The Biochronic Equation. 669 
sun can have shone for only about 20 million years with 
approximately the same energy as that with which it 
shines now; and since this is the first condition of life 
on the earth, we must assume that its duration has been 
about the same as this period. The most authoritative 
estimate for the total thickness of the geological strata, 
and of the speed with which they have been laid down, 
is that of 80 kilometers, laid down at a rate of 30 centi- 
meters per century, and this leads to an estimate of 26 
million years for the whole period. 
Therefore, about 20 to 40 million years is the period 
of the duration of life upon the earth ; and LORD KELVIN, 
who a few years ago subjected the data, on which this 
estimate is based, to a critical reconsideration, came to 
the conclusion that the duration of life on the earth may 
provisionally be put at about 24 million years. 1 
\Ye will therefore now adopt this figure as a basis 
for our further arguments. 
The second question is this : How quickly have the 
individual periods of mutation followed on one another? 
We have very few data which enable us to arrive at any 
conclusion on this point. As is well known, the parts 
of plants which have been preserved in the sepulchers 
of the pyramids along with the mummies, and in other 
monuments of the same period, such as flowers, leaves, 
fruits, cereals, straw and weeds of the fields, prove the 
great antiquity of many species which are still existing. 
Numerous species are no doubt older than the pyramids, 
and have therefore remained unchanged for a period of 
at least 4000 years. The remains of lake dwellings, 2 
1 See the review in the Phil. Mag., Jan. 1899. 
2 Osw. HEER, Die Pflanzen dcr Pfahlbautcn. Schweiz. Natnrf. 
Gesellsch., 1866, No. LXVIIT, with one plate; also C. SCHROTER and 
J. HEER, Lcbensbild von Oswald Hccr, Zurich, 1885. 
