122 RESPIRATORY EXCHANGE OF ANIMALS AND MAN 



and perhaps the heat produced were measured, would be sure to give 

 results of considerable value. 



4. Environmental factors. In warm-blooded animals the "en- 

 vironment" of the cells is maintained practically constant with regard 

 to temperature, concentration of hydrogen ions, salts, oxygen, sugar, 

 and probably other sources of energy. The cells have lost conse- 

 quently their power of adaptation to changes in all these respects, 

 and if one of the regulating mechanisms breaks down the death of the 

 organism results. It is only natural, therefore, that in these animals the 

 standard metabolism is a constant which is independent of changes 

 in the outside world, because these changes are always toned down to 

 insignificance by the regulating mechanisms before reaching the cells. 



In cold-blooded animals we have an extremely wide diversity 

 of conditions. Some probably possess regulating mechanisms which 

 are not very inferior to those of the warm-blooded (the temperature 

 regulation excepted), while in others any and every condition of life 

 may vary within wide limits. In such forms we must expect that 

 the intensity of the metabolic processes is distinctly influenced by 

 numerous environmental factors besides the well-defined ones which 

 have already been mentioned : temperature, oxygen pressure, etc. 

 Want of food is probably one of these factors in most lower animals. 

 The amount of water in the tissues depending in many cases simply 

 upon the humidity of the air must be a very important factor, notably 

 in many terrestrial molluscs, in which the life appears to become latent 

 and the metabolism to drop to a minimum when they are deprived of 

 water. 



Certain animals have periods of suspended activity, the determin- 

 ing causes of which are unknown, but during which the standard 

 metabolism must certainly be reduced. Thus several small flies, the 

 larvae of which- live in fungi, have a number of generations during the 

 summer, but in the autumn chrysalides appear in which development 

 cannot be induced by any temperature, however high, before the spring 

 (February), and the writer has found that the metabolism remains low 

 until this occurs. 1 



The same appears to be true also with regard to many other 

 chrysalides and some insect eggs. In the case of the silk-worm 

 eggs a beginning has been made by Luciani and Piutti [1888] to 



1 We have here a close analogy to the resting period of numerous plant seeds and 

 buds. The determining causes are unknown in both cases, 



