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THE AMERICAN NATURALIST [Vol. L1V 



mice are frequently weighed, and from time to time the whole 

 series is weighed. 



In the beginning of January an epidemic started in our 

 mousery. Our mice were at that time housed in approximately 

 seven hundred cages containing several thousand mice, both the 

 size-inheritance and other series of breeding experiments. The 

 cages of all the series were mixed and arranged on shelves in 

 three adjacent rooms. The infection apparently swept through 

 the entire colony, notwithstanding our attempts to limit it to one 

 room. The Japanese mice were distributed over all the stacks in 

 all three rooms, most of them mated to big mice or hybrids of 

 different generations. All these mice fell victim to the epidemic, 

 excepting three which we kept for a little while longer, by 

 taking th'-m into the living house at the beginning of the trouble. 

 To our surprise the white mice of Robertson's strain proved im- 

 mune. Even where the dead Japanese were partially eaten by 

 their mates, these latter remained in good health. 



It is clear that the main circumstance, which made it possible 

 for us to see the clearcut segregation about to be described, was 

 the rapid spread of the epidemic. All the Japanese mice were 

 dead before the virulence of the microorganism was materially 



The rapid course of the disease made it possible to <lisi inguish 

 simply between dead and surviving mice. As a rule we found 

 tli.it animals contracting the disease presented the hunched up ap- 

 pearance and walked with the small, prancing steps familiar to 

 students of paratyphoid in small rodents. They would he visibly 

 ill for one, two, or exceptionally three days before death. We 

 do not remember having seen one recover. 



Professor Hall, of the department of bacteriology, of the Uni- 

 versity of California, was kind enough to make a bacteriological 

 examination of the dyiim- animals, and was able to isolate the 

 same staphylococcus from the blood of the heart of four animals. 



If we count the proportion of the animals which succumbed 

 to the epidemic, we have to limit our countings to groups which 

 are comparable. Immunity can never be anything but relative, 

 and if we want simply to use the fact of survival as a criterion 

 for immunity we must exclude as far as possible other causes of 

 death. Of these the two main causes are death or illness of the 

 mother, causing starvation of the young, and troubles in par- 



In our study of the inheritance of immunity to this staphylo- 



