6 THE COMMON RATS 



large. In western Europe and other regions in which the house 

 rat population is waning, a careful search by several investi- 

 gators during 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 Institute, we have in addition to this, 

 a closely inbred strain reared by Dr. King. Strains of "ex- 

 tracted" Albinos are maintained in some laboratories. 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. 



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 pig- 

 mented strain, characteristically pigmented descendents, the 

 markings of which can be predicted. 



W T e are naturally concerned with the gametic composition of 

 the general population of Albinos constituting the different 

 colonies to-day. As these colonies stand, the Albinos com- 

 posing them do not form a strictly homozygous population, 



