16 PIEBALD RATS AND SELECTION. 



This can have but one meaning. The genetic character of the hooded 

 rat is in a general way correctly indicated by its somatic character. 

 Selection is therefore immediately effective, whether plus or minus in char- 

 acter, and whether or not preceded by selection in the same direction or in an 

 opposite direction. But regression may be expected from the character 

 of aberrant parents back toward the normal of the previous generation, 

 yet this regression will in general be less than the departure of the 

 aberrant parents from the normal of their generation. If one desires 

 in such a case to obtain continuous and progressive departure from the 

 normal in either a plus or a minus direction, he need only select con- 

 tinuously in the desired direction. 



CROSSES WITH WILD RATS. 



As a further test of the permanency of the modification effected by 

 selection in the hooded pattern of rats, crosses have from time to time 

 been made of the selected races with a pure wild stock, i. e., with ordi- 

 nary wild animals caught in traps. In all cases the wild animals used 

 were known to be homozygous as regards gray coat and self pattern, 

 since when crossed with black-hooded animals they produced only graj^ 

 self offspring. In such crosses the hooded pattern is recessive, the FI 

 offspring being indistinguishable from ordinary wild gray rats except 

 for the possession of a white patch of varying size upon the belly, but 

 even this may be lacking. (See Plate 2, d 1 8000, 8018, and 8021.) 



The grade of the hooded young extracted from a cross with wild ani- 

 mals corresponds in a general way with the grade of the hooded animal 

 used in making the cross, as the following cases will show. (Compare 

 also Plates 2 and 3.) 



A female of grade -1.87, belonging to generation 2|, minus series 

 (compare Tables 2 and 3), was crossed with a wild male. (See Plate 2, 

 9 6176.) Among her F 2 descendants (cf . Plate 2, 8070 to 8078) occurred 

 62 hooded individuals, whose grade distribution is shown in Table 42, 

 first row. Their mean grade is +0.31, although the uncrossed race of 

 the same grade and generation gave offspring of mean grade about 

 -1.20. The cross, therefore, had apparently increased the pigmenta- 

 tion of the extracted hooded recessives. This idea is supported by the 

 result of a control mating of the particular female used in making the 

 cross. When she was mated with a hooded male of the same grade as 

 herself, she produced three young, all of grade --1.00. The extracted 

 recessive grandchildren, as a group, show greatly increased pigmenta- 

 tion as compared with this, but vary greatly in the extent of the increase. 

 Some show very little modification, others very much, the most extreme 

 individual being of grade +3.50. It was undoubtedly out of just such 

 modified recessive individuals as this that the material for our initial 

 plus selections arose; to this point we shall return later. 



