594 General and Applied Biology • 



migration of a number of individuals from the overcrowded area. (2) 

 There can be an extermination of a number of individuals by parents, 

 brothers, sisters, or other species. (3) There can be a natural death for 

 a sufficient number so that a balance again can be realized. (4) The 

 problem of overpopulation may also be regulated by a reduction in the 

 number of offspring produced. This is a factor which cannot easily be 

 controlled, especially among the lower animals. (5) A different type of 

 food can be utilized if the struggle should develop around this very im- 

 portant factor. 



The principle of definite habitats shows that the home or habitat of a 

 particular species is determined by such physical factors as the following: 

 ( 1 ) The quantity and quality of foods. The herbivorous animals, such 

 as deers, must be near suitable vegetation. The carnivorous animals, 

 such as tigers and lions, must live near a source of suitable flesh foods. 

 The omnivorous animals, such as man, can usually be more widely dis- 

 tributed, although they must also be distributed so as to get the proper 

 types of both vegetable and animal foods. (2) The quantity and quality 

 of water also affect the selection of a habitat by a particular species. A 

 certain amount of water is essential for all animals because their bodies 

 are made of 50 to 95 per cent water. Many forms in dry climates prevent 

 excessive evaporation by some type of thick covering. Many species 

 found under rocks are not always there in order to shun light but for 

 moisture and protection. The depth, salinity, and hydrogen-ion concen- 

 tration of the water also are important factors in influencing the selection 

 of the proper habitat. (3) The quantity and quality of the air are also 

 influential in determining the habitat selected by a certain species of ani- 

 mal. This is particularly true for terrestrial forms, although the avail- 

 ability of air in the water also affects the aquatic types. (4) The quantity 

 and quality of light, including sunshine, is a factor which is influential 

 in the distribution of many animals. Some types shun light for protec- 

 tion and for reduction of heat produced in their bodies. Others actually 

 require certain amounts of light for their various normal metabolic activ- 

 ities, (5) The presence or absence of an optimum temperature may be 

 a determinative influence in animal distribution. It is well known that 

 animals will tend to seek the temperature for which they are particu- 

 larly fitted. Many animals living in tropical regions pass the summer in 

 a condition of aestivation or semitorpid condition of semiactivity. Cer- 

 tain animals living in colder climates pass the winter in various ways: 

 (1) Hibernation, or a period of inactivity in some protected location. 

 Such animals as frogs, turtles, snakes, the larvae and pupae of insects. 



