JENNINGS: CHANGES IN HEREDITARY CHARACTERS 289 



In no other organism have heritable variations been studied 

 so thoroughly as in Drosophila, and no other body of men have 

 been more thoroughgoing upholders of mutationism and of the 

 multiple factor explanation of the effects of selection, than the 

 students of Drosophila — Morgan, Sturtevant, Bridges, Dexter, 

 Muller, MacDowell, and the others. We may therefore turn to 

 the evidence from Drosophila with confidence that it will be 

 presented with fairness to the mutationist point of view. We 

 shall first ask (1) what we learn from the work on Drosophila 

 as to the possibility of finding finely graded variations in a 

 single unit character. Next we shall inquire (2) as to the re- 

 lation of the assumed modifying factors to changes in hereditary 

 constitution; to the nature of the effects of selection. 



1 . First, then, what are the facts as to numerous finely graded 

 variations in a single unit factor? Here we have certain remark- 

 able data as to the eye-color of Drosophila; data that are of 

 great interest with relation to the nature of evolutionary change. 

 This fruit fly has normally a red eye. Some years ago a variation 

 occurred by which the eye lost its color, becoming white, a typical 

 mutation. Somewhat later, another variation came, by which 

 the eye color became eosin. By those wonderfully ingenious 

 methods which the advanced state of knowledge of the genetics 

 of Drosophila have made possible, it was determined that the 

 mutations white and eosin are due to changes in a particular 

 part of a particular chromosome, namely, of the so-called 

 X-chromosome, or chromosome I. And further, it was discovered 

 that the two colors are due to different conditions of the same 

 locus of the chromosome; in other words, they represent two 

 different variations of the same unit. Moreover, the normal 

 red color represents a third condition of that same unit. 



Somewhat later a fourth condition of this same unit was found, 

 giving a color which lies nearer the red, between the red and eosin; 

 this new color was called cherry. So we have four grades or 

 conditions of this single unit character. 



And now, with the minute attention paid to the distinction of 

 these grades of eye color, new grades begin to come fast. In the 

 November number of Genetics, Hyde (1916), adds two new grades, 



