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THE AMERICAN NATURALIST [Vol. LI 



ing the rats in cubical houses of four feet in each direc- 

 tion, made of asbestos slates, and filled with rubbish for 

 the animals to hide in. In the beginning very many 

 animals refused to breed even in these cages, and as the 

 animals were crossbred from the very start, we believe 

 that a sort of very rigorous natural selection must have 

 been the reason for the fact that after a few generations, 

 every couple chosen could be relied upon to breed in as- 

 bestos cages, four feet deep and sixteen inches high and 

 wide. These cages were covered with small mesh netting 

 only on one half of the front, and they opened upon a sort 

 of corridor which was nearly completely dark, and could 

 be darkened entirely. In J ava some of our rats even bred in 

 small tin cages of the size of kerosene tins. In Buiten- 

 zorg the Department of Agriculture has constructed a rat- 

 house from plans furnished by us, composed of a series 

 of concrete rooms, so made, that the animals can be ob- 

 served from a darkened corridor without knowing it, and 

 a series of masonry tanks with wire covers. This house 

 is used for a biological study of rats, and for experiments 

 in cross-breeding, to determine the status of doubtful 

 material. 



It is not necessary to clean the cages very often, if only 

 they are well filled with dry straw and not overpopu- 

 lated. Disturbing the animals keeps them from breeding 

 freely. It happened that rats of this group bred in open 

 wire-netting cages, but in these cages the danger exists 

 that the mother can not make the nest sufficiently dark 

 and secluded to prevent disturbance by the male. It is 

 our experience, that a young female who has once neg- 

 lected or destroyed her Utter, is almost certainly lost for 

 further work. 



As ,'i rule tlie frinnlos do not leave the nest for the 

 first two or ill ice <lays, or as long as the young are cry- 

 ing. Afterwards, tliey cover the young in the evening, 

 bury tlie nest under earth, if they have it, to dig it up 

 again at the end of the night. When the young are three 

 days old, the mother permits young from an earlier litter 



