No. 633] SHORTER ARTICLES AND DISCUSSION 361 



tained in the ordinary way but in terms of factorial units and 

 not facet units. The standard deviations are used directly as 

 coefficients of variation. 



The biological validity of the factorial method as given is of 

 course dependent upon the correctness of the view that eye facet 

 numbers have such a relation to environmental and germinal 

 factors as is indicated. The normality of the factorial distribu- 

 tion has already been mentioned. General embryological con- 

 siderations favor proportionate action of factors rather than 

 action by accretion. But I wish to mention particularly the 

 definite experimental proof that at least one factor, temperature, 

 is in agreement with the hypothesis. Seyster' lias shown that in 

 bar eye facet number decreases with increase in the temperature 

 at which the larvae of Drosophila are reared. This decrease fol- 

 lows van't Hoff 's law if an inhibitor of facet number is assumed 

 as the effective agent upon which the temperature acts. Krafka 5 

 has demonstrated that this general law applies to ultra-bar as 

 well as to bar eye and that for the different bar stocks the effect 

 of a degree of change in temperature is roughly proportional to 

 the mean value of the stock and the same is approximately true 

 for the effects of a degree of change in temperature t h f< miu-1i« mt 

 the range of a single stock. The following table gives the facet 

 values for ultra-bar, low selected bar and unselected bar at 15° 

 and 25° : 



Representing the effect of a ten-degree difference for ultra-bar as 

 unity, low selected bar has 4.4 times and unselected bar 5.7 times 

 this difference. It is obvious that difference in facet number is 

 not a good measure of the value of the temperature factor. 



On the other hand, if facet values are reduced to factorial 

 values according to the method given above the results are as 

 follows : 



* Seyster, E. W., "Eye Facet Number as Influenced by Temperature in 

 the Bar-eyed Mutant of Drosophila melanogaster (ampelophUa) ," Biol. 

 Bull, 1919, 37: 168-182. 



s Krafka, Joseph, Jr., "The Effect of Temperature upon Facet Number 

 in the Bar-eyed Mutant of Drosophila," J. Gen. Physiol, 1920. (In press.) 



