LIFE AND FORCB. 99 



known forces. The ctieiiiical changes whicb ai'e coa- 

 stantly going on in any living creature produce heat, 

 which is very apparent in the case of the higher verte- 

 brateSj often spoken of as the warm-blooded, or homo- 

 thermous animals, in which the same temperature is 

 kept up, whatever the outside temperature may be. 

 The blood-heat of man is about 98° P., that of many 

 of the more active mammalia is somewhat more, while 

 birds, from the great activity of their life, are the 

 warmest animals of all. Animals in which the heat 

 produced is less, so that they have a ^ temperature 

 varying with that of the medium they live in, are 

 called cold-blooded, or poikilothermous ; but many 

 of these show a considerable tendency to be warmer 

 than the surrounding temperature. Efforts have been 

 made to show that the heat produced bears a definite 

 relationship in quantity to the chemical changes which 

 go on in the body ; but those efforts must be said to 

 have been unsuccessful, perhaps only from the extreme 

 complication of the problem involved, but probably 

 also from other causes. 



Light and electricity are also generated, under cer- 

 tain circumstances, by animal organisms. Light ex- 

 hibited by animal bodies is called phosphorescence, 

 because it resembles in appearance the light presented 

 by certain compounds of phosphorus in the dark. The 

 name does not necessarily imply that the light is 

 evolved by means of the compounds of phosphorus 

 existing in the fats of the organism, although some 

 have supposed this to be the case. The same term is 



