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 96^ when the temperature of the air was 60, and as 

 high as 102 when the air was at 82. As a variation of 5J 

 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 621 ; 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 



