660 



PHYSIOLOGICAL ZERO 



Yet Plough reared larvae and pupae at 5°C. Similar deviations are 

 apparent in all of Krogh's curves. 



These deviations at the lower temperatures further support the 

 hypothesis that the linear curve 



y oo X 



is in reality a modified exponential curve 



y oo 2* 



Fig. 1. Curves representing the relations between the temperature and the 

 rate of development of the fruit fly, Drosophila melanogaster. The experimental 

 points X fall along the linear curve y^x between certain limits. Projecting this 

 curve to the base line the physiological zero of Peairs is obtained at 10°C. The 

 point P represents an observation establishing development below this by Plough. 

 The curve y <» 2^ is a theoretical van't Hoflf curve involving two experimental 

 values, while the modified exponential is shown as the middle one at each end. 



