94, THE THEORY OF THE GENE 



Conclusions. 



An analysis of the evidence at hand does not justify 

 the view that the actual loss of some character present in 

 the original type must be interpreted to mean that a cor- 

 responding loss has taken place in the germinal material. 



Even by extending the literal interpretation of the pres- 

 ence and absence idea so that the postulated connection 

 between the loss of the character and the loss of the gene 

 means the effect produced by other genes, the assumption 

 of a loss still has no advantage over the alternative 

 view that a mutation is due to a change of some sort in 

 the gene. Furthermore, the occurrence of mutation in the 

 reverse direction (omitting the special case of bar rever- 

 sion), while not sufficiently established as yet, is in better 

 accord with the view that genes may mutate by a change 

 in their constitution without that change being neces- 

 sarily a loss of the whole gene. And finally, the evidence 

 from multiple allelomorphs seems more consistent with 

 the view that each is due to a change in the same gene. 



The theory of the gene as here formulated, regards the 

 wild type genes as specific elements in the chromosomes, 

 that are relatively stable over long periods. There is at 

 present no evidence that new genes arise except by 

 changes in the constitution of the old genes. The total 

 number of the genes remains on the whole constant over 

 long periods. Their number may be changed, however, 

 by a process of doubling the full set of chromosomes and 

 perhaps in other similar ways. The effect of changes of 

 this sort will be considered in later chapters. 



