024 



THE AMERICAN NATURALIST [Vol.XLV 



beneath all the cages to catch the excrement and removed 

 as occasion demanded. 



In all the cages, siphon watering troughs were used; 

 i. e., bottles held upright with their mouths dipping into 

 shallow dishes. Open vessels were objectionable because 

 rats drop their excrement as they climb about in the 

 cage, thus polluting the water. 



The experiment was started November 5, 1909, with 

 seven pairs of rats which had just been caught. For 

 convenience, each pair was numbered, the first four 

 being in the revolving cages and the other three in the 

 stationary. The pairs numbered 1, 2 and 5 were old 

 (adults), the females apparently having given birth 

 to young. The others were probably in their third to 

 sixth months and had not given birth. 



31, 1909, was severely injured by her mate. The wound 

 was not yet healed September 12, 1910, when she was 

 killed, at which time she was found pregnant. The fol- 

 lowing table is the reproductive record of the other six 

 pairs. 



