EFFECT OF TEMPERATURE ON CROSSING OVER 201 
is surely no mirror-image relation in the control series. In 
addition, the coincidence is subject to so high a probable error 
that it would take very marked and constant differences to 
establish such a conclusion as the one stated. It seems clear 
that much weightier evidence than that quoted must be given 
before it can be established that age and temperature act in 
different ways on the crossing-over process. It is more con- 
sistent with the results here given that each causes a variation 
in the actual freedom of crossing over and that the changes in 
coincidence recorded are without significance. 
^ SUMMARY 
1. It has been shown that a temperature of 31.5°C. causes 
little or no effect on crossing over in any part of the sex chromo- 
some, nor is there any significant variation with the age of the 
female. 
2. Crossing over in the sepia-spineless region of chromosome 
III is increased by a temperature of 31.5°C., the effect being 
most marked between Dichete and spineless. 
3. The same region shows a variation in crossing over with 
the age of the female parent. 
4. Crossing over in the remainder of chromosome III is influ- 
enced neither by temperature nor age. 
5. The chromosomal regions which are 'sensitive' to temper- 
ature and to age all give a very high ratio of double to single 
crossing over. 
6. This is interpreted as indicating that the effects of environ*- 
ment cause observable differences in crossing over only where 
crossing over occurs least freely. 
7. It is shown that the view that temperature and age act 
on crossing over in different ways is not established. 
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THE JOURNAL OF EXPERIMENTAL ZOOLOGY, VOL. 32, NO. 2 
