Genetic and Non-Genetic Variation 283 



be induced with a high degree of specificity by radiation, 

 temperature shock or chemical treatment at particular stages 

 of development. Rapoport (1947) claimed that enzyme 

 inhibitors were especially efficient in inducing specific pheno- 

 copies, and found that almost 100 per cent of a treated popu- 

 lation responded to chemical treatment by showing the same 

 phenocopy. However, these results have been only partly 

 confirmed (Bertschmann, 1955). Further, Goldschmidt never 

 found phenocopies of mutants which affect colour characters, 

 but only phenocopies of mutants affecting size or shape. 

 Landauer's extensive work on phenocopies induced in 

 chickens by insulin may be cited as giving other examples 

 (Landauer, 1954). It is characteristic of phenocopies that they 

 can be specifically induced, that they seem to be irreversible, 

 non-adaptive, and that they are not inherited, i.e. not trans- 

 mitted to sexual offspring. 



Returning to our model, I suggest that modifications and 

 phenocopies (with certain reservations) are both "receptor 

 events". In bacterial biology it is not sure that the term 

 phenocopy is very useful and even the modification concept 

 is used so loosely that it has become almost obsolete. How- 

 ever, the specific induction of enzyme synthesis by an exo- 

 geneous inducer could be ascribed to events taking place at 

 the receptor level. Also the phenomenon of "clonal varia- 

 tion", described by Yudkin and by Hughes, as well as the 

 more vaguely defined phenomenon of "physiological adapta- 

 tion" in Hinshelwood's sense may belong here. However, we 

 should be less concerned with what to call these phenomena 

 than with deciding at which level they take place. 



The second problem is : what events, observed at the pheno- 

 typic level, may be at the level of the decoding system? The 

 phenocopies might well belong here, but I preferred the 

 receptor level for them. I am more inclined to regard the 

 cytoplasmic mutations — e.g. phenotypic variants showing 

 cytoplasmic inheritance — as possible candidates. The dif- 

 ference between a cytoplasmic mutation and a phenocopy is 

 that the former is transmitted to the sexual offspring through 



