Inheritance of Quantitative Characters 89 



ing the location and general shape of a hood). This hooded pat- 

 tern bred approximately true and behaved as a simple IMendelian 

 recessive in crosses with rats of the ''wild" type. These facts 

 naturally led Castle to beUeve that hooded was a simple Men- 

 delian unit character, represented in the germ plasm by a single 

 gene. 



Castle then commenced selection. For twelve generations 

 selections were made from this new race without a single outcross, 

 that is, every generation was inbred (brother and sister matings) , 

 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 another series selection was made for 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. 

 Castle pointed out that: (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 suc- 

 ceeding generation there was a decreasing tendency to revert to 

 the original average type; (2) advance in the upper limit of varia- 

 tion 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 was like the progressive 

 shifting of the center of a circle; the diameter of the circle did not 

 change but the position of the circle, determined of course by its 

 center, was gradually changing. These were the two important 

 facts which Castle brought out and they have been stated approx- 

 imately in Castle's own words. 



Fig. 15 will help make the situation clear. The average 

 amount of variation in any one generation of the pure stock (the di- 

 ameter of the circle referred to) is indicated by ,^-£->. Of course, 

 even "pure stock" varies somewhat, since no two individuals are 

 exactly alike, biology recognizing what is called "individuality." 

 The point is that the comparatively small variation in a pure stock 

 is not due to germinal differences, but to responses called out by 

 varying external conditions, such as nutrition, light, etc. These 

 response variations, usually called tluctuations, vary with different 

 individuals, but the hereditary capacity of all of them remains 



