374 THE ORIGIN OP THE OLDEST FOSSILS. 



itors aud enemies of each individual are determined in great part by 

 accidents. We accordingly iind, even now, that the evolution of 

 pelagic animals is often linear instead of divergent, and ancient forms, 

 such as the sharks, often live on si<le by side with the later and more 

 evolved forms. The radiolarians and medusje and siphonophores fur- 

 uish 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 Saliva 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 evi- 

 dence of the easj^ character of the conditions of pelagic life. 



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

 nuiltiplication and to lengthen the time during which l)uds remain 

 united to and nourished by their parents, and to crowd individuals of 

 the same species together and to cause comi^etition between relations. 

 We have in this and other obvious peculiarities of life on the bottom a 

 sufficient explanation of the fiict that since the first establishment of 

 the bottom fauna, evolution has resulted in the elaboration and diver- 

 gent si)ecialization of the types of structure which w^ere 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 tiight from the 

 crovrded spots and a return to the open water above; just as in later 

 tinfes the Avhales 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 have carried everything before 

 them and i)roduced a great change in the pelagic fauna. 



The rapid intellectual development whicli has taken place among the 

 mammals since the Middle Tertiary, and the rapid structural changes 

 which took place in animals and ])lants when the land fauna and Hora 

 were established, are well known, but the fact that the discovery of the 

 bottom initiated a much earlier and probably more important era of 

 rapid development in the forms of animal life has never been pointed out. 



If this view is correct the i^rimitive fauna of the bottom must have 

 had the following characteristics: 



(1) It was entirely animal, without plants, and it at first depended 

 directly upon the pelagic food supply. 



(2) It was established around elevated areas in Avater deep enough 

 to be beyond the influence of the shore. 



(."'>) The great groups of animals were rapidly established from pelagic 

 ancestors. 



(4) The animals of the bottom rapidly increased in size and hard 

 parts were quickly acquired. 



