§2] LIMITING CONDITIONS OF METABOLISM 275 



attacks a different constituent of the protoplasm. In this way 

 the dissimilar substances in protoplasm are made strikingly 

 apparent. Not only does the protoplasm of one cell show this 

 differentiation, but that of different cells of the body stains very 

 diversely. Another line of evidence for the complexity of pro- 

 toplasm is gained from the study of the effect of poisons. We 

 have seen that the same poisonous substance acts very differently 

 upon allied species of organisms {e.g. of bacteria) and upon the 

 various organs of the body, — a fact which in many cases can 

 only be accounted for on the ground of dissimilar composition. 

 Again, most protoplasm must contain substances which are 

 acted upon specifically by the different agents ; for instance, 

 certain highly explosive compounds which are set off by contact, 

 certain others which are disturbed by light, and still others 

 which are especially changed by heat. Each compound, again, 

 must form an inconsiderable part of the whole, for (if the 

 action be not too intense or prolonged) the "stimulus" of the 

 agent results in no disturbance of the activities in general. 

 Likewise, the facts of acclimatization, according to which, ap- 

 parently, certain substances in the protoplasm may be destroyed 

 without other important change in activities, give additional 

 insight into protoplasmic complexity. Finally, the same agent 

 acts in varying degree on closely related protoplasm, and this 

 indicates that, even when the general composition is the same, 

 the proportions of the different substances vary. From the 

 facts of protoplasmic staining and of the varied effects of poi- 

 sons, from the diverse effects of other stimulating agents, and 

 from the facts of acclimatization of organisms, we conclude 

 that in dealing with protoplasm we are not always dealing with 

 the same thing, but, on the contrary, with very diverse combi- 

 nations, which have this in common, that they exhibit life. 



§ 2. The Limiting Conditions of Metabolism 



Metabolism is life. To know the limits within which it can 

 occur is to know the vital limits. It is impossible to define 

 these limits closely, however, for, at either extreme, metabolism 

 graduates insensibly into inaction. It will be necessary, conse- 

 quently, to place our limits very far out. 



