THE MUTABLE UNIT OF HEREDITY 



47 



general solution to the difficult mathematics of the actual distribution. 

 In some cases, the distributions observed fit very well the unique distri- 

 bution predicted on the assumption that mutations occur during growth 

 at a constant rate (Figure 2.4). These experiments provide evidence for 

 the statistical randomness of mutation. 



Statistical Randomness 



The statistical randomness of mutation, or the constancy of the chance 

 that it will occur, has two components, one of time and one of space. 

 Lea and Coulson assumed for their calculations that this chance was the 

 same at all periods during growth and for all bacteria in the populations. 



TABLE 2.2 



Number of Streptomycin-Resistant E. co/i in Samples from 20 Independent 



Cultures and from 15 Similar Samples from a Single Culture 



(From Demerec, 1948, /. Bad., 56:63) 



Experimental 

 Samples from Independent Cultures 



Control 

 Samples from a Single Culture 



°The sum of the differences from the mean (average) squared and divided bv the number 

 of entries minus one [2k/"/(n - 1) = standard deviation' = a^]- 



