6 A. C. R. Dean and Sir Cyril Hinshelwood 



The details need not be repeated here, but a very crude 

 analogy may be cited in illustration. Suppose we have a tank 

 with an inflow and an outflow of water. A certain level is 

 established. If now the outflow is restricted the head of water 

 rises until eventually outflov/ equals inflow once more. The 

 kinetic theory of the automatic adjustments in cells is less 

 crude than this analogy but still, no doubt, far cruder than 

 reality. Nevertheless, it is very general, and one point which 

 is worth emphasizing is that if certain quite general and 

 extremely likely conditions are fulfilled the development of 

 drug resistance becomes a predictable phenomenon. If the 

 mechanism just mentioned does not operate in nature there 

 should be a positive explanation of why it does not. 



In principle, such adjustments are reversible when the 

 original conditions are restored, and a major clash of opinion 

 has occurred in this connexion. Drug resistance and proper- 

 ties such as ready utilization of substrates are often rather 

 persistent, and are often cited as manifestations of "stable 

 heredity". We differ from this view in two ways. In the first 

 place, we believe these phenomena in fact to be essentially 

 reversible, on experimental grounds which will be sum- 

 marized later. The reversion can be slow, for reasons not 

 wholly unlike those which account for the delay of many 

 other chemical transformations. In the second place, we con- 

 sider that the term "heredity" is a rather ambiguous one to 

 use of organisms which multiply by binary fission. With such 

 organisms new individuals do not develop from special cells 

 which are a minute fraction onh^ of the total somatic make-up 

 of the progenitors. When the two new cells are formed by 

 division there is no question of parent and offspring. Each is 

 roughly half of the original cell and retains its cytoplasmic 

 make-up. If there is an inertial lag to adjustments in this 

 (and reasons can be imagined why there should be) the exist- 

 ing organization will persist. But the persistence might just 

 as well be called the stability of the physiology of an individual 

 as a hereditary quality. 



In fact this stability is not absolute, and in some examples 



