132 THE AMERICAN NATURALIST [Vol. LIV 



which are unrelated so far as the pedigrees show back to 

 our original stock. This stock came from two sources, a 

 few animals received from Professor Castle, of Harvard 

 University, and somewhat less than a dozen young ani- 

 mals supplied us by our veterinary department, but ob- 

 tained from a dealer. This stock had multiplied to about 

 forty individuals at the time records were begun on it. 

 The pedigrees show that in all probability there was orily 

 one individual, a male, in the Castle stock which might 

 have brought in the palsy character, and since Professor 

 Castle informs us that he has never noticed it in his ani- 

 mals, this individual may with considerable certainty be 

 ruled out as the source of the defect in our experiments. 

 The young animals received from the dealer were all of 

 about the same age and were white spotted, very similar 

 in appearance, which suggests that they may have been 

 related. We are accordingly led to conclude that the 

 character was introduced with this stock and that in all 

 probability it may have traced back to one, or at least 

 only a few, heterozygous animals, and that, furthermore, 

 if there were more than one they were probably related. 



Inheritance of the Palsy Character 

 The factor for normality appears to be completely dom- 

 inant and we have found it impossible to distinguish ani- 

 mals carrying the defective trait from those which do not 

 on the basis of observable behavior or any other char- 

 acters. The only method of separating the two classes is 

 therefore by breeding tests. Owing to the fact that the 

 affected (recessive) individuals always die, it has been 

 necessary to conduct the experimental tests by the round- 

 about method of always mating animals to be tested to 

 others known to be heterozygous. If the individual being 

 tested was a homozygous normal no defective offspring 

 would be produced by such a mating, whereas if it was 

 heterozygous they should appear in the usual ratio for 

 Mendelian recessives. We have therefore conducted ex- 

 tensive experiments to determine, (1) the ratio of palsied 



