88 PROCEEDINGS OP THE AMERICAN ACADEMY. 



tlie other (No. 39) of 4 pigmeuted to 2 albino mice. Neither litter shows 

 the ideal proportion, but taken together the total result is 9 pigmented to 

 3 albino mice, the exact Mendelian ratio. Other cases show less close 

 approximation, though the totals for several pairs come within a very 

 small number of the 3 : 1 expectation. 



It is clear, then, that these heterozygotes from the spotted stock 

 crossed with albinos tend to produce the two sorts of gametes in equal 

 numbers, so that when intercrossed the ratio of pigmented to albino 

 young is approximately 3:1. 



Table H gives in condensed form the result of a number of crosses in 

 which an albino was bred to a heterozygote of spotted and albino parentage. 

 In all these cases, the expected result is an equality of the albino and the 

 pigmented classes, by the Mendelian formula (6). In 11 out of 32 litters 

 this expectation is precisely fulfilled. Sixteen of the litters contain an 

 uneven number of young, so that exact equality of the two classes is not 

 possible in the separate lots, though their totals aggregate 38 pigmented 

 to 40 albino mice. The totals for both classes show a very close approx- 

 imation to the 1 : 1 ratio which the Mendelian law requires. As .seen ia 

 the other tables, the separate litters of young frequently show a consider- 

 able deviation from the 1 : 1 ratio of the two classes. In only two cases 

 (Nos. 1, 14), where the numbers were very small (2 and 1 respectively), 

 were there no pigmented individuals. In ten litters there was an excess 

 of pigmented young, while in ten others the albinos were in excess. The 

 excess in the latter group, however, was usually small, ranging from 0.5 

 to 1.0 individuals, save in one case (No. 4), a litter of 1 pigmented and 6 

 albino young, where the latter were 2.5 too many. The discrepancies 

 tend to counterbalance each other as the totals grow larger, so that in the 

 entire lot of 138 individuals there is actual equality. We may, then, con- 

 clude that heterozygotes, whether male or female, arising from a cross 

 between a pied and an albino stock, tend to produce in equal numbers 

 gametes representing the pigmented and unpigmented conditions. 



An account of the writer's experiments on the heredity of complete 

 albinism having been given in what precedes, a review is now added of 

 experiments along similar lines recorded by others. 



Crampe ('77) gives the partial results of certain crosses with rats, and 

 in a later, more elaborate paper ('85) gives a summary of his crosses for 

 a period of some ten years. This latter paper, th'ough containing an 

 immense accumulation of data, is unfortunately so obscure in many places 

 as to cause considerable uncertainty as to what sorts of animals were used 

 in the various crosses. It seems clear that in many cases three stocks, a 



