No. 607] 



BATS AXD EVOLUTIOX 



413 



The liouse-rat, European as well as Javanese, is dariiiu-. 

 calculating and relatively little nervous, and on bein-- 

 persecuted, looks out very well for possible hiding places, 

 in which the animal will remain immobile for hours. An 

 escaped house-rat is very often found with the utmost dif- 

 ficulty. Their disposition makes them relatively easy to 

 tame. • 



The tree-rats, Egyptian as well as Javanese, try to es- 

 cape from a persecutor by climbing. They are excep- 

 tionally aggressive, we have certainly been bitten more 

 times by tree-rats than by all our other rats combined. 



In our breeding experiments we used for all rats a card 

 catalogue. Every animal has its own card, on which are 

 noted its number, the numbers of its parents, eventual 

 outline drawings of its markings, and the numbers of the 

 animals it has been mated with, together with the num- 

 bers of its young born from these matings. In moving a 

 rat from one cage to the other its duplicate card was 

 moved with it to a receptacle attached to the cage. On 

 the card on file the cage number is noted in pencil. Ani- 

 mals which are dead get a distinctive mark, or their cards 

 are moved to the back of the file. "With such cards it is 

 very easy to find out the ascendants and the descendants 

 of every given animal, and it is easy to arrange the card 

 on a big table in the form of a pedigree. 



We started our experiments with Eattus rats, by taking 

 over some animals from Dr. Lewis Bonliote. From our 

 experiments^, whicli we are about to describe, it l)ecame 

 clear, tliat ci-ossinu- is not only tlie means of recombining 

 <']iara«-t(M'- ot' tlie -]>('.-i('- cro^-ed, a^ many English 



