io6 Plant Genetics 



white coat pattern behaved as a simple unit character 

 in inheritance. Then, starting with a single pair of 

 rats of this new pure strain, the following breeding 

 experiments were performed. For twelve generations 

 selections were made from this new strain without a 

 single outcross, that is, every generation was inbred, 

 thus insuring the constant purity of the stock. In one 

 series selection was made for an increase in the extent 

 of the pigmented areas; in the other series selection 

 was made for a decrease in the extent of these areas. 

 The result was that the areas in the one series steadily 

 increased while in the other they steadily decreased. 

 Thus far nothing very unusual is involved. Castle 

 points out, however, the following important facts which 

 were developed: (i) with each selection the amount of 

 regression ("running back") grew less; that is, the 

 effects of selection became more permanent; in other 

 words, in each succeeding generation there was a 

 decreasing tendency to revert to the original average 

 type; (2) advance in the upper limit of variation was 

 attended by a like advance of the lower limit. The 

 total range of variation, therefore, was not materially 

 changed, but there was a progressive change in the 

 point about which the variation occurred. In other 

 words, it is Uke a progressive shifting of the center of a 

 circle; the diameter of the circle does not change but the 

 position of the circle, determined of course by its center, 

 is gradually changing. These were the two important 

 facts which Castle brought out and they have been 

 stated approximately in Castle's words. 



Fig. 27 will help to make the situation plain. The 

 average amount of variation in any one generation of 



