118 THE AMERICAN NATURALIST [Vol. LII 



mutations we shall call them, have been most thoroughly 

 investigated in the common fruit fly, Drosophila ampelo- 

 phila, in which species Morgan and others have discov- 

 ered over 150 factor mutations, each of which is inherited 

 in strict conformity with Mendel's laws, when tested in 

 contrast with its normal mate as it exists in the wild 

 type. This has become the classical evidence for the 

 theory of factor mutations. Furthermore, it is now com- 

 mon practise to refer all Mendelizing characters to their 

 hypothetical representatives in the germ cell, the genes 

 or factors; and we speak of a pair of contrasted charac- 

 ters which are inherited according to Mendel's rule for 

 the monohybrid as due to a single factor difference. 



It is well known that Mendelizing characters exist gen- 

 erally throughout all groups of sexually reproduced or- 

 ganisms. Therefore it appears that factor mutations are 

 of general occurrence. The data on factor differences will 

 undoubtedly continue to increase in volume, and as they do 

 our knowledge concerning the relative frequency of fac- 

 tor mutations will become more precise. In general Men- 

 delian phenomena have been observed mostly in conspicu- 

 ous characters, but certain minute character differences, 

 such as forked bristles in Drosophila and size and shape 

 of starch grains in peas, are inherited in Mendelian 

 fashion. Factor mutations, therefore, are sufficient to 

 explain the origin of all differences between varieties, and 

 doubtless they provide the necessary point of departure 

 in the origin of new races. If the new characters thus 

 produced are beneficial or advantageous, then natural 

 selection will cause them to be preserved. Sumner has 

 recently discovered a number of interesting mutations in 

 deer-mice (some as yet unpublished) and has shown that 

 isolation may assist in differentiating local races. The 

 writer is not unmindful of the earlier discussions of Wag- 

 ner and others, and later of Jordan and others, on isola- 

 tion as a cause of evolution. Many biologists are still 

 inclined to think of geographical differences as the deter- 

 minative condition in the production of new species. For 



