OF MUTANT CHARACTERS. 195 



case are the data conclusive. A calculation of the coincidence shown 

 by Plough's black purple curved 2-day tube-cultures has provided 

 data considerably more satisfactory, though still subject to a high 

 probable error. On the basis of all the data it is probable, not only 

 that coincidence varies with age, but that the curve of age-variation 

 in coincidence is roughly the mirror image of the curve of age variations 

 in crossing-over. While it seems probable that at least part of the 

 explanation of the age-variations both in crossing-over and in coinci- 

 dence has been found in an internode variation as suggested, yet in 

 any case there is provided evidence of a common cause that should 

 repay further analysis. 



A second problem involving purple and very closely allied to the 

 age-variation in conception, material, methods, and bearing is that 

 of the temperature- variation described by Plough (1917). Since the 

 genie constitution of a female showing the age-variation is constant 

 throughout the course of this variation, the immediate causes of the 

 variations must be regarded as environmental differences arising 

 through rhythmic changes in the physiological processes of nutrition 

 and development. While the crossing-over variations due to age and 

 to specific genes were effected through environmental changes arising 

 internally, they suggested the possibility that similar variations might 

 be initiated by environmental changes arising externally. Plough 

 found that exposure to abnormally high or low temperature actually 

 did produce linkage change even more extreme than those due to age 

 changes. Black-purple-curved back-cross cultures were paired at 

 various temperatures from 9 to 32 C. When the black-purple cross- 

 over values were plotted it was seen that at a low temperature (9), 

 crossing-over is very free (14 per cent) and becomes even more free at 

 13 (18 per cent). The amount of crossing-over then falls away very 

 rapidly, and at 18 is nearly the normal value (6.0 per cent). This 

 value is maintained to about 27, or throughout the range of "room" 

 temperature at which the breeding work is ordinarily conducted. At 

 29 the crossing-over is slightly freer than normal, but between 29 

 and 31 the amount of crossing-over nearly trebles (18.2 per cent). 

 This extraordinarily sharp and extensive rise is followed by a slight 

 fall at 32 (15.4 per cent). Above this temperature and below 9 C. 

 it was found that the flies either died or produced too few offspring to 

 be workable. It seems probable that here also these two sharply 

 marked maxima, separated by a long interval of no or slight change, 

 may represent two distinct phenomena. 



When the coincidences are calculated for these various temperatures 

 it is seen that the curve of temperature- variation of coincidence is a 

 slightly rising but practically straight line cutting alike through both 

 of the maxima and the normal interval. This is a significant difference 



