ADAPTATION FOR THE INDIVIDUAL 99 



they return to it. When we come to the backboned 

 animals there is a little more tendency to a stationary 

 location. The sun fish may frequent the same reach 

 of the stream, the trout may haunt the same pool, 

 year after year, but a great majority of fishes doubt- 

 less move indiscriminately up and down the stream 

 or about the lake or ocean and are not found two 

 successive days in the same place. The same may be 

 said of frogs. For a time a particular frog may have 

 a fondness for a special bend in the stream, but it is 

 only a temporary fondness, I believe. 



Our own need for shelter is the prime motive in 

 leading us to build a home, and this necessity arises 

 first of all because of our warm blood. What we are 

 accustomed to call cold-blooded animals are not truly 

 so. Their blood holds practically the temperature of 

 their surroundings. As the air or the water in which 

 they live grows warmer or colder the bodies of these 

 creatures alter with it. Consequently they are active 

 when the temperature is high and grow more slug- 

 gish as the thermometer falls. When the day grows 

 distinctly cold the animals may go practically dor- 

 mant. 



Only the birds and mammals have warm blood, and 

 of these the birds are distinctly the warmer. Whereas 

 the temperature of the mammals runs from about 

 ninety-eight to a hundred degrees Fahrenheit, that of 



