208 Life and Death, Heredity and Evolution 



In no other organism have heritable variations been stud- 

 ied so thoroughly as in Drosophila, and no other body of 

 men have been more thoroughgoing upholders of mutation- 

 ism and of the multiple factor explanation of the effects of 

 selection, than the students of Drosophila Morgan, Sturte- 

 vant, 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 relation 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 remarkable 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 these wonderfully ingenious meth- 

 ods which the advanced state of knowledge of the genetics 

 of Drosophila has 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 dis- 

 covered 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. More- 

 over, the normal red color represents a third condition of 

 that same unit. 



Somewhat later a fourth condition of this same unit was 



