130 NATURE AND LIFE. 
and extended analysis has thus resulted in an instructive 
synthesis, which is one of the most signal acquisitions of the 
experimental method. - 
{. 
All animals have a temperature above that of the gas- 
eous or fluid media in which they live; that is to say, they 
all possess the faculty of producing heat. Warm-blooded 
animals maintain an almost constant temperature in all 
latitudes and all climates. Thus, in polar regions, man, 
mammals, and birds, mark only one or two degrees less than 
they do at the tropics. The mean temperature of birds is 
41° (cent.), and that of mammals 37°. Those animals called 
cold-blooded produce heat also, though in a less degree; 
but their temperature follows the variations of that of the 
surrounding medium, keeping, however, a temperature a 
few degrees higher than it. In reptiles, this excess varies 
from 5° to half a degree; in fish and insects, it is still 
smaller; and, in the wholly inferior species, it rarely reaches 
half a degree. In fine, with animals that vary in tempera- 
ture, the power of resistance to external causes of refrigera- 
tion increases in proportion to the perfection of the organ- 
ization. It is observed, too, that in these beings vital 
activity and the force of respiration have a direct relation to 
the thermometric state; thus, in a medium of 7°, lizards 
consume eight times less oxygen than at 23°. With ani- 
mals of constant temperature, the reverse is the case; the 
colder it is, the more active is their respiration: a man, for 
instance, who, in summer, consumes only a fraction over an 
ounce of oxygen an hour, in winter consumes more than an 
ounce and a half. Apart from the state of the surrounding 
medium, many different circumstances exert a perceptible 
influence on animal heat, and produce tolerably regular 
variations in it. The seasons, the times of day, sleep, di- 
gestion, mode of nourishment, age, etc., are thus constant 

