SEQUEL TO SYSTEMATIC GEOLOGY. 577 



The terms which have been formed by geolo- 

 gists in recent times have been drawn from sources' 

 similar to those of the older ones, and will have 

 their fortune determined by the same conditions. 

 Thus Mr. Lyell has given to the divisions of the 

 tertiary strata the appellations Pleiocene, Meiocene, 

 Eocene, accordingly as they contain a majority of 

 recent species of shells, a minority of such species, 

 or a small proportion of living species, which may 

 be looked upon as indicating the dawn of the 

 existing state of the animate creation. But in this 

 case, he wisely treats his distinctions, not as defini- 

 tions, but as the marks of natural groups. "The 

 plurality of species indicated by the name pleiocene, 

 must not," he says 4 , "be understood to imply an 

 absolute majority of recent fossil shells in all cases, 

 but a comparative preponderance wherever the 

 pleiocene are contrasted with strata of the period 

 immediately preceding." 



Mr. Lyell might have added, that no precise 

 per-centage of recent species, nor any numerical 

 criterion whatever, can be allowed to overbear the 

 closer natural relations of strata, proved by evidence 

 of a superior kind, if such can be found. And this 

 would be the proper answer to the objection made 

 by Mr. De la Beche to these names ; namely, that it 

 may happen that the meiocene rocks of one country 

 may be of the same date as the pleiocene of an- 

 other; the same formation having in one place a 



4 Geol iii. 392. 

 VOL. III. P P 



