SALPA IN RELATION TO EVOLUTION OF LIFE. 169 



fauna did not live upon the outer shore, facing the ocean, but on 

 the shores of interior seas, straits, or lagoons that occupied the 

 intervals between the several ridges that ran from the central plat- 

 form east and west of the main continental land-surface of the 

 time." 



The lower cambrian fauna was rich and varied, but it was not 

 self-supporting, for no fossil plants are found, and the primary 

 food-supply was pelagic. Animals adapted for a rapacious life at 

 the surface, such as the pteropods, were abundant, and they prove 

 the existence of a rich supply of pelagic animals. All the forms 

 are either carnivorous animals, such as medusae, corals, Crustacea 

 and trilobites, or they are adapted, like the sponges, brachiopods 

 and lamellibranchs, for straining minute organisms out of the 

 water, or for gathering up those which rained down from above, 

 and the conditions under which they lived were obviously very 

 similar to those on the bottom at the present day. 



Walcott's studies show that the earliest known fauna had the 

 following characteristics : 



1. So far as the record goes it consisted of animals alone, and 

 these animals were dependent upon the pelagic food-supply for 

 support, 



2. While small in comparison with many modern animals, they 

 were gigantic in size as compared with primitive pelagic animals. 



3. The species were few, but they represented a very wide range 

 of types. 



4. All the types have modern representatives, and most of the 

 modern types are represented in the lower cambrian. 



5. The habitat was not the bottom of the deep ocean, but the 

 submerged surface of a sinking continent, under water of con- 

 siderable depth. 



Remains of bottom animals are found in rocks below the cam- 

 brian, and Walcott believes that while the olenellan fauna adds 

 a little more to our knowledge of the rate of convergence back- 

 wards in geological time of the lines representing the evolution of 

 animal life, it also proves, at the same time, that an immense 

 interval has elapsed between the beginning 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, it 



