Tlic Biochroiiic pAiuatioii. 669 



snn can have shone fur (jnl\' ahoul 2U niilhon }ears with 

 approximately the same ener^^^y as tliat with whicli it 

 shines now; and since this is the hrst C(jnchtion of hfe 

 on the earth, we must assume that its (hirati(jn has Ijeen 

 about the same as this period. The most authoritative 

 estimate for the total thickness of the geological strata, 

 and of the s])ce(l with which they have been laid down, 

 is that of 80 kilometers, laid down at a rate of 30 centi- 

 meters per century, an.d this leads to an estimate of 26 

 million years for the whole period. 



Therefore, about 20 to 40 million years is the j^eriod 

 of the duration of life u])on the earth ; and Lord Kkiain, 

 wdio a few years ago sul)jecte(l the data, on which this 

 estimate is based, to a critical reconsideration, came to 

 the conclusion that the duration (j\ life on the earth may 

 provisionally be put at about 24 million years. ^ 



We will therefore now ado])t this lioure 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 i)arts 

 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 tlie 

 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 ])erii»d ^A 

 at least 4000 years. The remains of lake dwelling'^. - 



^ See the review in the Phil Mag., Jan. i!^. 



^Osw. PTeer. Die Pflanzcn dcr Pfalilbautni. Sdnvciz. Natitrf. 

 Gesellsch.. i866. No. LXVIIT. with one plate: also C Schrotfr and 

 J. Heer, Lchcnshild von Osn'ald Ilccr, Zurich. 18S5. 



