174 JOHNS HOPKINS UNIVEESITY MORPHOLOGICAL MONOGRAPHS. 



Even at the present day things are somewhat different in the open 

 ocean, and they must have been very different in the primitive 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 the early steps 

 in the series often live on side by side with the later and more evolved 

 forms. The radiolarians and the medus* and the siphonophores furnish 

 many well-known illustrations of this feature of pelagic life. 



No one is much surprised to find in the South Pacific or in the Indian 

 Ocean a salpa, or a pelagic crustacean, or a surface fish which has pre- 

 viously been known only in 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 geo- 

 graphical distribution which limit land animals is additional evidence of 

 the easy character of the conditions of pelagic life. We have seen that 

 one of the first results of life upon the bottom was to increase asexual 

 multiplication and to lengthen the time during which buds remained 

 united to and nourished by their parents. One result of this is the 

 crowding together of individuals of the same species, and competition 

 between relations. We have in this and in other obvious peculiarities 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 elabo- 

 ration and divergent specialization of the types of structure which were 

 already established, rather than in 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 cetacea and sea-birds have gone back from the land to the 

 ocean. These emigrants, like the civilized men who invade the homes of 

 peaceful islanders, brought with them the improvements which had 

 come from fierce competition, and they carried everything before them 

 and produced a great and rapid change in the character of the pelagic 

 fauna. 



The rapid intellectual improvement which has taken place among 

 the mammalia since the middle tertiaries, and the rapid structural 

 development which took place in animals and plants when the land 

 fauna and flora were first established, are well known, but the fact that 

 the discovery of the bottom initiated a much earlier, and probably much 



