468 THE JOURNAL OF GEOLOGY. 



selves 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 sub-tropical seas, and it may always be found skim- 

 ming over the surface of mid-ocean 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 shown by the fact that 

 the larvae of innumerable animals from the bottom and the shore 

 have retained their pelagic habit, and I shall soon give reasons for 

 believing that the larva of a shore animal is safer at sea than 

 near the shore. 



There was little opportunity in the primitive pelagic fauna 

 and flora 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 fauna of 

 the surface had been enriched by all the marine animals which 

 have become secondarily adapted to pelagic life, competition 

 with those improved forms brought about improvements in those 

 which were strictly pelagic in origin, like the siphonophores, and 

 those wanderers from the bottom introduced another factor into 

 the evolution of pelagic life, for their bodies have been utilized 

 for protection or concealment and in other ways, and we now 

 have fishes which hide in the poison curtain of Physalia, Crus- 

 tacea which live in the pharynx of Salpa or in the mouth of the 

 manhaden, barnacles and sucking fish 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 very simple, and while 

 competition was not entirely absent the possibilities of evolution 

 must have been extremely limited and the progress of divergent 

 modification very slow so long as all life was restricted to the 

 waters of the ocean. 



There can be no doubt that floating life was abundant for a 



