288 JENNINGS: CHANGES IN HEREDITARY CHARACTERS 



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The method in which these modifying factors are conceived to 

 operate is doubtless familiar to you : their application to Castle's 

 work with selection in rats will serve as an example. There is 

 conceived to be a single "main factor" which determines whether 

 the "hooded pattern" shall or shall not be present. In addition 

 to this there are a considerable number of "modifying factors" 

 which, when the "hooded pattern" is present, increase or de- 

 crease the extent of pigmentation. When many of the positive 

 factors of this sort are present, the rat's coat has much pigment ; 

 when fewer are present the extent of pigment is less, and so on. 

 The process of changing the extent of pigmentation by selection 

 consists, according to this view, merely in making diverse com- 

 binations of these factors, by proper crosses. 



This same explanation is applied to a great variety of cases. 

 Castle had carried the war into the enemy's country by predict- 

 ing (or at least suggesting) that the so-called unit characters in 

 Drosophila would be found to be modifiable through selection. 7 

 Later research by MacDowell (1915), Zeleny and Mattoon (1915), 

 Reeves (1916), Morgan (1917), and Sturtevant (1917) actually 

 verified this prediction; it has indeed been found that the Dro- 

 sophila mutations can be modified by selection. Again the 

 mutationists counter the blow with their explanation of multiple 

 modifying factors, which are segregated in the process of selec- 

 tion; and they give some real evidence that such is actually the 

 case. 



Now, into the merits of that particular question, as to whether 

 the apparent effects of selection are really due to modifying 

 factors in the manner set forth, I do not propose to enter. Castle 

 maintains that they are not, and I doubt not that he will show 

 you reason for that point of view. At this point my own dis- 

 cussion will diverge from what I judge that he will be likely to 

 give. What I am going to do is to abandon the ground that 

 Castle would defend, proceed directly into the territory of the 

 enemy, accept the conditions met there, then see where we come 

 out in relation to the nature of variation, the effects of selection, 

 and the method of evolution. 



7 See Castle, 1915, p. 39. (See Bibliography.) 



