NATURAL SELECTION, AND THE ANTIQUITY OF LIFE 21 7 



brain of a modern mammal is more than twice as large, compared 

 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 paleozoic 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 evolution 

 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 abun- 

 dant long before, the evolution of animals likely to be preserved 

 as fossils took place with comparative rapidity, and that the zoologi- 

 cal 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 repre- 

 sented 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 ptero- 

 pods and gasteropods and lamellibranchs, of Crustacea and medusae, 

 echinoderms and brachiopods, that he now has at a marine labora- 

 tory ; that his studies would have followed the same lines then 

 that they do now, and that most of the record of the past which 

 they make known to him would have been ancient history then. 

 Most of the great types of animal life show by their embryology 

 that they run back to simple and minute ancestors which lived at 

 the surface of the ocean, and that the common meeting point must 

 be projected back to a still more remote time, before these ancestors 

 had become differentiated from each other. 



After we have traced each great line of modern animals as far 

 backward as we can through the study of fossils, we still find these 



