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CRITICAL EPISODE IN EVOLUTION 
fossils, and give a more perfect record of the early history of 
life than paleontology. 
While the history of life, as told by fossils, has been slow and 
gradual it has not been uniform, for we have evidence of the 
occurrence of several periods when modification was comparatively 
rapid. 
We are living in a period of intellectual progress; and, among 
terrestrial animals, cunning now counts for more than size and 
strength. Fossils show that while the average size of mammals 
has diminished since Mid Tertic times, the size of their brains 
has increased more than one hundred per cent. The brain of a 
modern mammal is more than twice as large, as compared with its 
body, as the brain of its Mid Tertic ancest6rs. Measured in years 
the Mid Tertic Period is very remote, but is very modern com¬ 
pared 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 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 Early Cambric record, 
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 during Early Cambric time are of such 
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. 
To the zoologist nothing brings home more vividly a picture of 
the diversity of the Early Cambric fauna, and of its intimate 
relation to the fauna of the bottom of the modern ocean, than 
the thought that he would have found on the old Cambric shores 
the same opportunity to study the embryology and anatomy of 
the Pteropods and Gasteropods and Lammellibranchs, of the 
Crustaceans and Medusae, Echinoderms and Brachiopods that he 
now has at a marine laboratory; -that his studies would have 
