614 PROCEEDINGS OF THE AMERICAN ACADEMY. 



fawn-colored mice (likewise lighter below). The two fawn-colored mice 

 were borne by a single " pure-bred" mother. Gray mice were borne by 

 all five " pure-bred" mothers. 



This result indicates that not all albinos breed alike when crossed with 

 the same pigmented stock, a conclusion which our own experiments fully 

 substantiate. Darbishire's white mice clearly show individual differences 

 in the way in which they breed. These differences are even more strik- 

 ing among the " pure-bred " than among the " cross-bred " mothers, 

 doubtless because the former were obtained from different breeders, 

 whereas the latter all came from one source. 



Darbishire is entirely right in concluding that the ancestry of white 

 mice does "make a difference" in their breeding capacity, but he is cer- 

 tainly wrong when he surmises that " the more in-bred an albino is the less 

 power it has of transmitting its whiteness," unless he is willing to limit 

 this statement to 1\\q first hybrid generation, which, however, he has not 

 done. The truth probably is that in crosses between albino and spotted 

 races, reversion to the ancestral form of pigmentation is more complete 

 the purer the white stock is. But the ability of the white parent to 

 transmit its whiteness to generations other than the first is certainly not 

 diminished by inbreeding. For, while von Guaita employed a stock of 

 white mice which had been inbred for many generations, and which 

 showed in consequence a considerably diminished fertility, the white 

 stock used by us gave no indications of extensive inbreeding ; and yet in 

 the experiments of von Guaita, as in our own, complete albinism is in- 

 herited in slightly more than the proportions demanded by Mendelian 

 principles. The experiments of Cuenot also show a slight excess of 

 albinos over expectation. 



Inbreeding, then, does not affect the inheritance of complete albinism 

 in crosses ; whether it affects the character of the pigmentation of the 

 hybrid individuals formed is an entirely different question, one which can 

 be tested only by the use of albino individuals resulting from a cross be- 

 tween unrelated pure albino stocks. Such a cross should serve to coun- 

 teract, at least in part, the effect of any previous inbreeding either in one 

 or in both albino stocks. Our present opinion is that purity of the 

 albino stock, rather than inbredness, is of consequence in determining the 

 extent of reversion to the primitive gray pigmentation in the primary 

 cross with mosaic individuals ; but further experiments are needed to 

 settle this point. 



In the foregoing pages we have used such expressions as impure albino 

 and impure recessive, expressions which seem incompatible with the hy- 



