No. 607] RATS AXD EVOLUTION 4 1 1 



to return to the nest, but exclusively the femak's. < )iily 

 when the young open the eyes at fourteen to sixteen days, 

 the father is permitted to return into the nest, Tlie young 

 males are kept out of the nest until the young are weaned. 



To observe the habits of rats of this group, an endless 

 patience is required, as the animals, which are extremely 

 sensitive to hardly noticeable sounds and movements, 

 habitually are active only at night. If it is possible to 

 darken the room completely, it is possible to observe the 

 animals in the daytime by the light of a small lantern, 

 after rousing them. Weak artificial light seems to make 

 hardly any impression upon rats or mice. 



We have seen wild rats, mating and foraging, to con- 

 tinue eating or playing, even when a small lamp was 

 waved to and fro under their very noses, whereas the 

 same animals would be disturbed by the falling of the 

 head of a match, A good plan i§ to feed caged rats only 

 once a day, at a set hour, to which they accustom them- 

 selves very rapidly, as in this way they can be counted 

 upon to be up and doing at a time when it is most con- 

 venient to observe them. Even wild-living rats and mice 

 accustom themselves to a fixed hour of feeding, A draw- 

 back of the system is that when the supply of food is not 

 more than abundant, delay in feeding of only two hours 

 may cause the death of recently weaned, sometimes even 

 of half -grown rats. The discouragement may be looked 

 upon as being partly the reason of this, for these rats are 

 extremely nervous animals. A shock, a sudden fright, 

 may cause them to lose consciousness for a long time, and 

 fright will often kill them outright. 



To be able to observe rats of this group at our ease we 

 tried to tame some of them. Young Mns decnmanus taken 

 at the time of weaning become tame, or rather stay tame 

 without more trouble. It is impossible to get them tame 

 by taking tliem at an age of six weeks to two months, 

 wlKMi tlu'v aiv wild and a])! to bito. Fidl ,-rown animals 

 are ,-a>ior to lame, cvm if wild r-au-lit. 



To make Mus raff us tame, it is necessary to handle the 



