CAUSES OF GENETIC VARIATION 225 



From these observations, Tower concludes that the determi- 

 nation of dominance, and the ensuing type of behaviour, is clearly 

 a function of the conditions incident upon the combining germ 

 plasms. 



It will be observed that expts. (i) and (3) gave identical 

 results but (2) and (4), though much the same conditions were 

 applied, are at variance, for (2) gave all intermediates, while 

 (4) gave all signaticollis. In Amer. Nat., XLIV, 1910, p. 747, 

 Professor T. D. A. Cockerell commented on this paper of Tower's 

 and pointed out that there must be an error somewhere, for when 

 he discusses these experiments Tower speaks of (2) and (4) as 

 confirming each other. To this Tower replied 12 that there had 

 been a mistake. He states that in preparing the paper "certain 

 minor experiments were taken from a larger series and combined 

 to illustrate a general point in the behaviour of alternative 

 characters in inheritance," and that expt. (2) was introduced 

 inadvertently in place of another which he desires to substitute. 

 In this, which I number (5), signaticollis 9 X diversa cf from 

 exactly the same stocks as those used in (i), were mated at the 

 lower temperatures specified for (2), day average 75 F., night 

 average 50 F. These gave all of the signaticollis type with a 

 narrow range of variability, which bred true, in some cases to F. 

 Tower says he has repeated this experiment six times with identi- 

 cal results. 



Nevertheless he proceeds to say that the description of expt. 

 (2), which was repeated eleven times with identical results, was 

 correct "as far as given." That experiment was " from a second 

 series of cultures parallel to the one given, but in which there are 

 other factors involved, which in H. 410 [my (2)] are productive 

 of a typical Mendelian behaviour." He adds he does" not care 

 at this time to make any statement of what these factors are, 

 nor of their relations to the behaviours given in the H. 409, H. 41 1 , 

 H. 409/11 series [my (i), (5) and (3)-(4)] which are the simplest 

 and most easily presented series obtained in the crossing of 

 signaticollis and diversa." 



Professor Cockerell's intervention has thus elicited the fact 



Biol. Bull., XX, 1910, p. 67. 

 16 



