360 T. Davidson - Tertiary Brachiopoda. 



of Tertiary species in conjunction with those still existing is a 

 subject of considerable interest, and it did not fail to attract the 

 attention of our justly-celebrated geologist Sir Charles Lyell at the 

 very dawn of his geological career, and it is mainly to bis travels, 

 and admirable researches, and to the publication of the third volume 

 of his renowned " Principles of Geology " in 1833, that we are 

 indebted for the first clear light that has been thrown upon the 

 subject of Tertiary geology. In 1829 (as stated in his preface) Sir 

 C. Lyell had, after repeated journeys in Italy and in other regions, 

 arrived at the conclusion that the Tertiary period might be con- 

 veniently subdivided into three or four principal divisions, for which 

 he invented the designations of Eocene, Miocene, older and newer 

 Pliocene — these divisions being founded on the relative number of 

 species in each group which were identifiable with species now 

 living. 



In 1830 Sir C. Lyell became acquainted with the eminent French 

 conchologist, M. Deshayes, who had, previous to his introduction to 

 Sir Charles, been arranging the numerous Tertiary shells in his 

 possession into three groups, which, in the main, agreed with the 

 divisions arrived at by our English geologist after several years of 

 labour in the field ; and, in order to give greater weight to these 

 divisions. Sir Charles induced M. Deshayes to prepare a series of 

 tables to be appended to the third volume of the "Principles" 

 already quoted. 



Sir Charles Lyell and M. Deshayes were, however, perfectly 

 aware in 1833 that the per-centage test would continually require 

 modification, as every fresh discovery would, in a measure, alter 

 the numerical results they had provisionally published ; but they 

 also felt that the main fact would remain unaltered, namely, that as 

 our Tertiary formations approach in age to the recent period, they 

 will be found to contain a larger proportion or per-centage of living 

 forms. Moreover, as was surmised by Sir Charles himself, geologists 

 would subsequently find it necessary to subdivide each of his large 

 divisions into two, three, or more stages. Indeed, as has been so 

 often observed by geologists as well as by palaeontologists, lines of 

 demarcation are established more for convenience of reference than 

 in order to denote any absolute interruption in the regular sequence 

 of sedimentary deposition, or of life on the surface of om- globe. 

 These supposed breaks are very often due to local conditions, which 

 may occur in one and not in another field of observation. We are 

 likewise far from having discovered the laws which regulate the 

 gradual succession of life ; and we are, I fear, much too apt to guess 

 at the origin of species, and to interpret those unknown laws from a 

 small number of incomplete observations. The assiduous researches 

 which for many years I have made among the living and fossil 

 species of Brachiopoda, have to a certain extent imbued my mind 

 with the idea that an individual species may have been gradually 

 very much modified in time so as to suit the conditions under which 

 it had to exist, but at the same time everyone who has studied with 

 any degree of care any class composing the animal kingdom, must 



