170 W. K. BROOKS. 



any, doubt. The few traces known of it prove little of its charac- 

 ter, but they prove that life existed in a period far preceding lower 

 Cambrian times, and they foster the hope that it is only a question 

 of search and favorable conditions to discover it." 



No one can question the validity of the basis for Walcott's hope, 

 for pelagic animals have undoubtedly established themselves on the 

 shores of elevated tracts again and again, during the oscillations of 

 the sea-bottom, and we have every reason to expect and look for 

 their remains. 



If, however, it is true that the primitive stem-forms were pelagic 

 and minute, there is little hope of finding their delicate microscopic 

 remains in the sedimentary rocks of the shore. 



The Cambrian fauna is usually regarded as a half-way station in 

 a series of organisms which reaches back into the past for an im- 

 measurable period, and it is even stated that the history of life 

 before the cambrian is longer, by many fold, than its history since. 



So far as this opinion rests on the diversity of types in cambrian 

 and silurian times it has no good basis, for if the view which I have 

 advocated is correct, the evolution of the ancestral stem-forms took 

 place at the surface, and alj the necessary conditions for the rapid 

 production of types were present when the bottom fauna first became 

 established. 



As we pass backwards towards the lower cambrian we find closer 

 and closer agreement with the biological conception of the primi- 

 tive life at the bottom. 



We cannot regard the olenellan fauna as the first bottom fauna, 

 for it contains forms which have been secondarily adapted for a 

 pelagic life, such as the pteropods. 



We may, however, feel confident that the first bottom fauna re- 

 sembled that of the lower cambrian in its physical conditions, and 

 in its most distinctive peculiarity, the abundance of types and the 

 slight amount of differentiation among the representatives of these 

 types. 



Far from seeing in the lower cambrian fauna a half-way station 

 in a long series of bottom animals, the biologist must regard it as 

 an unmistakable and decided aj^proximation to the primitive fauna 

 of the bottom, beyond which life was represented only by simple 

 and minute pelagic organisms. 



