THE ORIGIN OF THE OLDEST FOSSILS, ETC. 271 



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 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 abundant long 

 before, the evolution of animals likely to be preserved as fossils 

 took place with comparative rapidity, and that the zoological fea- 

 tures of the lower Cambrian are of such a character as to indicate 

 that it is a decided and unmistakable approximation to the primi- 

 tive 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 

 marine laboratory ; 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 

 backwards as we can through the study of fossils, we still find 

 these lines distinctly laid down. The lower Cambrian Crustacea, 

 for example, are as distinct from the lower Cambrian echinoderms 

 or pteropods or lamellibranchs or brachiopods as they are from 

 these of the present day, but zoology gives us evidence that the 



