110 THE NEW PHYSIOLOGY. 



and the more definitely alive any part of an organism 

 appears to be, the more marked is this characteristic. 

 Not only is matter being taken up or given off molecule 

 by molecule; but whole cells or groups of cells are 

 being given off and developed anew. Moreover, we 

 cannot draw any really definite line between an organism 

 and its environment. For instance, the blood and 

 lymph may be regarded either as environment or as 

 parts of the body ; and the same is true of the contents 

 of body cavities, such as the gas in the lungs or the 

 food in the alimentary canal, or even of we know not 

 how much of the " contents " of living cells. The 

 environment and the different parts of the body are 

 constantly influencing one another in different ways ; 

 but on the whole these activities and influences tend 

 to remain constant or oscillate round a characteristic 

 normal standard. Normal structures, normal environ- 

 ment, and normal activity are clearly bound up 

 together. 



When we seek for a mechanistic explanation of the 

 life of an organism the only course is to look for it in the 

 peculiar structure of the body and the nature of the 

 reactions between this structure and the environment. 

 The wonderful delicacy and definiteness in the reactions 

 which are constantly occurring within the body seem 

 to imply a corresponding delicacy and definiteness of 

 structure. But we are soon met by the difficulty that 

 the more delicate the structure must be supposed 

 to be, the more evidently does its existence from 

 moment to moment depend on its immediate environ- 

 ment and on the activities which the influence of the 

 environment is constantly producing in it. We are thus 



