252 ON VARIABILITY AND ADAPTATION 



ordinary complexity, and in a state of constant unsatisfaction, 

 built up by linking on other simple molecules, and as constantly, 

 in the performance of function, giving up or discharging into 

 the surrounding medium these and other molecular complexes 

 which it has elaborated. 



In my Principles of Pathology, published in 1908, I called 

 attention to certain defects in Ehrlich's theory, and pointed 

 out how it might be brought into fuller conformity with actual 

 knowledge of general cell activities and of enzyme action. I 

 laid down that throughout the whole study of immunity we deal 

 with " the methods by which living matter, whether animal 

 or vegetable, reacts towards other living matter, whether animal 

 or vegetable, and towards the products of the same which come in 

 contact with it, and although," I continued, " the statement may 

 at first encounter appear to be both novel and extreme, further 

 thought will confirm it ; the problems of immunity narrow 

 themselves down to special problems bearing upon the assimila- 

 tion and digestion of unusual proteid matter." I might have 

 added, for my treatment of the subject was consistently along 

 these lines, that they resolve themselves into studies of enzyme 

 action. 



Long years ago, in the early 'eighties, I remember Sir Michael 

 Foster teaching us that the assimilation of food-stuffs evidently 

 demanded that more complex molecules of food-stuffs were not 

 built into the cell direct, but were first broken down into their 

 simpler constituent molecules, and of these the molecules common 

 to the constitution of the foreign food-stuffs and to the cells are 

 taken up by the latter, and used for building-up purposes. This 

 view to-day is universally accepted, more particularly as the 

 result of Emil Fischer's studies upon the chemical constitution 

 of proteins, and his demonstration that these are complexes 

 formed by the union of varying proportions of mono- and diamino- 

 acids. The Germans speak of this as the " Baustein," or building- 

 stone theory. 



It is a familiar fact that proteids taken as food, while 

 they disappear from the intestinal contents, are not to be 

 detected in the blood and lymph coming from the intestinal 

 tract. And this is true not only of the fully formed proteins, 

 but also of the peptones and proteoses of the alimentary canal 

 resulting from the disintegration of the larger protein molecules 



