A COMMENTARY ON LAMARCKISM 



In very brief outline the evidence may be summarized thus. 

 An individual Paramecium aurelia belongs to a variety — 

 essentially a species; to a 'type\ which is an assembly defined 

 by mating compatibilities and so equivalent to a sex; and to a 

 stock. A stock is the progeny of a single homozygous individual. 

 Within a stock, an individual may display one and (except 

 while a tranformation is actually afoot) only one of a distinct 

 set of surface antigens defined and labelled by their power to 

 elicit specific antibodies from the rabbit. Diff'erences of anti- 

 genic composition between the individuals of a stock are herit- 

 able, but they depend upon diiferences of cytoplasm and not 

 upon differences of nuclear genes. Different stocks are dis- 

 tinguished by different combinations of the antigenic char- 

 acters that may be displayed by their constituent members, 

 and these differences of antigenic potential are governed by 

 differences of nuclear genes. (It seems likely that the same gene 

 loci are represented in all the stocks of a given variety, and 

 that differences of antigenic composition between stocks 

 depend upon different representations of the alleles of these 

 loci.) Within a given stock, however, it is an inherited cyto- 

 plasmic difference that discriminates between the range of 

 antigenic possibilities governed by the prevailing nuclear 

 constitution. 



If an individual or an assembly of similar individuals is 

 exposed to a sub-lethal concentration of the antibody directed 

 against the prevailing surface antigen, a heritable transforma- 

 tion is brought about, in consequence of which the prevailing 

 antigen is replaced by another member of the set characteristic 

 of the stock. The effect of the transformation is to confer 

 resistance to an antibody upon the progeny of an individual 

 which was formerly susceptible to it. It is of some importance 

 that such transformations may also be brought about, though 

 (so far as present knowledge goes) more slowly, by a variety of 

 'non-specific'' stimuli such as changes of temperature or 



101 



