APPENDIX I 



For the table on p. 155 I am indebted to the kindness of 

 Mr H. T. J. Norton of Trinity College, Cambridge. It affords 

 an easy means of estimating the change brought about through 

 selection with regard to a given hereditary factor in a population 

 of mixed nature mating at random. It must be supposed that 

 the character depending upon the given factor shews complete 

 dominance, so that there is no visible distinction between the 

 homozygous and the heterozygous forms. The three sets of 

 figures in the left-hand column indicate different positions of 

 equilibrium in a population consisting of homozygous domi- 

 nants, heterozygous dominants, and recessives. The remaining 

 columns indicate the number of generations in which a popu- 

 lation will pass from one position of equilibrium to another, 

 imder a given intensity of selection. The intensity of selection 

 is indicated by the fractions \l>^, i^f, etc. Thus ^-^ means 

 that where the chances of the favoured new variety of surviving 

 to produce offspring are 100, those of the older variety against 

 which selection is operating are as 75 ; there is a 25 % selection 

 rate in favour of the new form. 



The working of the table may perhaps be best explained by 

 a couple of simple examples. 



In a population in equilibrium consisting of homozygous 

 dominants, heterozygous dominants and recessives the last 

 named class comprises 2-8 % of the total: assuming that a 

 10 % selection rate now operates in its favour as opposed to 

 the two classes of dominants — in how many generations will 

 the recessive come to constitute one-quarter of the population ? 

 The answer is to be looked for in column B (since the favoured 

 variety is recessive) under the fraction ^-. The recessive 



