No. 602] SHORTER ARTICLES AND DISCUSSIONS 



105 



of the recessive race before selection began. This result suggested 

 that the regression observed might be due to removal by the cross of 

 modifying factors, which selection had accumulated in the hooded race. 

 If this view was correct, it was thought that further crossing of the 

 extracted hooded animals with the same wild race should result in 

 further regression, and that if this further regression was not obsen-ed 

 a different explanation must be sought for the regression already noted. 



The entire experiment has accordingly been repeated from the be- 

 ginning, with the same result as regards regression in the first F, gen- 

 eration, but with no regression of the same sort in a second F, contain- 

 ing twice-extracted hooded animals. So far from observing further 

 regression as a result of the second cross with wild rats, we have unmis- 

 takable evidence that the movement of the mean, mode and range of 

 the hooded character has been in the reverse direction. So the hypothe- 

 sis of modifying factors to account for the regression and for the pro- 

 gressive changes observed under selection becomes untenable. 



In repeating the experiment of crossing hooded rats of our selected 

 races with wild rats, great care has been taken to employ as parents 

 individuals of the greatest racial purity and to inbreed the offspring 

 brother with sister, thus precluding the possibility of introducing 

 modifying factors from other sources. In making the second set of 

 crosses, the extracted individual has, wherever possible, been crossed 

 with its own wild grandparent. In the few cases in which this was 

 impossible, wild animals of the same stock have been used. This stock 

 consisted of a colony of wild rats which invaded the basement of the 

 Bussey Institution apparently from a near-by stable. Owing to faulty 

 construction of the building they were able to breed in spots inaccessible 

 to us, and it took many months of continuous and persistent trapping 

 to secure their extermination. During this period we trapped a hun- 

 dred or more of them, all typical Norway rats, colored all over, without 

 even the white spot occasionally seen on the chest of wild rats. Two 

 generations of rats from this wild stock have been reared in the labora- 

 tory, and all have this same self-colored condition. 



The hooded animals used in the experiments to be reported on in 

 this connection consisted of 4 individuals of the plus selected series, 

 a male and 3 females, as follows: 



TABLE 140 



^ See figure 35, plate 7 for significatii 



