20 BIOLOGICAL ASPECTS OF HUMAN PROBLEMS 



It is not claimed that there are no differences, but 

 it appears in a high degree probable that these 

 differences are ones of degree and not of essence or 

 type. What we particularly miss in our laboratory 

 imitations of the animal oxidations is the regulation 

 of the stages of combustion. But these differences 

 are doubtless due to the complexity of the organic 

 oxidizing ferments or enzymes and probably to their 

 specificity. We know that when plant bacteria fix 

 nitrogen for the use of plants they do so by stages. 

 One group of bacteria oxidizes the nitrogen to nitrites, 

 and a distinct group oxidizes the nitrites to nitrates. 

 These bacteria doubtless work by means of oxidative 

 enzymes or oxidases and thus afford us a suggestive 

 illustration for the differentiation of oxidative labor 

 that probably goes on in the living cells. 



A third feature of difference between the animal 

 organism and the mechanism of human make is the 

 power of reproduction. Clearly the process of 

 reproduction has no analogy in the world of un- 

 organized things. The process cannot be explained 

 on any mechanistic or physicochemical grounds at 

 present within our ken, even in the case of the 

 simplest process of reproduction, as where a yeast 

 plant divides by budding and fission. In the case 

 of reproduction in the higher animals the changes in 

 the egg cell that attend and follow impregnation are 

 of an extremely complex nature, and it can hardly 

 be considered surprising that the dynamics of cell 

 division should be wrapped in obscurity. But the 



