SALPA IN RELATION TO EVOLUTION OF LIFE. 167 



upon the bottom was to increase asexual multiplication and to 

 lengthen the time during wliich 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, evolu- 

 tion has resulted in the elaboration 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 more important era of rapid development in 

 the forms of animal life has never received the attention which it 

 so well merits. 



If the views which I have advanced are correct, the primitive 

 bottom fauna 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 water deep enough 

 to be beyond the influence of the shore. 



3. The great grou]>s of metazoa were rapidly established from 

 pelagic ancestors. 



4. There was a rapid increase in the size of the bottom animals 

 and hard parts were quickly acquired. 



