THE ORIGIN OF THE OLDEST FOSSILS, ETC. 457 



occurrence of several periods when modification was compara- 

 tively rapid. 



We are living in a period of intellectual progress, and, among 

 terrestrial animals, cunning now counts for more than size or 

 strength, and fossils show that while the average size of mam- 

 mals has diminished since the middle Tertiary, the size of their 

 brains has increased more than one hundred per cent. ; that the 

 brain of a modern mammal is more than twice as larafe, com- 

 pared with its body, as the brain of its ancestors in the middle 

 Tertiary. Measured in years the middle Tertiary is very remote, 

 but it is very modern compared with the whole history of the 

 fossiliferous rocks, although more of brain development has been 

 effected in this short time than in all preceding time from the 

 beginning. 



The later palaeozoic and early secondary fossils mark another 

 period of rapid change, when the fitness of the land for animal 

 life, and the presence of land plants, brought about the evolu- 

 tion of terrestrial animals. 



I shall give reasons for seeing, in the lower Cambrian, another 

 period of rapid change, when a new factor, the discovery of 

 the bottom of the ocean, began to act in the modification of 

 species, and I shall try to show that, while animal life was 

 abundant long before, the evolution of animals likely to be pre- 

 served as fossils took place with comparative rapidity, and that 

 the zoological features of the lower Cambrian are of such a 

 character as to indicate that it is a decided and unmistakable 

 approximation to the primitive fauna of the bottom, beyond 

 which life was represented only by minute and simple surface 

 animals not likely to be preserved as fossils. 



Nothing brings home more vividly to the zoologist a picture 

 of the diversity of the lower Cambrian fauna, and of its intimate 

 relation to the fauna on the bottom of the modern ocean, than 

 the thought that he would have found on the old Cambrian shore 

 the same opportunity to study the embryology and anatomy of 

 pteropods and gasteropods and lamellibranchs, of Crustacea and 

 medusae, echinoderms and brachiopods that he now has at a 



