47 6 THE JOURNAL OF GEOLOGY. 



be sought in the records of the past or reconstructed by com- 

 parison. 



Even at the present day things are somewhat different in the 

 open ocean, and they must have been very different in the prim- 

 itive ocean, for a pelagic animal has no fixed home, one locality 

 is like another, and the competitors and enemies of each individ- 

 ual are determined in great part by accidents. 



We accordingly find, even now, that the evolution of pelagic 

 animals is often linear instead of divergent, and ancient 

 forms, such as the sharks, often live on side by side with the 

 later and more evolved forms. The radiolarians and medusae 

 and siphonophores furnish many well-known illustrations of this 

 feature of pelagic life. 



No naturalist is surprised to find in the South Pacific or in the 

 Indian Ocean a salpa or a pelagic crustacean or a surface fish or 

 a whale which was previously known only from the North Atlan- 

 tic, and the list of species of marine animals which are found in 

 all seas is a very long one. The fact that pelagic animals are so 

 independent of those laws of geographical distribution which 

 limit land animals is additional evidence of the easy character of 

 the conditions of pelagic life. 



One of the first results of life on the bottom was to increase 

 asexual multiplication and to lengthen the time during which 

 buds remain united to and nourished by their parents, and to 

 crowd individuals of the same species together and to cause 

 competition between relations. We have in this and other 

 obvious peculiarities of life on the bottom a sufficient explana- 

 tion of the fact that since the first establishment of the bottom 

 fauna, evolution has resulted in the elaboration and divergent 

 specialization of the types of structure which were already estab- 

 lished rather than the production of new types. 



Another result of the struggle for existence on the bottom 

 was the escape of varieties from competition with their allies 

 by flight from the crowded spots and a return to the open water 

 above ; just as in later times the whales and sea birds have gone 

 back from the land to the ocean. 



