204 THE WORLD OY LIFE 



terns " of the geologist. These phenomena are admirably ex- 

 plained in Professor James Geikie's attractive and well-illus- 

 trated volume on '^ Earth Sculpture or the Origin of Land 

 Forms," published in 1898. Here I can only attempt to 

 sketch in outline the successive stages of life which are ex- 

 hibited in the rocks, and point out some of their most striking 

 features with the conclusions to which they lead us. 



During the latter part of the eighteenth century geologists 

 were beginning to obtain some detailed knowledge of the 

 earth's crust and its fossils, and arrived at a first rude di- 

 vision into primitive, secondary, and tertiary formations. 

 The first were supposed to represent the epoch before life ap- 

 peared, and comprised such rocks as granite, basalt, and 

 crystalline schists. 'Next above these came various strata of 

 sandstones, limestones, and argillaceous rocks, evidently of 

 aqueous origin and often containing abundant fossils of 

 marine, fresh-water, or terrestrial animals and plants. The 

 tertiary were clearly, of more recent origin, and contained 

 shells and other remains often closely resembling those of liv- 

 ing animals. It was soon found, however, that many of the 

 rocks classed as " primitive " either themselves produced 

 fossils, or were found overlying fossiliferous strata; and, by a 

 more careful study of these during the early part of the nine- 

 teenth century, the three divisions were more precisely limited 

 — the first or '^ Primary,'' as containing the remains of Mol- 

 lusca, Crustacea, and some strange fishes and amphibians; 

 the '' Secondary," by the first appearance of reptiles of many 

 strange forms ; and the ^' Tertiary," by abundance of Mam- 

 malia of all the chief types now existing, with others of new 

 and apparently primitive forms, or serving as connecting links 

 w^ith living groups. 



It is a very remarkable fact, not sufiiciently dwelt upon in 

 geological treatises, that this first grouping of the whole of 

 the life-forms of the past into three great divisions, at a time 

 when our knowledge of extinct animals and plants was ex- 

 tremely scanty as compared with what it is now, should still 



