JENNINGS: CHANGES IN HEREDITARY CHARACTERS 293 



means of multiple modifying factors, whereas Castle believes 

 that in the rat it is by actual alterations of the hereditary con- 

 stitution! 



But what are these modifying factors? And here we come to 

 the astonishing point. These modifying factors are themselves 

 alterations in the hereditary constitution. Bridges leaves no doubt 

 upon this point. He lists and describes them specifically as muta- 

 tions; as actual changes in the hereditary material. 



Where then is the difference in principle between the condi- 

 tion in Drosophila and that in the rat? In Drosophila there 

 occur minute changes in the germinal material, such as to give, 

 so far as our present imperfect knowledge goes, seven diverse 

 grades of a color which is itself only one grade of another series 

 of seven known grades. By means of these graded changes 

 one could obtain, by the mutationist's own statement, the con- 

 tinuously graded results which selection actually gives. What 

 more can the selectionist ask? 



There are indeed certain differences in detail, in the notions 

 entertained by the different investigators as to exactly where 

 the changes occur. Castle believes that in the rat the changes 

 occur all in one unit — in one chromosomal locus — giving a series 

 like the primary series for eye color in Drosophila. The sup- 

 porters of multiple modifying factors believe, on the other 

 hand — if we are to accept Bridges' account of such factors as 

 typical (and it is the only account we have) — they believe, I 

 say, that these minute changes have occurred in some other 

 part of the germinal material. But this difference is one of mere 

 detail; it does not touch the fundamental question. 



This fundamental question is as to the occurrence of these 

 minute changes in the hereditary constitution, and as to the 

 possibility of getting therefrom by selection various grades of a 

 given external characteristic. In this, so far as I can see, there 

 is complete agreement. 



Now, doubtless, there is a further diversity in the mental proc- 

 esses of the two sets of men, in that the mutationist thinks of 

 all these numerous grades as after all essentially discontinuous, 

 as a series of steps so minute that the difference between one 



