SALPA IN RELATION TO EVOLUTION OF LIFE. 145 



The food-supply of the ocean consists of a few species of uni- 

 cellular microscopic plants, and of a few simple protozoa which 

 feed upon them. This supply is inexhaustible, and it is the only 

 source of food for all the inhabitants of the ocean, except a few 

 which live upon floating sargassum and the littoral algse, and the 

 drainage from the land. 



Many marine animals are adapted for direct subsistence upon 

 these organisms, and some of them, like Salpa, are universally 

 distributed and are found in enormous numbers in all parts of 

 the ocean. 



The food-supply is not only inexhaustible, it is also primeval, 

 and all the life of the ocean has gradually taken shape in direct 

 dependence upon it during the history of its evolution. 



In view of all these facts we cannot but be profoundly impressed 

 by the thought that all the highly organized marine animals are 

 products of the bottom, or of the shore, or of the land, and that 

 while the largest animals on earth are pelagic, the few which are 

 primitively pelagic are very small and very simple. 



The reason is obvious. The conditions of pelagic life are so easy 

 that tiiere is no 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 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 remote- 

 ness of an enemy rather than upon defense or protection, and sur- 

 vival 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 

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