iv PLEISTOCENE MAMMALIA. 



Nomenclature — Definition of terms. 



The rest of this chapter must be devoted to the difficult question of the nomen- 

 clature of the great divisions of past time, with which we shall have to deal, and 

 of the geological periods which preceded them. The Tertiary or Camozoic strata 

 were divided by Sir Charles Lyell, in 1833, 1 into three great groups, according 

 to the percentage of existing Mollusca, which was presented in a comparison of 

 3000 fossil with 5000 living forms. The Eocene (»jwc, dawn, Kaivdg, new) or the 

 earliest group contained about 3^ per cent, of living shells, and thus, to speak metaphori- 

 cally, was characterised by the dawn of the testaceous fauna now living in the sea. 

 In the Meiocene {^uwv kciivoq) group alone, the existing forms were much more 

 abundant, being about 17 per cent, of the whole, and considerably less than one half. 

 The upper group was termed Pleiocene (ttXziwv, more, Kaivog, new), presented from 

 35 to 50 and even 90 per cent, of living Testacea. The vast number of fossil species 

 which have since been added to those which formed the basis of this classification has not 

 materially altered its value, but merely rendered the strict definition of the percentages 

 impossible. 



This system of classification, based on the examination of the creatures living in the 

 sea, cannot, strictly speaking, be applied to the dwellers on the land. In the Eocene 

 strata, for example, no living species of Mammalia has ever been discovered, nor even in the 

 Meiocene. Nor, indeed, does it apply to the land vegetation. The Laurel, Sequoia, and 

 Poplar, found in the Chalk of Kansas and Nebraska by the American geological surveyors, 

 are so like those which flourished in Europe during the Meiocene that Professor Heer 

 had no hesitation in assigning them to the Meiocene age. It follows, therefore, that the 

 change in terrestrial life does not proceed at the same rate as that of the marine fauna, 

 and that it is impossible to reason from the one to the other. But, nevertheless, the 

 names are so useful in classification that it is more convenient to adopt them as mere 

 names, without any idea of percentages, than to discard them as being literally untrue 

 according to their original definition. They embalm the important truth of the gradual 

 evolution of animal and vegetable life, and will remain in the literature of geology as a 

 monument of the sagacity of their inventor. 



So far the nomenclature is very simple and accepted by all naturalists; but the 

 names of the rest of the divisions by which geology is linked on to history are very 

 complex and badly defined. Sir Charles Lyell made the Tertiary period to end with the 

 Pleiocene, 2 drawing a hard and fast line between it and the succeeding or Post- 

 pleiocene age. It is undoubtedly true that, looking at the marine shells only, some few 



1 ' Principles,' 1st edit., 3rd vol., 1833 ; 'Antiquity of Man,' 1st edit., p. 3. 



2 'Antiquity of Man,' 1st edit., chap, i ; ' Student's Elements of Geology,' pp. 108-9 (1870). 



