EFFECTS OF INBREEDING ON FERTILITY AND VIGOR 365 



In the inbred rats of the earlier generations the brain and spinal 

 ■ cord were decidedly below the normal weight of these organs in 

 stock animals of like age and body weight. ''From the fourth to 

 the tenth generation the relative brain weight remained, on the 

 average, constant at six and one-half per cent less than that of the 

 normal control rats" (Basset, '14). The habit formation in a 

 number of rats that belonged in the sixth and in the seventh in- 

 bred generations was tested at Johns Hopkins University by 

 Basset ('14), who found that these animals were inferior to stock 

 rats in their ability to form habits, and that they show less reten- 

 tion of a habit, and were longer in relearning it, than were the 

 controls. 



Inbred rats belonging in the twelfth and in the fourteenth gen- 

 erations were sent to Harvard University where Mrs. Yerkes 

 ('16) studied their, behavior and compared it with that of stock 

 albino rats obtained from The Wistar Institute colony and from a 

 different source of supply. The general conclusion reached by 

 Mrs. Yerkes was that ''inbred rats learned a trifle more slowly 

 than the stock rats, both in the maze and in the discrimination 

 experiments, but that they carried discrimination of lightness and 

 darkness further, and showed the most pronounced difference 

 only in their greater timidity and instability of behavior." 



Temperamental differences between stock Albinos and inbreds 

 of the fourteenth and the fifteenth generations were investigated 

 at Harvard by Utsurikawa ('17). The results obtained showed 

 that inbred rats were less active and more savage than the out- 

 bred rats, and that they responded more quickly and in greater 

 amount to momentary auditory stimulation than did outbred 

 rats. The two strains were found to differ also in "restlessness 

 or continuity of response." Inbred rats showed the greatest 

 restlessness" in case of momentary and repeated auditory stimu- 

 lation and less in case of continued stimulation, whereas for the 

 outbred animals the reverse is true." These temperamental 

 differences between inbred and stock rats would seem to indicate 

 that inbred rats are more 'high strung' nervouslj^ than are out- 

 bred rats. Nervousness is a trait manifested by many thorough- 

 bred animals, and it is particularly characteristic of the racehorse. 



