W. K. BROOKS ON THE GENUS SALPA. 177 



the same time, that an immense interval has elapsed between the begin- 

 ning of life and the epoch represented by the olenellan fauna. He 

 says: "That the life in the pre-olenellus seas was large and varied, 

 there can be little, if any, doubt. The few traces known of it prove little 

 of its character, 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 immeasur- 

 able 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 all 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 primitive 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 resem- 

 bled 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 approximation to the primitive fauna of the 

 bottom, beyond which life was represented only by simple and minute 

 pelagic organisms. 



