MUTATION 251 



back in the germ-tract to give rise, after reduction, both 

 to pollen and to ovules, each one carrying the mutated 

 genes. Such an interpretation is supported by the evi- 

 dence from Drosophila, where, although mutations are 

 much more numerous, no such cases have been observed, 

 and none such would be expected if mutation occurs in a 

 single chromosome at a time, since here the germ-cells 

 come from separate individuals. 



Probably the most important evidence bearing on the 

 nature of the genes is that derived from multiple allelo- 

 morphs. Now that the proof has been furnished that the 

 phenomena connected with these cases are not due to nests 

 of closely linked genes, we can properly appeal to these as 

 crucial cases. As already explained, in ever-increasing 

 numbers of animals and plants, series of genes have been 

 found in each of which mutant characters with the same 

 normal allelomorph have been found. These mutant char- 

 acters of each series are also allelomorphs of one another 

 ■ — only two ever existing in the same individual. Ob- 

 viously, not all such mutants can be due to the absence 

 of a factor present in the germ-plasm of the wild type, 

 since only one kind of absence is thinkable. If to save the 

 situation for the theory of presence and absence it be 

 assumed that only a part of the original gene is absent, 

 and a different part in each case, then nothing is gained by 

 the admission; and while this may be true it is equally 

 possible that the genes change in other ways. It is not 

 essential that we should specify the nature of the change, 

 but simpler to look upon the mutant gene as due to 

 some kind of change or changes that have taken place 

 in the original germ-plasm at a specific locus — ^there is 

 nothing known at present to furnish even a clue as to the 

 nature of this change. 



The demonstration that multiple allelomorphs are 

 modifications of the same locus in the chromosome, rather 

 than cases of closely linked genes, can come only where 

 their origin is known, and at present this holds only in 



