VARIATION IN LINKAGE 



141 



31° C, it is high again. The control values for sister 

 flies, at normal temperature (22° C), are given in the 

 next table. 



The figures given in this table were obtained as a con- 

 trol for the last results, and from these data the results 

 of crossing over are reduced to the same scale. These 

 weighted crossing-over values for the first regions give 

 the curve drawn in Fig. 56. The curve begins at a high 

 level and drops rapidly. The first maximum is reached at 

 about 13° C, and then falls to 17.5° C, where the level 

 remains nearly constant for ten degrees more (27° C). 

 It rises rapidly at about 28° and reaches a second maxi- 

 mum at 31° to 32° C. Afterwards it is seen to fall until 

 sterility occurs at 35° C. 



The temperature curve of crossing over seems to show 

 that the phenomenon is not a simple chemical reaction, 

 for if it were we should expect for every rise in 10° C. the 

 amount of change in crossing over to be approximately 

 tripled. It would appear, therefore, that the phenomena 

 might be due to the physical state of the materials involved 

 in crossing over. Plough calls attention to the similarity 

 of this curve to that shown by the amount of contraction 

 of a frog's muscle. Here there is an increase from zero 

 to 9° C, when a maximum is reached. After this, the 

 amount of contraction decreases, reaching a low point 



