INTRODUCTION 



The mediaeval alchemists conceived the idea of the trans- 

 mutation of metals and dreamt of changing the baser metals 

 into gold. The task which baffled them the scientists of our own 

 generation seem destined to achieve. The transmutation of 

 bacteria is a problem of more recent date but it bears a certain 

 resemblance. If silver and gold are the currency of wealth 

 by means of which it changes hands, bacteria represent the 

 currency of disease by means of which this also is passed from 

 one person to another. The resemblance, however, goes much 

 deeper than this, for just as the metals have hitherto been 

 regarded as "elements" of matter so the functions of the 

 unicellular organism have been thought to represent the 

 "elements" of life. The physicist has learnt that the so-called 

 "elements" of matter are themselves composed of infinitely 

 small particles or "ions"; the pathologist is learning that the 

 functions of bacteria in many cases result from the activity of 

 ultra-microscopic bodies, of the nature of "enzymes." The 

 occurrence of transmutation in the case of bacteria would 

 prove as revolutionary in our conception of disease as its 

 occurrence in the case of certain rare metals is already proving 

 in our conception of matter. 



The idea of the permanence of characters in the animal 

 world is at least as old as the question "Can the Ethiopian 

 change his skin or the leopard his spots?" but it is only in 

 recent times that the fixity of animal species has been 

 scientifically demonstrated. 



Amongst the less highly organised structures of plant life 

 variation is of more frequent occurrence and, though it is 

 not possible to "gather figs from thistles," it is generally 

 acknowledged that "species" in the case of plants are less 

 rigidly defined than in the animal world. 



In the realm of bacteriology still simpler forms are met 

 with in which are recognised the beginnings of both animal 



D. 1 



