584 



WHEELER P. DAVEY 



that the points follow a smooth curve, which is the integral of a 

 probability curve. If now the slope of this curve is plotted 

 against its abscissae, a probability curve may be obtained. If 

 the beetles represented by two such curves have been gathered 

 from the same brooder at the same time, the corresponding 

 points on the two curves may be compared, for they represent 

 beetles of corresponding resistance to the action of the X-rays. 

 It will be noticed that the curve approaches the zero and the 

 100 per cent lines asymptotically. This is in agreement with the 

 well known fact in toxicology that some individuals are expecially 

 susceptible to a given harmful agent, so that a very small dose 

 causes death, while other individuals are especially resistant to 

 the same agent so that they continue to live for a comparatively 

 long time, even when given large doses. The steepness of the 

 curves as plotted in figure 2a is a measure of the idiosyncrasy. 



I e 3 4 5678 9 10 imams 



Figure 2 A 



