4i8 



THE GENERAL PRINCIPLES OF ZOOLOGY 



life is the most complete of all. Again, the fossiliferous rocks 

 which are readily accessible for investigation are relatively few ; 

 vast masses are still buried deeply beneath the water ; those 

 which form the dry land are exposed to a very small extent 

 only, and we must often rely on excavations for roads and 

 canals and similar constructions for bringing buried strata to 



light. Thus it will 

 be at once apparent 

 that only a very 

 small number of 

 the fossils which 

 the earth contains 

 are in the posses- 

 sion of man ; great 

 areas of the land 

 have not been ex- 

 plored at all, and 

 our knowledge of 

 this subject will be 

 vastly increased by 

 future researches. 

 It has been esti- 

 mated that the 

 stratified rocks con- 

 stitute a mass which 

 ranges from 60,000 

 to 90,000 meters 

 thick. Of this two 

 thirds, or from 

 40,000 to 60.000 

 meters, belong to 



Fig. 395. Pterichthys testudinarius. A, dorsal aspect; B, 

 ventral ; C, lateral, c.f, caudal fin ; d.f, dorsal fin ; pet./, pec- 

 toral tin. (From the Brit. Mus. Cat. of Fossil Fishes.) 



the Archaean era, 

 in which there are no fossils. The composition of these rocks 

 shows us, however, that they once contained organic remains, 

 but all the rocks of this group have become so altered that there 

 is not the slightest evidence from the rocks themselves as to 

 what the fossils were which they contained. Thus the first, 

 the most important, and the longest chapter in the record of 

 prehistoric life is absolutely closed, but much remains, and it 



