No. 602] MENDELIAN FACTOR DIFFERENCES 



95 



in the constant and continual production of a number of 

 distinct forms which display for the most part a compli- 

 cated behavior when tested with the parents from which 

 they arise and with other forms. Such forms are often 

 widely different from the parent forms in all or nearly 

 all of their characters. 



3. Chromosome duplications resulting in duplication of 

 one or more or even all of the chromosomes to the ex- 

 tent of tetraploidy in some forms. 



The first of these categories may be rigidly distin- 

 guished from the other two, both of which may be funda- 

 mentally expressions of some inherent condition in the 

 ''mutating" individual. The existence of this first type 

 of mutation can scarcely be denied in the face of the ex- 

 tensive evidence accumulated both on the plant and on 

 the animal side. In addition to this long series of past 

 observations, we have now the extensive work of Morgan 

 and his associates (Z. c.) in which the origin of over a 

 hundred such factor mutations has occurred under obser- 

 vation in Drosophila cultures. These investigations indi- 

 cate clearly that such mutations are fundamentally de- 

 pendent upon actual changes in the germinal substance. 

 They are the loss-mutations of genetic literature, but with 

 the abandonment of the presence and absence hypothesis 

 such a classification, of course, loses its significance. 

 Practically always only one locus is involved in such a 

 change, and the new form displays a consistent alterna- 

 tive behavior when tested with the form from which it 

 arose. Such mutations are usually recessive, although a 

 few dominant ones have been secured. When obtained in 

 pure culture they show no tendency to revert to the parent 

 form, at least not more frequently than they tend to 

 change in entirely ditferent directions. They are rela- 

 tively rare, they do not recur in any definite considerable 

 ratio in any strain, and they are not clearly due to any 

 specific cause. In CEnoflieya, ruhricahjx api>ears clearly 

 to be a dominant mutation of this type (Gates, 1914). 

 There seems to be little occasion for confusing these strict 



