THE UNIQUENESS OF THE INDIVIDUAL 



antigenic types were to be subjected under carefully calculated 

 conditions to the action of an antiserum directed against only 

 one, then the resulting transformation of the assembly con- 

 sidered as a whole would represent the outcome of two pro- 

 cesses: (a) the continued propagation, at first numerically 

 favoured, of individuals lacking the antigen against which the 

 antiserum was effective, and (b) the transformation of formerly 

 susceptible individuals into different antigenic types. Denied 

 the use of a microscope, it would have been very much more 

 difficult — it has not in any case been easy — to distinguish 

 between the contributions of the two processes, or, indeed, to 

 be certain that both occurred. It may be agreed, then, that 

 micro-organisms show heritable adaptive transformations of 

 individual cells, and that these are superimposed upon, and act 

 within limits governed by, the Mendelian mechanism of 

 genetic inheritance. 



There seems to be no great mystery about the significance 

 for Paramecium itself of the type of inheritance illustrated so 

 well by antigenic transformations. Paramecium is a very 

 vulnerable organism, with a very short interval between suc- 

 cessive generations. It is to its advantage not only to be able 

 to transform itself in the face of adverse circumstances but 

 also to be able to take several generations to do so, if the 

 change cannot be accomplished in one. Moreover, the com- 

 pleted change is passed on ready-made to succeeding genera- 

 tions; it would be a hopeless arrangement, from Paramecium's 

 point of view, to start from scratch in each generation. 



But what is the wider significance of the fact that Para- 

 mecium enjoys two systems of inheritance: the nuclear or 

 gene-determined, and the cytoplasmic or gene-limited? It is 

 possible that of the two collateral systems of inheritance dis- 

 played by protozoa the nuclear system persists in the mech- 



104 



