TEMPERATURE OF WARM-BLOODED ANIMALS. 335 
would otherwise be too rapidly conducted away; their bodies 
being clothed with a thick and soft down, which is rendered 
impenetrable to fluid by an oily secretion applied with the 
bill. The temperature of Mammals generally seems to range 
from 96° to 104°; but that of the Bat , and probably of 
other hybernating species, appears to be frequently much 
below the lower of these limits, even when the animals are in 
their ordinary activity. The mean or average heat of the 
body of Man is about 100°; but it has been observed as 
low as 96Jr° when the temperature of the air was 60°, and as 
high as 102° when the air was at 82°. As a variation of 5^° 
may occur when the range of the external temperature of the 
air is only from 60° to 82°, it is probable that observations 
made in cold climates will show that the temperature of the 
body may be still further lowered, when that of the air around 
is much depressed. But it seems that, in Man, as in other 
animals, the lower the temperature of the air around, the 
greater is his power of generating heat within his body, to 
keep up the necessary standard; and no observations yet 
made indicate that the temperature of the body ever falls 
below 95° in health. 
408. The young of warm-blooded animals have usually less 
power of maintaining an independent heat than adults. The 
embryo, whether in the egg, or within the body of the parent, 
is dependent upon external sources for the heat necessary to 
its full development. The contents of the egg, when lying 
under the body of its parent, are so situated, that the germ- 
spot (§ 756) is brought into the nearest neighbourhood of 
the source of warmth. It is not usually until some weeks 
after the hatching of Birds, or the birth of Mammals, that 
the young animals have the power of maintaining an inde¬ 
pendent temperature. Thus young Sparrows, taken from the 
nest a week after they were hatched, were found to have a 
temperature of from 95° to 97°; but this fell in one hour to 
66J°, the temperature of the atmosphere being at the same 
time 62^°; and the rapid cooling was proved not to be due 
to the want of feathers alone. There are some birds, how¬ 
ever, which can run about and pick up their food the moment 
they are hatched : these come into the world in a more 
advanced condition than the rest, and can maintain their 
temperature with little or no assistance. We find the same 
