60 GENETICS 



aberrations, is dependent upon, or associated with, 

 abnormal quantitative differences in the chromosomes. 



The other category of mutations, factor mutations, 

 is qualitative and concerns the character of hereditary 

 units or genes rather than quantitative groups of these 

 genes as they are assembled in the chromosomes. 



Whatever it is that causes the character of a gene 

 to change in quality, with the resultant expression in 

 the somatoplasm, is still apparently beyond the pale 

 of scientific proof. Some investigators find satisfac- 

 tion in assigning external environmental causes to the 

 solution of the problem while others prefer to conceal 

 their ignorance under the blanket of "internal causes," 

 whatever these may be. At least it is reasonable to 

 say when a new variety appears suddenly in a bottle 

 full of flies or in a field of plants in the same environ- 

 ment with all of its unmodified fellows, that mutation 

 can arise somehow without outside interference. 



The wild jungle fowl presents a large and useful 

 series of mutations which have cropped out in poultry 

 under the spell of domestication while the goose, on 

 the contrary, although domesticated for an equally 

 long period, has remained practically the same. The 

 nature of the plastic hen must be different from that 

 of the more conservative goose. 



Meanwhile the secret of the real causes of muta- 

 tions remains a challenge to every geneticist and suc- 

 cess surely awaits some clever workman who knows 

 how to use skilfully the indispensable tools of obser- 

 vation and experimentation. 



The bearing of the whole matter of mutation upon 



