168 LYELL'S ELEMENTS OF GEOLOGY. 



Classification of the Tertiary Formations. 



explained ; namely, the comparative resemblance of the testa- 

 ceous fauna of each period to that of the neighbouring seas. 

 There can be no cabalistic virtue in such numbers as 3. 17. or 

 40., which were at first imagined to express correctly the pro- 

 portional number of identical species in three of the tertiary 

 periods ; but until the time arrives when we can obtain the gene- 

 ral acquiescence of conchologists as to the real proportional 

 numbers, we must endeavour to find some readier method of 

 estimating the relation of one fauna to another ; a method not 

 involving the question of the identity or non-identity of every 

 fossil with some known recent species. 



Now, it has been suggested by Dr. Beck that, in order to form 

 such an estimate of the comparative resemblance of the faunas 

 of different eras, we may follow the same plan as would enable 

 us to appreciate the amount of agreement or discrepancy between 

 the faunas now existing in two distinct geographical regions. 



It is well known that, although nearly all the species of mol- 

 lusca inhabiting the temperate zones on each side of the equator 

 are distinct, yet the whole assemblage of species in one of these 

 zones bears a striking analogy to that in the other, and differs in 

 a corresponding manner from the tropical and arctic faunas. By 

 what language can the zoologist express such points of agree- 

 ment or disagreement, where the species are admitted to be dis- 

 tinct? 



In such cases it is necessary to mark the relative abundance 

 in the two regions compared of certain families, genera, and 

 sections of genera ; the entire absence of some of these, the 

 comparative strength of others, this strength being sometimes 

 represented by the numbers of species, sometimes by the great 

 abundance and size of the individuals of certain species. It is, 

 moreover, important to estimate the total number of species 

 inhabiting a given area; and also the average proportion of 

 species to genera, as this differs materially according to climate. 

 Thus, if we adopt comprehensive genera like those of Lamarck, 

 we shall find, according to Dr. Beck, that, upon^an average, there 

 are in arctic latitudes nearly as many genera as species ; in the 

 temperate regions, about three or four species to a genus ; in the 

 tropical, five or six species to a genus. 



The method of which the above sketch conveys but a faint 

 outline, is the more easy of application to the tertiary deposits 

 of Europe, because the conchological fauna of the Eocene period 

 indicates a tropical climate; that of the Miocene strata, a climate 

 bordering on the tropics; and that of the Older and Newer 

 Pliocene deposits, a climate much more closely approaching to, 

 if not the same as, that of the seas in corresponding latitudes. 



