ARTEMIA SALINA 



293 



The 3rd selected generation may be ignored owing to insufficient data, as 

 shown by the large standard error, but the decrease in resistance of the 

 S2 generation seems to be real. 



The different behaviour of these two lines might be ascribed to the pos- 

 sibility that in the latter no genetic variability affecting survival in the test 

 , condition was present, whereas in C6 great differences in genotype were at 

 first available, leading to a rapid selective response. We suggest, however, 

 that, as each generation may be predominantly bred from so few as three pairs 

 of parents, the element of chance in the selection of these latter may be the 

 more important factor in causing discrepancy. On this view in the one 

 line selection has been successful in securing parents of high innate resistance 

 for the propagation of the stock, while in the second case parents of mediocre 

 or low resistance have happened to be selected owing to unintentional 

 inequality of the conditions to which different broods have been subjected. 



Table III. 



Table IV. 



Generation. 



Unselected 

 1st selected 

 2nd do. . 

 3rd do. . 



50 per cent. 

 Death Point. 



8-524 ± 0-543 

 7-436 ± 0-132 

 4-675 ± 0-427 

 0-761 ± 5-474 



Regression. 



0-245 

 0-316 

 0-224 

 0-061 



The behaviour of some of the other lines seems to confirm this view. 

 (See Tables V-IX.) 



A feature of some interest is shown by the steepness of the gradient of 

 mortality with respect to poison concentration (called the regression in 

 Tables IV and V). Other things being equal, this gradient will be higher 

 the more uniform the material. In line C6 it may be observed that the 

 regression increases in successive generations as the resistance is heightened. 

 In other lines it also appears that, when an advance has been made, by 

 selection, it is generally accompanied by an increase in the regression. It 

 would appear that the increase in genetic resistance is generally accompanied 

 by a decrease in variability, or, in other words, that the stocks are becoming 

 more homozygous for the genes favoured by the selection. In view of this 

 effect it will be of especial interest to cross the two lines now available after 

 five generations of selection, and to determine whether the variability is 

 thereby restored without loss of resistance. 



The Committee asked to be reappointed with a grant of £20. 



