226 THE FOUNDATIONS OF ZOOLOGY 



on the bottom or on the shore. Embryology also gives us good 

 ground for believing that all these animals are still more remotely 

 descended from minute and simple pelagic ancestors, and that 

 the history of all the highly organized inhabitants of the water 

 has followed a roundabout path from the surface to the bottom 

 and then back into the water. When this fact is seen in all its 

 bearings, and its full significance is grasped, it is certainly one of 

 the most notable and instructive features of evolution. 



The food supply of marine animals consists of a few species 

 of microscopic organisms which are inexhaustible and the only 

 source of food for all the inhabitants of the ocean. The supply 

 is primeval as well as inexhaustible, and all the life of the ocean 

 has gradually taken shape in direct dependence upon it. In view 

 of these facts we cannot but be profoundly impressed by the 

 thought that all the highly organized marine animals are products 

 of the bottom or the shore or the land, and that while the largest 

 animals on earth are pelagic, the few that are primitively pelagic 

 are small and simple. The reason is obvious. The conditions of 

 life at the surface are so easy that there is little fierce competition, 

 and the inorganic environment is so simple that there is little 

 chance for diversity of habits. 



The growth of terrestrial plants is limited by the scarcity of 

 food, but there is no such limit to the growth of pelagic plants 

 or the animals which feed on them, and while the balance of life 

 is no doubt adjusted by competition for food, this is never very 

 fierce, even at the present day, when the ocean swarms with highly 

 organized wanderers from the bottom and the shore. Even now 

 the destruction or escape of a microscopic pelagic organism de- 

 pends upon the accidental proximity or remoteness of an enemy 

 rather than upon defence or protection, and survival is determined 

 by space relations rather than a struggle for existence. 



The abundance of food is shown by the ease with which wan- 

 derers from the land, like sea-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 skim- 

 ming over the surface of mid-ocean as much at home as a Gerris 



