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 
