284 PHYSIOLOGY OF BACTERIA 



Another attempt at an explanation for this is that 

 made by Henderson Smith (1921). He raises the 

 well-grounded objection that in death by chemical 

 poisoning, differences in resistance might come from 

 differences in the thickness of the cell membrane. Our 

 only measure of resistance is the killing time ; but killing 

 times will correspond to the grades of resistance only 

 if the penetration of the poison is directly proportional 

 to the thickness of the wall. If it would take the 

 poison four times as long to penetrate a membrane 

 twice as thick, the killing times could not be considered 

 as indicators of uniformly graded resistance. 



However, if retarded penetration were the reason for 

 the logarithmic order of death of bacteria, then they 

 must show an order equal to that of higher organisms 

 when death is caused by agencies which penetrate with 

 immeasurable speed, such as light or ultraviolet rays. 

 This is not the case, however. With these agencies, 

 we find the logarithmic order established just as plainly 

 as in chemical disinfection (p. 371). 



Quite different again is the interpretation of Rahn 

 (1929c, 30, 31a) who started from the idea that the 

 property of growth is condensed in the chromosomes 

 and genes (p. 259). The genes are very small, beyond 

 the limit of microscopic visibility. Considering the 

 large size of the complex protein molecules of living 

 protoplasm, each gene may contain only one, or a very 

 few, molecules. If only one molecule of one of the 

 genes is inactivated, the cell cannot multiply. Cell 

 division requires the doubling of all chromosomes, 

 and of each gene. 



In computing the variability of the growth rate 

 (p. 267), Rahn had assumed that the gene-type molecules 

 of a large number of uniform cells under uniform environ- 



