288 THE ORIGIN OF THE OLDEST FOSSILS, ETC. 



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 individual 

 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 siphono- 

 phores 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 Atlantic, 

 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 pecuHarities 

 of life on the bottom a sufficient explanation of the fact that since 

 the first establishment of the bottom fauna, evolution has resulted 

 in the elaboration and divergent specialisation of the types of 

 structure, which were already established, rather than the pro- 

 duction 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. These emigrants, like the civilised men 

 who invade the homes of peaceful islanders, brought with them 

 the improvements which had come from fierce competition, and 

 they have carried everything before them and produced a great 

 change in the pelagic fauna. 



