GENETICS — MORGAN 349 



picture the molecular or aggregate structure of the gene and furnish 

 a clue concerning its mode of division. 



Since by infinite subdivisions the genes do not diminish in size or 

 alter as to their properties, they must, in some sense, compensate by 

 growing between successive divisions. We might call this property 

 autocatalysis, but, since we do not know how the gene grows, it is 

 somewhat hazardous to assume that its property of growth after 

 division is the same process that the chemist calls autocatalytic. The 

 comparison is at present too vague to be reliable. 



The relative stability of the gene is an inference from genetic evi- 

 dence. For thousands — perhaps many millions — of subdivisions of its 

 material it remains constant. Nevertheless, on rare occasions, it may 

 change. We call this change a mutation, following de Vries' termi- 

 nology. The point to emphasize here is that the mutated gene retains, 

 in the great majority of cases studied, the property of growth and 

 division, and more important still the property of stability. It is, 

 however, not necessary to assume, either for the original genes or 

 for the mutated genes, that they are all equally stable. In fact, there 

 is a good deal of evidence for the view that some genes mutate oftener 

 than others, and in a few cases the phenomenon is not infrequent, 

 both in the germ-cells and in somatic tissues. Here the significant 

 fact is that these repetitional changes are in definite and specific 

 directions. 



The constancy of position of genes with respect to other genes in 

 linear order in the chromosomes is deducible, both from genetic evi- 

 dence and from cytological observations. Whether the relative posi- 

 tion is no more than a historical accident or whether it is due to some 

 relation between each gene and its neighbors cannot be definitely 

 stated. But the evidence from the dislocation of a fragment of the 

 chromosome and its reattachment to another one indicates that acci- 

 dent rather than mutual interaction has determined their present 

 location; for, when a piece of one chromosome becomes attached to 

 the end of a chain of genes of another chromosome or when a section 

 of a chromosome becomes inverted, the genes in the new position hold 

 as fast together as they do in the normal chromosome. 



There is one point of great interest. So far as we can judge from 

 the action of mutated genes, the kind of effect produced has as a rule 

 no relation to location of the gene in the chromosome. A gene may 

 produce its chief effect on the eye color, while one nearby may affect 

 the wing structure, and a third, in the same region, the fertility of 

 the male or of the female. Moreover, genes in different chromosomes 

 may produce almost identical effects on the same organs. One may 

 say, then, that the position of the genes in the hereditary material is 

 inconsequential in relation to the effects that they produce. This leads 



