NATURAL SELECTION, AND THE ANTIQUITY OF LIFE 235 



the types of structure which were already established, 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. 



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 every- 

 thing before them and produced a great change in the pelagic 

 fauna. 



The rapid intellectual development which has taken place 

 among the mammals since the Middle Tertiary, and the rapid 

 structural changes which took place in animals and plants when 

 the land fauna and flora 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 primitive We of the bottom must 

 have had the following characteristics : 



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

 pended 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 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. 



(5) The bottom fauna soon produced progressive development 

 among pelagic animals. 



(6) After the establishment of the fauna of the bottom, elabo- 

 ration and differentiation among the representatives of each primi- 

 tive type soon set in and led to the extinction of connecting 

 forms. 



Many of the oldest fossils like the pteropods are the modified 

 descendants of ancestors with hard parts, and there is no reason 

 to suppose that the first animals which were capable of preserva- 



