cii.sp. I. SUBJECTS TREATED OF IN THIS WORK. 3 



barrier to all attempts to trace farther back into the past the 

 signs of the existence of man upon the earth. 



In the concluding chapters I shall offer a few remarks on 

 the recent modifications of the Lamarckian theory of pro- 

 gressive development and ti'ansmutation, which are sug- 

 gested by Mr. Darwin's work on the " Origin of Si)ecies, by 

 Variation and Natural Selection," and the bearing' of this 

 hypothesis on the different races of mankind and their con- 

 nection w^ith other parts of the animal kingdom. 



Nomenclature. — Some preliminary explanation of the 

 nomenclature adojoted in the following pages will be indis- 

 pensable, that the meaning attached to the terms Eecent, 

 Post-pliocene, and Post-tertiary may be correctly understood. 



Previously to the year 1833, when I published the third 

 volume of the " Principles of Geology," the strata called 

 Tertiary had been divided by geologists into Lower, Middle, 

 and Upper ; the Lower comprising the oldest formations of the 

 environs of Paris and London, Avith others of like age ; the 

 Middle, those of Bordeaux and Touraine; and the LTpper, all 

 that lay above or were newer than the last-mentioned group. 



When engaged, in 1828, in preparing for the press the 

 treatise on geology above alluded to, I conceived the idea of 

 classing the whole of this series of strata according to the 

 different degrees of afiinity which their fossil testacea bore 

 to the living fauna. Having obtained information on this 

 subject during my travels on the Continent, I learnt that 

 M. Deshayes of Paris, already celebrated as a conchologist, had 

 been led independent!}^, by the study of a large collection of 

 recent and fossil shells, to very similar views respecting the 

 possibility of arranging the tertiaiy formations in chrono- 

 logical order, according to the proportional number of species 

 of shells identical with living ones, which characterized each 

 of the successive groups above mentioned. After comparing 

 3000 fossil species with 5000 Hving ones, the result arrived at 



