THE ORGANISM AND ITS ENVIRONMENT 377 



were nearer to its location. The term "bee line" is descriptive of the 

 fact that bees orient themselves with great rapidity and return di- 

 rectly toward the hive. Whatever the essential nature of such 

 reactions may be, they illustrate a most remarkable sensitivity of the 

 organism to its environment. 



In Man, behavior is complicated by the ability of the brain to 

 predetermine a response, to exercise will power and imagination. 

 Thus the human behavior pattern is most intricate and as yet only 

 partially studied. Individually the behavior of a human appears to 

 be unique among animals and is only partially predictable. But 

 human populations behave as a whole as do populations of other 

 types of animal life. In both human and other animal population 

 movements one notes the effects of food supply or lack of food, the 

 instinct for perpetuation of the species and for physical protection 

 or escape from impending disaster. 



Animal Communities. From our daily experience we know 

 that certain types of animals are to be found only in certain localities 

 and that no single type is completely isolated from other animals. 

 The study of the relation of the animal to its habitat and to other 

 animals in its environment has for many years been known under 

 the broad term of nature study. During the first quarter of the 

 current century, however, the method of study has been enlarged 

 to include not only observation and survey of the occurrence and 

 distribution of animals, but experimentation and exact physical and 

 chemical study of the characters of the habitat that determine its 

 fauna. Thus ecology has come to take a prominent place in the 

 science of Biology. 



We may begin with the most conspicuous of all environmental 

 factors, namely, temperature, and consider that the average yearly 

 temperatures of regions beginning with the Equator and ending 

 with the Poles constitute a temperature gradient, with its high point 

 at the Equator and its low point at the Poles. Along this gradient it 

 is possible to mark off broad regions that include definite types of 



