THE UNIT OF BIOLOGICAL ACTIVITY 149 



'The physiology of unicellular organisms, although of considerable 

 importance, is not to be regarded as a general physiology," because the 

 physiology of microorganisms leads only to the point where one phase 

 rule must give way to another phase rule the physiology, in fact, of 

 any restricted number of organisms harmoniously related can never 

 cover the field of general physiology. Let us examine this more 

 particularly for the purpose of securing the most effective attitude in 

 the study of microbial physiology. 



The differences which separate unicellular physiology from the 

 specific human physiology are worthy of consideration. Neither the 

 one nor the other can be considered general or comparative physiology 

 but both have values which are of interchangeable advantage. The 

 unicellular organism enters upon its nutritional career as a free cell 

 found in an environ of food which, in its specific manner, it must 

 prepare for absorption and assimilation. The human first brings its 

 food into a canal, a tube, the alimentary canal, lined with specialized 

 cells which contribute to the preparation, the absorption and assimila- 

 tion of the food. Enzymes are secreted by the unicellular form into the 

 food medium undergoing preparation for absorption, just as enzymes 

 are secreted by the cells of the walls of the stomach and intestines of 

 the human species. The same purpose is apparent in each case. The 

 food is absorbed through the cell-wall or directly into the protoplasm in 

 the unicellular organism, while in man the cells lining the alimentary 

 tract operate in much the same manner. In the human the distribution 

 takes place by means of a carrier system, the circulation, in order to 

 reach the distant points w r hile with the single-celled organisms the process 

 is one of diffusion. Enzymes are present in both organisms engaged in 

 the process of converting food material into protoplasm and the 

 production of waste. Aside from the distributive method, the nutritive 

 processes are much the same in both. It now remains to conclude 

 which is the more simple to study, the more accessible for study, and 

 the more adaptable to study. In this the single-celled organism has 

 many advantages. This does not constitute, however, general or 

 comparative physiology for either one of these assumes a large number 

 and a varied number of living forms into which creep specific differences. 

 The similarity paralleled in nutrition could be carried into other 

 functions but this is not the purpose. It is desirable, mainly, to em- 



