J. Henderson Sjmitii 



33 



frequency table), and then plot the results graphically, the curve so 

 obtained will conform more or less closely to the normal curve. This 

 is in accordance with a large mass of biological experience, both ana- 

 tomical and physiological; and is likely to be true, whatever may be 

 the nature of the resistance offered — whether, e.g. it has a structural 

 basis, such as thickness or composition of the spore coat, or depends on 

 some such property as a capacity to neutralise or destroy the phenol 

 as it enters the spore. If this be granted, then the sigmoid survivor 

 curve is what we should expect to find. For in the mass of spores 

 exposed to the phenol those will die first which have the lowest grade 

 of resistance: and these are few in number. In each successive time- 

 interval further grades of resistance will be overcome, and these grades 



10 



12 



Fig. 2. Tomato moth larvae and lead arsenate spray (Lloyd). 

 © © 3 lbs. per gallon. X X 6 lbs. per gallon. 



contain more and more individuals, so that in the successive intervals 

 more and more spores will die: until the middle grade of resistance is 

 passed. After that, in successive intervals more and more grades of 

 resistance will be overcome, but as the individuals in these grades are 

 less and less in number, fewer and fewer spores will die, until eventually 

 only a very few individuals are left in each remaining grade. A curve 

 expressing the numbers left ahve after each time-interval, i.e. a survivor 

 curve, will have a symmetrical sigmoid shape, and will resemble the 

 survivor curve we obtain with 0-4 per cent, phenol, shown in Fig. 1. 



The idea of explaining the different hfe-times of the spores by 

 variations in the resistance of the individual spores was put forward at 

 least as long ago as 1889 (16), and if we assume the variations to conform 



Ann. Biol, vm 3 



