456 EDWIN CARLETON MacDOWELL 



bristle number, this mutation causes a spreading of the wings, a 

 character more diagnostic than the modified bristle number. 

 Unlike either of Payne's factors, dichaet gives simple ratios 

 that prove it to be primarily dependent upon a single gene; its 

 linkage shows it to lie in the third chromosome. Modifiers 

 of the bristle numbers are located in the second and third 

 chromosome; in each of these chromosomes a modifier was 

 shown to cross over. Selection separated out two races; 

 crossing these two races together resulted in increased variabiHty 

 in the F2; return selection was successful when started from the 

 early generations of the advance, and usually unsuccessful when 

 started from the later generations. The influence of the environ- 

 ment was recognized as causing a significant part of the varia- 

 bility observed. Small flies had fewer bristles, both small flies 

 and fewer bristles appeared when food conditions were bad, as 

 at the end of a culture. 



A searching discussion based on the following penetrating 

 questions concludes Sturtevant's paper: 



Does selection use germinal differences that are already present, or 

 differences that arise during the experiment, or both? 



In the case it uses new differences, does it cause them to occur more 

 frequently, and does it influence their direction? 



Are differences, already present or arising de novo more likel}^ to 

 occur in the locus of the gene under observation, or in other loci? 

 (p. 36). 



After citing twelve cases formerly cited as evidence favoring 

 the theory of genetic contamination, he concludes that such an 

 hypothesis is unproved, unnecessary and confronted with some 

 directly contrary evidence. The first paragraph of the general 

 conclusions should be read in full: 



That many characters may be influenced by more than one pair of 

 genes has long been recognized, and this is the essence of the mul- 

 tiple-factor view. That genes exist that require the action of other 

 genes before they produce visible effects has also been long known. 

 Furthermore, that there are genes that produce very slight visible 

 effects is now another commonplace. Given these three facts and the 

 hypothesis (which is supported by much specific evidence) that most 

 races are heterozygous for many such genes is all that is required to 



