48 THE AMFAilCAN NATURALIST [Vol. LV 



families, 13, 35, 39, 2 and 32, is far from being the order 

 in frequency of litter or the mortality records. Taken 

 as a whole, family 2 seems to have been rather the easiest 

 family to keep going. Its regularity in producing litters 

 and success in raising the young which are bom alive are 

 factors in this, but even more important seems to be 

 another factor, probably correlated with that last named, 

 the longevity of the animals after the matings are made, 

 in which it far surpasses all of the others. Families 13 

 and 35 have also been relatively easy to maintain. This 

 leaves families 32 and 39 as those most difficult to keep 

 up to a desirable strength. 



In spite of the better record of family 35 in the four 

 years, 1916-19, as compared with the single year 1919, 

 there is still no close relation between resistance to tuber- 

 culosis and vigor in other respects. In size, in both ele- 

 ments of fertility, and in the percentage of the young born 

 alive, family 35 is still inferior to families which are dis- 

 tinctly more susceptible to tuberculosis. It may, how- 

 ever, be significant that family 35 led in the percentage 

 raised of the young bom alive and that family 2, second 

 in resistance, is also second in this respect. 



It may also be significant that while family 35 does not 

 stand out in any particular element of vigor, unless in 

 that last named, it stands relatively high in all, and so is 

 the best family in the number of young produced per 

 year by a mating, the product of frequency and size of 

 litter, and is also the best family in the total percentage 

 of the young raised, the product of the percentage bom 

 alive and the percentage of these raised. Moreover, 

 family 2 is second in both respects as in resistance. These 

 results suggest that while resistance to tuberculosis is 

 not related to the most important factors which deter- 

 mine the various elements of vigor, it is a contributing 

 factor to a sufficient extent to make the total efficiency of 

 a resistant family higher than that of a susceptible 

 family. 



In this connection, the low standing of family 35 dur- 



