192 ELEMENTS OF BIOLOGY 



A second avenue o£ heat loss in the human body is by means of 

 the warm air that is expired from the lungs. Not only is the air 

 warmed by its stay in the lungs but in this air the water on the 

 moist surfaces of the respiratory apparatus evaporates; the process 

 of vaporizing water absorbs body heat. In the dog, which has no 

 sweat glands, this is the chief method of cooling during exercise or 

 on warm days. Another avenue of heat loss is by way of the urine 

 and faeces. The specific heat of water is high; ridding the body of 

 more than a quart of water at body temperature daily involves a 

 considerable loss of heat. In mammals that lack sweat glands loss 

 of body heat is by radiation from the skin, by ventilation of the 

 lungs and by the heat loss in excretory and faecal materials. 



The striking difference between the homoiothermic birds and 

 mammals and the poikilothermic lower forms inclines one to 

 search for intermediate conditions. Not many animals are known 

 to be intermediate between warm- and cold-blooded. It is true 

 that some mammals during the cold season hibernate and that 

 during their winter stupor the body temperature decreases consid- 

 erably. Some old observations on the body temperature of an in- 

 cubating female python (a snake) indicate that during this period 

 the reptile does conserve heat, for the body temperature is twenty 

 or more degrees above that of the air. Recent observations of the 

 body temperature of newly hatched birds show that they at this 

 time resemble cold-blooded animals in that their temperature varies 

 with that of the air. 



Reproduction. Animals reproduce in two ways, sexually, in- 

 volving the union of two types of cells, and asexually, involving a 

 single cell or group of cells. They are more properly termed 

 GAMETIC and AGAMIC. In plants and in animals that reproduce asexu- 

 ally, various sorts of organs are specialized for the purpose of de- 

 veloping the reproductive element. For examples, the sori on the 

 leaves of the fern (Fig. 174) are the organs of development of the 

 reproducing spores; in the coelenterate hydroids the reproductive 



