366 HELEN DEAN KING 



The nervousness of the horse is undoubtedly the result of continued 

 selection, since breeders consider that an animal must have this 

 trait highly developed if it is to be a success on the track. If 

 nervousness is a trait that is transmitted by inheritance and 

 amenable to selection it is probably also a trait that would tend 

 to be intensified by close inbreeding, and therefore it might 

 be expected that rats closely inbred for many generations would 

 be somewhat more nervous than outbred stock controls, as Utsur- 

 ikawa found to be the case. 



When the last two series of investigations were completed the 

 animals used were sent to The Wistar Institute where they were 

 killed and carefully examined by Dr. Hatai. It was found, as 

 Mrs. Yerkes states, that the inbred rats had a somewhat greater 

 body length and body weight than the stock rats, and that they 

 showed a brain weight in relation to body length and body weight 

 that was only from 0.002 per cent to 0.006 per cent less than that 

 of stock rats. Since the inbred rats of the sixth and of the sev- 

 enth generations had a brain weight about six and one-half per 

 cent less than the normal (Basset, '14), it w^ould appear, from Mrs. 

 Yerkes' findings, that somewhere between the seventh and the 

 twelfth generations the animals entirely recovered from the 

 effects of malnutrition and became normal again with respect to 

 the relative weight of the central nervous system. They have 

 remained normal in this regard up to the present time, as autopsies 

 made at various periods on animals of the later generations have 

 shown. 



With the return of the central nervous system to its normal 

 weight relations, the inbred rats must have regained much of 

 their lost mental vigor, since in behavior tests animals of the 

 fourteenth generation were found to be inferior to stock animals 

 only in that they were slower and less active. The lesser activity 

 of the inbred rats Mrs. Yerkes ascribes to " a greater timidity and 

 a greater susceptibility to environmental conditions." Savage- 

 ness, wildness, and timidity are heritable behavior complexes, 

 according to R. Yerkes ('13), and since no attempt was made in 

 the course of these experiments to eliminate these traits by selec- 

 tion, it is not surprising that they were manifested in a somewhat 

 intensified form after many generations of close inbreeding. 



