W. K. BROOKS ON THE GENUS SALPA. 155 



but there is no such limit to the growth of pelagic plants or the animals 

 which feed upon them, and while the balance of life is undoubtedly 

 adjusted, competition for food is never very fierce even at the present 

 day, when the ocean swarms with highly organized animals which have 

 become secondarily adapted for a pelagic life. Even now the destruction 

 or escape of a microscopic pelagic organism depends upon the accidental 

 proximity or remoteness of an enemy rather than upon defense or pro- 

 tection, and survival is determined by space relations rather than by a 

 struggle for existence. 



The abundance of food is shown by the ease with which wanderers 

 from the land, like birds, find places for themselves in the ocean, and the 

 rapidity with which they spread over its whole extent. 



As a marine animal, the insect, halobates, must be very modern as 

 compared with most pelagic forms, yet it has spread over all tropical and 

 subtropical seas, and it may always be found skimming over the surface 

 of the water as much at home as a gerris in a pond. I never found it 

 absent in the Gulf Stream when conditions were favorable for collecting. 



The easy character of pelagic life is also shown by the fact that the 

 larvas of innumerable animals from the bottom and the shore have 

 retained their pelagic habit, and I shall soon refer to facts which prove 

 that the larva of a shore animal is safer at sea than it is near the shore. 

 The absence of fierce competition in the open ocean is well shown by the 

 simultaneous existence in the modern ocean of graded stages in the 

 evolution of a type, such as the series of Pelagidae; and also by the 

 persistency of a stem form like the elasmobranch, side by side with, 

 and often in competition with, various improved lines of divergent 

 descendants. 



In the primitive pelagic fauna and flora there was little opportunity 

 for an organism to gain superiority by seizing upon an advantageous 

 site or by acquiring peculiar habits, for one place was like another, and 

 peculiar habits could count for little in comparison with accidental space 

 relations. 



After the pelagic fauna had been enriched by the addition of all the 

 marine animals which are secondarily pelagic, competition with these 

 improved forms from the bottom brought about improvements in those 

 which were strictly pelagic in their origin, and through this competition, 

 complicated animals of considerable size, like the siphonophores, have 

 been evolved at the surface, but while their whole history has thus been 

 pelagic they are not primitively pelagic ; that is, they are not the out- 



