CLASSIFICATION 9 



At present Albinos of the house rat appear to be not uncom- 

 mon in India (Lloyd, '12) where the house rat population is large. 

 In western Europe and other regions in which the house rat pop- 

 ulation is waning, a careful search by several investigators dur- 

 ing the last decade .has failed to reveal a living albino specimen. 



At the present time, therefore, the Albino of Mus norvegicus 

 is the only albino variety generally found. In these pages this 

 form is designated Mus norvegicus albinus when the name is 

 given in full, but where possible the single word Albino is used 

 for it. 



When the albino variety is mentioned here the strain as com- 

 monly reared is the one meant. As a rule this strain is far re- 

 moved from its wild ancestor and moderately inbred. It may 

 be conveniently designated as the common albino strain. In the 

 colony at The Wistar Institute, we have in addition to this a 

 closely inbred strain reared by Dr. King and also a strain of 

 'extracted' Albinos. These latter are the Albinos descended 

 from the F 2 generation of hybrids from the wild Norway and 

 the domesticated Albino. 



During the first few generations after their appearance, these 

 extracted Albinos show clearly certain Norway characters, which 

 distinguish them from the rats with a longer albino ancestry. 

 With the peculiarities of either the inbred or of the extracted 

 strain, we are however not specially concerned at the present time. 



While all Albinos breed true as to color, the composition of 

 the gametes is undoubtedly different among them in accordance 

 with their remote ancestry. Mudge ('10) recognizes thirteen 

 gametic types. The gametic dissimilarity of various Albinos 

 in respect to hair color is shown by the fact that in breeding 

 tests (Doncaster, '06 and Mudge, '10) Albinos extracted from 

 ancestors with characteristic differences in pigmentation will 

 reveal their origin by producing, when crossed with the pigmented 

 strain, characteristically pigmented descendents, the markings of 

 which can be predicted. 



We are naturally concerned with the gametic composition of 

 the general population of Albinos constituting our colonies today. 

 As the several colonies stand, the Albinos forming them do not 



