26 



THE AMERICAN NATURALIST [Vol. LV 



ent case, there was very little conscious selection but a 

 considerable amount of natural selection was, of course, 

 unavoidable. 



On crossing two inbred lines, each, as a rule, supplies 

 some of the dominant factors lacking in the other, with 

 the consequence that there is increased vigor in so far as 

 the character in question depends on the heredity of the 

 animal itself. In the next generation if brother-sister 

 matings are made, there should be a decline as compared 

 with the first generation in characters which depend 

 wholly on the animal itself. The decline from this cause 

 may, however, be balanced or more than balanced by the 

 improvement due to the influence of the crossbred dam. 



Experiments on Resistance to Tubeeculosis 

 Since April, 1919, all surplus individuals in five inbred 

 families (2, 13, 32, 35 and 39) and a random selection 

 from certain crossbreeding experiments {CO, CA, AC, 

 and B) have been shipped from Washington to Phila- 

 delphia to be tested for resistance to tuberculosis. The 

 animals were shipped in lots of 30 to 60 every two or 

 three weeks. They were from 5 to 8 weeks old at the 

 time of shipping. They usually arrived in good condi- 

 tion but with some loss in weight. The first two lots 

 were not tested because of heavy mortality before inocu- 

 lation due to feed conditions. Lot 6 was not tested be- 

 cause of heavy mortality due to excessive heat during 

 shipment. Data on the shipment and inoculation are pre- 

 sented in Table I. The deaths within 15 days after in- 

 oculation are probably not attributable to tuberculosis and 

 are not considered in the later work. The experiments 

 were considered closed when all but two or three of the 

 animals had died. The last column gives the number of 

 days after iiiocnlation at this timo. T\w next to the last 



(•..luillll Uivr. th.' (l:i> l.> XNhicll :.() prl- (..nt. nf IIimm' which 



tliat the few hurvivor^ of the experiment died on thi day 

 following that given in the last column. 



