2XX ROBERT M. YERKES 



* 



kinds, defensive and offensive. Of each there are several indi- 

 cations. The former deserves a higher grade than the latter. 

 Defensively savage individuals are likely to jump at the observer 

 and cannot be safely handled even with the gloved hand. Offen- 

 sively savage rats may safely be handled: it is necessary only 

 to avoid hurting them. Wildness almost invariably accom- 

 panies savageness. Timidity may or may not. An extremely 

 savage and wild rat may exhibit little fear of the experimenter. 

 A savage and aggressive wild rat fights, whereas a timid rat 

 cowers, trembles, and chatters." 



The animals observed, numbering about three hundred (300), 

 consisted of wild rats, tame rats, and first and second genera- 

 tion hybrids. 



The wild rats were captured either in Belmont or in Cam- 

 bridge, Massachusetts, and were, with one exception, adult 

 males. Observations and tests on them, made in several in- 

 stances immediately after capture and again after they had been 

 in captivity for a year, indicated extreme savageness and wild- 

 ness, with variable timidity. The grade of 5 for savageness was 

 assigned to almost all of these individuals. In wildness, they 

 were graded either 4 or 5, and in timidity 3, 4, or 5. As a result 

 of their confinement in cages for a year, they exhibited a lower 

 grade of wildness and timidity, but their savageness remained 

 unchanged. It was impracticable and wholly unnecessary to 

 repeat frequently the tests on these wild individuals. 



The tame rats were taken from a strain in use for studies 

 of coat color at the Bussey Institution. This strain has been 

 bred in the Harvard Zoological Laboratory for at least ten 

 years. A brief account of some recent experiments with these 

 rats has been given by Professor Castle in a paper entitled 

 "Some biological principles of animal breeding." 1 



As a result of certain experiments in selective breeding, 

 two types of animal, each of which was used in my experiments 

 are distinguishable in this strain. They are known as wide (W) 

 and narrow (N) individuals. Both have black heads (hoods), 

 but in the wide the black extends further back than in the 

 narrow. The wide are known to have more wild blood than 

 the narrow, and in these experiments they prove to be wilder 

 and more savage. 



1 American Breeders' Magazine, 1912, vol. 3, no. 4. 



