EFFECTS OF INBREEDING ON FERTILITY AND VIGOR 367 

 3. DISCUSSION 



Wherever inbreeding has been practiced it has usually been 

 accused of producing anything and everything undesirable that 

 has appeared in the offspring. The following quotation from 

 Mitchell ('65) is quite typical of the belief that prevailed among 

 zoologists, as well as among the laity, until the past decade, 

 regarding the effects of consanguineous marriages: 



Consanguinity in parentage tends to injure the offspring. This 

 injury assumes various forms. It may show itself in diminished via- 

 bility at birth; in feeble constitutions, exposing them to increased risks 

 from the invasion of strumous disease in after life; in bodily defects and 

 malformations; in deprivation or impairment of the senses, especially 

 those of hearing and sight; and, more frequently than in any other way, 

 in errors and disturbances of the nervous system, as in epilepsy, chorea, 

 paralysis, imbecility, idiocy, and moral and intellectual insanity. 

 Sterility or impaired reproductiveness is another result of consanguinity 

 in marriage, but not one of such frequent occurrance as has been thought. 



Stock breeders, also, have been imbued with the idea that in- 

 breeding is always inimical to constitutional vigor and that it 

 leads to sterility. For these reasons most of them have opposed 

 the mating of animals related even in a remote degree. During 

 the past few years it has been shown by a number of carefully 

 controlled experiments that inbreeding does not necessarily 

 produce the evil effects that have been attributed to it, and that 

 the results obtained in any inbreeding experiment depend, pri- 

 marily, on the soundness of the stock that is inbred; secondarily, 

 on the selection of animals for breeding purposes, and, finally, 

 on the environmental conditions under which the animals live. 

 Haphazard inbreeding of inferior stock under unfavorable en- 

 vironmental conditions has produced many of the failures for 

 which inbreeding alone has been held responsible. 



Since the experiments of Crampe ('83), of Ritzema-Bos ('93, 

 '94), and of von Guiata ('98, '00) have furnished the classic ex- 

 amples of the dire effects of inbreeding on rodents, it may be well 

 to examine these experiments in some detail to see whether the 

 unfavorable results obtained cannot be traced to some cause 

 other than inbreeding per se. 



THE JOtRNAL OF EXPERIMENTAL ZOOLOGY, VOL. 26, NO. 2 



