Mefabolic Pathways 109 



genetic selection — need not concern us here. The point is 

 that the microorganisms by their very variety have pro- 

 vided the tools for the study of separate and alternative 

 metabolic pathways, which in higher cells are frequently 

 so mixed and intertwined that naught but confusion is the 

 result of laborious study. 



The second unique contribution of microbiology is that 

 in many aspects the microorganism is a simpler pattern 

 than the higher cell. Not only may the investigator select 

 a suitable metabolic material for study from among the 

 microorganisms, but also the nature of this material is 

 more under his control. It is tissue not normally under 

 nervous or hormonal control, as is that of the animal. Its 

 age may be more uniform, the functional relation of the 

 cells more constant. A slice of kidney contains glomerulae, 

 tubules, secreting and absorbing cells differing in function, 

 doubtless different in metabolism; their relative prepon- 

 derance and activity subject to forces of diet, tension, and 

 hormones, to competition with other organs, to a variety 

 of external matters that we rarely consider. In short, then, 

 the microbial cell is unquestionably more primitive than 

 that of higher animals, in the sense that it has little func- 

 tional integration with other cells, and that it functions 

 by and large for itself, not for a larger organism of which 

 it may be a specialized, perhaps even sacrificed, part. 



Studies of metabolic pathways provide us with the ma- 

 terial for judging pattern as well as mechanism. And the 

 microorganisms provide us with the tools whereby pattern 

 may be characteristically determined. It is not too great an 

 extension of our perspective to believe that over the hori- 

 zon the determinant of pattern will play the more im- 

 portant role. 



