146 W. K. BROOKS. 



tropical and subtropical seas, and it may always be found skim- 

 ming 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 con- 

 ditions were favorable for collecting. 



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

 the larvse 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 Pelagidse ; 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 oppor- 

 tunity 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 com- 

 parison 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 improve- 

 ments 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 >vhile 

 their whole history has thus been pelagic they are not primitively 

 pelagic ; that is, they are not the outcome of purely pelagic in- 

 fluences. The wanderers from the bottom have introduced another 

 factor in the evolution of pelagic life, for their bodies have been 

 utilized for purposes of protection or concealment or on account of 

 other advantages, and we now have fishes which shelter themselves 

 in the poisoned curtain of physalia ; Crustacea which live in the 

 pharynx of salpa ; barnacles and sucking fishes fastened to whales 

 and turtles, besides a host of external and internal parasites. The 

 primitive ocean furnished no such opportunity, and the conditions 

 of pelagic life must, at first, have been extremely simple. 



Among the higher metazoa and the higher plants size is, in 

 itself, an important factor in evolution. Variations in the con- 



