that, owimg to the friction of the tide-wave, the 
rotation of the earth is retarded, and is therefore slower now than 
it must have been at one time. Sir William Thomson contends 
that had the globe become solid some 10,000 million years ago, 
or indeed any high antiquity beyond 100 million years, the centri- 
fugal force due to the more rapid rotation must have given the planet 
a very much greater polar flattening than it actually possesses. 
He admits, however, that though 100 million years ago that force 
must have been about 3 per cent. greater than now, yet “nothing we 
know regarding the figure of the earth and the disposition of land 
and water would justify us in saying that a body consolidated when 
there was more centrifugal force by 3 per cent. than now might 
1 Trans. Roy. Soc. Edin. xxiii. p. 157. Trans. Geol. Soc. Glasgow, iii. p. 25, Professor 
Tait reduces the period to 10 or 15 millions, Recent Advances in Physical Science, 
p. 167. 
