1867.] Sir Charles Lijell and Modern Geology. 5 



commonly known as the "percentage test." The Eocene strata 

 were defined as having only about one-thirtieth part of their Mol- 

 lusca identical with living species, the Miocene as having about 

 one-fifth, the Older Pliocene from one-third to one-half, and the 

 Newer Pliocine nine-tenths. 



The terms Eocene, Miocene, and Pliocene were at once adopted, 

 together with the principle of classifying the Tertiary strata by 

 means of the percentage of recent shells winch they contain. But 

 of late years several geologists and palaeontologists have raised 

 objections to the percentage test, as did Mr. Charles worth when it 

 was first proposed. It may not be out of place, therefore, to dis- 

 cuss the advantages that have accrued from its adoption during the 

 last thirty years, and the probability of its eventually becoming 

 superseded by some other mode of classification. 



Sir Charles Ly ell's classification depends on two principles ; first, 

 that the Mollusca are typical of the rest of the animal kingdom, 

 and are, at the same time, the most convenient for the purpose ; and 

 secondly, that the percentage of recent species in a fauna varies in- 

 versely with its age. In the first place, it is certain that the fossils 

 which are most generally and completely preserved belong to the 

 class Mollusca. The shells of these animals are so durable, so easily 

 recognized, and, generally speaking, so characteristic of the species 

 to which they belong, that they form a much better medium of 

 comparison than the remains of land-animals or of plants, which 

 generally occur in a fragmentary condition, and the preservation of 

 which is usually the result of some fortuitous circumstance. Again, 

 other classes of marine organisms are either not so abundant in 

 species and individuals, or they are not likely to be preserved in 

 the fossil state. It is difficult to say whether the Mollusca are, or 

 are not, typical, in their duration, of the rest of the animal kingdom. 

 They are not so prone to change as higher organisms ; but, on the 

 other hand, species of shells do not, as a r-ule, exist through several 

 geological periods, like species of Foraminifera. We should imagine, 

 however, that while the scale furnished by each class of animals 

 is true, each scale has a value of its own, which has a certain ratio 

 to that of each of the rest. The scale furnished by the Mollusca 

 being neither too large nor too small, is on this account preferable 

 to several others ; therefore, from every point of view the Mollusca 

 seem more convenient for the purpose than any other group of 

 organisms. 



The proposition that the greater the age of a Tertiary fauna the 

 smaller is the proportion of recent species that it contains, is ex- 

 tremely difficult either to prove or to disprove ; but even if it can 

 now be shown to be untrue, it was at the time the percentage test 

 was proposed a most convenient fiction. Sir Charles Lyell urged 

 that the recent faunas formed a common point of departure in all 



