CHAPTER XVI 

 ENVIRONMENT AND DISTRIBUTION 



The environment of an animal includes biological factors, 

 mainly breeding and feeding conditions, as well as physical fac- 

 tors, such as temperature, moisture, light, altitude, etc., all of 

 which are reflected in its adaptations. Darwin, in discussing 

 the nature of the struggle for existence, illustrated the compli- 

 cated interplay of biological factors involved in the production of 

 red clover (Trifolium repens). The fertilization of red clover 

 depends upon the visits of bumblebees, which in their search 

 for nectar unwittingly carry pollen from one flower to another. 

 The number of bumblebees in any district is controlled in part 

 by the number of field mice, which destroy the combs and nests 

 of the bees. The number of field mice in turn is regulated by 

 the number of cats or other enemies of the field mice. Hence the 

 number of cats may become an important factor in determining 

 the size of the clover crop. Every living thing is beset with 

 biological hazards which it must surmount in order to survive. 

 The background for the interplay of biological forces is the 

 physical environment, some aspects of which will be briefly 

 summarized in the following paragraphs. The two major 

 physical environments are water and land. 



Water. — The most obvious adaptive features found in animals 

 living submersed are means of obtaining sufficient oxygen for 

 respiration and means of locomotion in a fluid medium. The 

 source of oxygen available for respiration is air dissolved in the 

 water and oxygen generated by aquatic chlorophyll-bearing 

 plants. The water of a well-aerated stream contains from 9 to 

 10 cc. of oxygen per liter, which is about 3^o of the amount of 

 oxygen in an equal volume of air. However, the amount of 

 oxygen available to aquatic animals may be greater than this 

 if the water contains green plants. Aquatic animals are adjusted 

 or adapted physiologically to a relatively low supply of oxygen 

 because they are poikilothermous, which eliminates the necessity 

 of maintaining a body temperature higher than that of the water. 



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