50 HELEN DEAN KING AND HENRY H. DONALDSON 



organs of these animals are not diseased, and that the dis- 

 eases from which they suffer are not those that affect fertility. 

 He shows, furthermore, that this infertility cannot be at- 

 tributed to failure of the sexual instinct, to a change in climate 

 or in food, nor can it be due to lack of exercise. His conclu- 

 sion is: "we can in no case precisely say what is the cause 

 of the diminished fertility of an animal when first captured. 

 We can only infer that it is caused by a change of some kind 

 in the natural conditions of life. ' ' 



The tendency to sterility, which Darwin showed is common 

 to many species of animals when first taken from a state of 

 nature, was exhibited in a marked degree in the case of the 

 wild Norway rats used as foundation stock for our colony of 

 captive Grays. Only six of twenty wild females bore young 

 during the period of their captivity, as far as known. The 

 reproductive power of the females that bred seemed little, if 

 at all, affected by the changed conditions of environment and 

 of nutrition, since the average number of litters they cast and 

 the average size of their litters compared favorably with 

 similar averages for litter production and litter size in later 

 generations (table 7). The fourteen sterile females lived for 

 some months in the colony and appeared to be in good physi- 

 cal condition during this time. Autopsies made on a number 

 of these animals showed no abnormalities or diseases of the 

 reproductive organs, although some of them were found 'to* 

 be suffering from the lung infection which frequently attacks 

 rats of any race after they are a year old. That the changed 

 conditions to which these rats were subjected in some way 

 adversely affected their fertility there can be little doubt. 



A factor which may have prevented breeding in many of 

 the wild rats, and one that may be responsible to a greater 

 or a less extent for infertility in other animals when first 

 brought into captivity, was the extreme fear of man which 

 wild rats show in a marked degree. Intense fear produces a 

 nervous tension which undoubtedly influences the activity of 

 many organs of the body, particularly those concerned with 

 secretion. It seems not improbable, therefore, that this nerv- 



