34 



INHlSRlTANCe OP COAT-PIGMENTS AND COAT-PATTERNS 



the hooded and Irish patterns, Doncaster (:o6), after an extended experience, 

 says (p. 216): "Only once have I had any hesitation in classing a rat as 

 belonging to one or the other." Yet we have seen that by cross-breeding 

 and selection these same discontinuous groups can be made continuous by 

 the production of any desired number of intermediate groups, each varying 

 continuously about a different mode. 



Again, though the inheritance is clearly Mendelian, when hooded and Irish 

 rats are crossed, the gametes formed by cross-breds are not pure, but modified, 

 each extracted pattern being changed somewhat in the direction of that 

 pattern with which it was associated in the cross-bred parent. This means 

 simply that the inheritance, though in the main alternative, is to some extent 

 blending. 



Since it is impossible to make a sharp distinction between continuous and 

 discontinuous variations, as well as between blending and alternative inherit- 

 ance, it is fallacious to assign all evolutionary progress to one sort of variation 

 or to one sort of inheritance. 



TABLES. 



Tabi,E I. — ■ Actually different classes into which we may expect the five visibly different 



classes of albino and partial-albino rats to fall. 



[Twenty-two of the twenty-seven classes enumerated have been proved to exist,] 



B, black; G, gray; H, hooded; I, Irish; W, total albinism. Symbols indicating unseen 

 recessive characters are placed within parentheses ( ), those indicating characters latent 

 in albinos are placed within brackets [ ]. 



When two symbols only are used the first refers to color, the second to coat-pattern. 



When more than two symbols stand together within brackets, those which refer to 

 color are placed at the left of a hyphen, those referring to pattern at its right. 



Total albinism is indicated by W. When albinism is recessive with other characters 

 in pigmented individuals, the W will be separated by a period from the symbols designating 

 the other characters. 



The numerals prefixed to the several class-designations indicate the expected frequencies 

 of the classes when individuals are mated inter se which have the characters indicated by 

 the designation GI (W.BH). 



