666 Geological Periods of Mutation. 
Let us now proceed to consider the relation between 
the degree of organization and the speed of the evolution 
from a more general point of view. For this purpose 
I shall deal with the several factors as briefly as possible, 
and propose to begin with biological time. 
Many investigators have attempted to reach an ap- 
proximate estimation of biological time, i. e., the dura- 
tion of life on the earth. Proceeding on entirely different 
lines, the best of them have arrived at results which 
agree in a most remarkable way. From this fact we 
may infer that the calculations, which from their very 
nature must be more or less vague, probably represent a 
fairly close approximation to the truth. 
I take the following, 1 partly from LORD KELVIN'S 
famous researches, and partly from the clear exposition 
given by W. J. SOLLAS in his address as President of the 
geological section of the British Association in the meet- 
ing of 1900; and further from the recent investigations 
of DUBOIS. 
LORD KELVIN based his first calculations on the in- 
crease in temperature in the successive depths of a mine. 2 
This increase, however, has been shown by more recent 
investigations to vary considerably. The older deter- 
minations gave from 25 to 37, or sometimes as much as 
50 meters for each degree Centigrade. In the neighbor- 
hood of the North American lakes, however, in a shaft 
of 1396 meters, an increase of l c C. per 122 meters has 
1 An exhaustive review of the subject can be found in Album 
der Natuur, Sept. 1901 ; and the -natter is also dealt with by H. 
CHARLTON BASTIAN, Studies on H erogenesis, London, 1901, pp. i-x. 
See also Nature, Sept. 1900 and Revue scientifique, April 1901. 
2 SIR WILLIAM THOMSON (afterwards LORD KELVIN), The Secu- 
lar Cooling of the Earth, Transact. Roy. Soc. Edinburgh, 1862, Vol. 
XXIII. 
