14 Introduction 



nowhere else. A considerable number of types of animals which 

 had their origin in, or became adjusted to life in, fresh water or on 

 land have entered the ocean. Fishes began in fresh water but now 

 range through the ocean at all depths. Many extinct and modern 

 reptiles took up life in the ocean. Whales, pinnipeds, and certain 

 birds, like the albatross and penguin, are truly oceanic, though their 

 ancestors probably lived on land. Though insects are ubiquitous, 

 aggressive, and dominant in many situations on land and occur in 

 considerable numbers in fresh water, they have invaded the ocean 

 very little. Alongshore are a few flies, midges, beetles, and apterous 

 insects. Water striders run over the surface of the ocean in warm 

 climates. Only one species of insect (a crane-fly) is submarine at all 

 stages of its Ufe cycle (Buxton, 1926; Tokunaga, 1930) . However, 

 if a pond is cut off from the ocean and becomes to some degree 

 fresh, it is speedily invaded by insects — Diptera, Coleoptera, Odo- 

 nata, Hemiptera, etc. (Pearse, 1932e) . Cold sea water is more pro- 

 ductive of life than that near the equator, perhaps because more 

 trihydrol and oxygen are present in it or because conditions for 

 nitrification and denitrification are more favorable. Trihydrol as 

 colloidal water particles perhaps has a catalytic effect which favors 

 the growth of aquatic organism.s (Barnes, 1932a) . 



Although there is more than twice as much water as land sur- 

 face on the earth (Fowler, 1928) , the fresh-water areas of the world 

 are isolated and limited in extent. Even the great lakes in Africa 

 and North America are minute compared with the ocean. Fresh 

 water is often variable in amount, as in swamps and rivers; is often 

 subject to considerable variations in temperature, daily and seasonal, 

 as in pools; and in various situations there may be wide differences 

 in organic and inorganic content and reaction. In a shallow, stag- 

 nant pond or swamp where the surface is covered thickly with float- 

 ing vegetation there may be no oxygen in the water at night. Such 

 a condition is especially characteristic in the tropics. In eutrophic 



