THE ORGANISM AS A MECHANISM 67 



in the metabolic tissues. The proteids of the food 

 are broken down into animo-substances in the ali- 

 mentary canal, and these animo-substances are 

 synthesised into the specific proteids of the animal's 

 body. Corresponding changes occur with the carbo- 

 hydrates and fats ingested. These rearrangements of 

 the molecular structure of the foodstuffs are the 

 object of the processes of digestion and assimilation ; 

 and when they are concluded, a certain proportion of 

 the food taken into the body has become incorporated 

 with, or has actually become a part of, the living tissues 

 (muscles, nerves, etc.) of the animal body. This living 

 substance, compounds of high chemical potential 

 (proteids, carbohydrates, and fats) undergoes trans- 

 formation into compounds of low chemical potential 

 (water, carbon dioxide, and urea). There is a difference 

 of energy, and this appears as mechanical energy, as 

 the chemical energy required for glandular activity, 

 and as heat. 



We must not, however, conclude that this heat of 

 the warm-blooded animal is comparable with the waste 

 heat of the steam-engine. The homoiothermic animal 

 maintains its body at a constant temperature, which is 

 usually higher than that of the medium in which it 

 lives, and this constancy of temperature obviously 

 confers many advantages. Chemical reactions proceed 

 with a velocity which varies with the temperature, so 

 that in the warm-blooded animal the processes of life 

 go on almost unaffected by changes in the medium. 

 The animal exhibits complete activity throughout all 

 the seasons of the year. It does not, or need not, 

 hibernate, and it can live in climates which are widely 

 different. We therefore find that the most widely- 

 distributed groups of land-animals are the warm- 

 blooded mammals and birds, while the largest and most 



