18 ECOLOGICAL FOUNDATIONS OF ZOOGEOGRAPHY 



dark coloration of animals of high mountains and at times in deserts 

 where there is little protection from the sun's rays. 



Oxygen and carbon dioxide. — Oxygen is of primary importance 

 to life, although a few animals, which are able to supply themselves 

 with energy by the decomposition of a rich food supply, can live 

 without it; certain annelid worms, fly larvae, and internal parasites 

 are good examples. The ability to live without free oxygen, therefore, 

 scarcely affects the problems of animal distribution except among 

 parasites and in stagnant or polluted waters. Though oxygen is nearly 

 everywhere available, there is a much more abundant supply in air 

 than in water, an important fact for the explanation of the differences 

 between water- and air-breathing animals. Oxygen is absent, or nearly 

 so, in a few aquatic situations. In the depths of the Black Sea, and in 

 many Norwegian fiords which are closed by a bar, the evolution of 

 hydrogen sulphide has combined all the available oxygen. In the inter- 

 mediate depths of tropical seas, where the vertical circulation is very 

 weak, poverty in this gas is the rule. Many fresh-water lakes are 

 without oxygen in their depths in summer. On account of the putre- 

 faction in sewage, the oxygen is often used up in rivers below large 

 cities. The Thames, whose waters contain 7.4 cc. of oxygen per liter 

 above London, has only 0.25 cc. per liter below the city. Only a few 

 animals are able to persist in such situations; these are forms which 

 can reduce their oxygen requirements. Most animals have a large 

 oxygen requirement with a narrow range of variation; these include 

 all air-breathers. In nature there is rarely too much oxygen for animal 

 life, but that given off by aquatic plants has been found to be toxic 

 to insect larvae. 25 



In fumaroles, where carbon dioxide escapes from the earth, it may 

 displace the air near the ground on account of its greater density; 

 the Grotto del Cane at Pozzuoli is an instance ; the death valley on the 

 Dieng plateau in Java, is another. 26 Such places are closed to animal 

 life. The bodies of small birds and mammals (finches and mice), which 

 have wandered into the carbon dioxide atmosphere, are frequently 

 found at the Mofetten on the eastern shore of Lake Laach near 

 Coblenz. The amount of carbon dioxide present in water is apparently 

 an important factor in the distribution of aquatic animals and is asso- 

 ciated with the hydrogen-ion concentration (pH) of the water. In the 

 sea, and in the deeper lake waters, there is usually an inverse relation 

 between the amount of oxygen and of carbon dioxide present. Fish 

 avoid waters having a high carbon dioxide tension more actively than 

 they do water deficient in oxygen. The ability of fish to utilize oxygen 

 when present in small amounts is decreased with the higher hydrogen- 



