592 



THE AMERICAN NATURALIST 



[Vol. XLVI 



of heat or cold, such extremes are not conducive to their 

 productivity. At 35° C. I have found breeding' to he 

 greatly impeded, and at a temperature as low as 2° C. 

 the young- horn are subject to a number of mortal ills 

 which practically prevent their reaching maturity. But 

 a constant temperature of either of the above extremes 

 is not so detrimental as is great fluctuation in tempera- 

 ture. A mouse taken from favorable conditions and sub- 

 jected to daily fluctuations of from 30° C. to 2° C. soon 

 becomes a different animal physiologically. The fur 

 which was sleek and glossy roughens, the exposed veins 

 in the ears and tail darken, and the animal is readily re- 

 duced to a condition which, if prolonged, not infre<|u<>ntly 

 terminates in death. If after having reached tins con- 

 dition, however, the animal be promptly restored to 

 equable conditions of temperature, its fur becomes sleek, 

 signs of anaemia disappear and the mouse regains its 

 normal health and vigor often with surprising rapidity. 



2. Parasitism. — If mice, even under the most favor- 

 able conditions of temperature, become badly parasitized 

 breeding ceases and unless they are ridded of the para- 

 sites the adult mice as well the young fall victims to this 

 pest. To test the effects of parasitism, I have taken 

 mice from fresh stock, kept under excellent conditions, 

 and have placed them in infested nests, with the result 

 that in a few days the mice became sluggish, and many 

 sooner or later died. 



When the two factors— parasitism and wide fluctua- 

 tions in temperature— are combined, the animals, espe- 

 cially the young, die in great numbers. 



B. Factors Essential to Intensive Breeding 

 1. Construction and Equipment of Cases— In general, 

 where a number of mice are to be kept together, the wire 

 and wood cases described by Yerkes (4) has been much 

 used. But for intensive breeding I have found it better 

 to keep few mice in a case, and to keep these under better 

 conditions of sanitation than is possible in the above case. 



I know of no better plan to insure sanitation than to 

 construct cases which will offer little surface upon which 



