282 M. Westergaard 



have originated in one of three ways, through a change in 

 (1) the information system, or (2) in the decoding system, or 

 (3) in the receptor system. This is probably too rigid a model, 

 and the decoding system and the receptor system especially 

 may be much more integrated. However, as a basis for discus- 

 sion, the model may serve its purpose. 



What we must know is how to decide whether a given 

 phenotypic variant originates from changes at the receptor, 

 translation, or information level. It would be very attractive 

 indeed if this problem could be decided on a chemical basis 

 (DNA-RNA-protein) but I think we all agree that today this 

 would be wishful thinking. Therefore, the best we can do 

 is probably to consider the genetical work on higher organisms 

 with our model in mind. 



There are two terms in which non-heritable variation is 

 described in higher organisms: "modifications" and "pheno- 

 copies ". Both types of events are induced by the environment 

 and neither is heritable in the sense that they are not trans- 

 mitted to the offspring through the sexual cycle. Perhaps the 

 best analysis of modification in higher organisms is the work 

 of Clausen, Keck and Hiesey (1948). They have transplanted 

 clones of various plants {Potentilla, Achillea and others) to 

 different altitudinal climates in California and studied over a 

 period of years the response of identical genotypes to drastic 

 climatic changes. The modifications induced mainly affect 

 quantitative characters, they seem to be largely adaptive (in 

 the ecological sense of the word) and they are reversible. If a 

 genetically heterogeneous population of plants is transplanted 

 to a new environment, the degree of modification depends 

 upon the genotype, some being more adaptive than others. 

 If a genetically homogeneous population is transplanted, the 

 response of the clones to the new environment is strikingly 

 uniform. 



The term "phenocopy", as defined by Goldschmidt (1935), 

 is a specific phenotypic change imitating a known Mendelian 

 mutant phenotypically, but not transmitted to the sexual 

 offspring. The phenocopy is an irreversible change. It can 



