THE NATURE OF THE GENE 309 



More specific arc Muller and Prokofieva (1935) when they 

 assume that position effects are due to a higher degree of inter- 

 action between locally more concentrated products of gene 

 activity as compared with the interacting of products from widely 

 separated points, which are either more diluted or changed (see 

 Schmalfuss, page 299). The same idea is also offered by Offer- 

 mann (1935) and is found in Sturtevant's first discussion (1925). 

 Muller (1935) will not exclude absolutely the possibility of inter- 

 action between the genes but thinks that it is more probable 

 that only the products of the genes are involved. In a lecture 

 before the Physiological Congress of 1935, he elaborated this idea 

 as follows. In the production of phenotypic effects, the gene 

 begins by interacting with cellular substances to produce specific 

 products which diffuse from the locus of origin and cause or affect 

 further physicochemical changes. Then follow the chains of 

 reactions and interactions, conceived in the same way as we 

 discussed them above. When such interactions involve the 

 immediate products of neighboring genes there, distance may 

 count. It is to be assumed that the highest concentration of the 

 gradient of gene-controlled primary products will be near the 

 gene, whereas with increasing distance these substances will be 

 altered by undergoing new reactions. This idea has been further 

 elaborated by Offermann (1935). 



The important point for the discussions of this chapter is that 

 it is generally assumed that the position effect has to be explained 

 within the present gene theory, which remains unaffected as such. 



Muller and collaborators (1935) once were tempted to go one 

 step further, when they argued: "As our studies of mutations in 

 the X and other chromosomes have shown that apparent replicas 

 of practically all known natural mutations in Drosophila may also 

 be obtained by X-rays, the further question is raised as to what 

 proportion of natural mutations in Drosophila may really be 

 minute rearrangements." The importance of trying to dis- 

 tinguish intragenic from intergenic changes is therefore urged. 



C. Is the Theory of the Gene Still Valid? 



The preceding sentences bring us now to the point where we 

 have to ask ourselves whether or not the theory of the gene as the 

 hereditary unit of actual separate existence is still tenable. The 

 facts regarding position effects, which we mentioned, have led to 



