BOWMAN LECTURE. LIX 



One of the first questions generally raised when heredity 

 is under discussion is the influence of consanguinity in the 

 parents or ancestors. The belief that kinship between 

 parents is a source of disease or degeneracy in the 

 children is widely spread and has its roots deep in the 

 past ; and yet we meet every day with marked differences 

 of opinion and practice in regard to the matter, between 

 one house or family and another. The real question is 

 this : Can the marriage of blood relations produce disease 

 of which neither the parents nor ancestors showed any 

 trace, or does the consanguinity operate simply by increas- 

 ing the likelihood that both of a pair of parents will contain 

 the seeds of the same undesirable, or it may be desirable, 

 character ? If the former be true no cousin- marriage can 

 be said to be safe. But if the latter be the correct position 

 and the results of all modern research appear to point 

 that way the outcome of the consanguineous union will 

 depend entirely upon whether the particular disease, or 

 other heritable character, is carried by both parents, by 

 only one of them, or by neither ; the consanguinity will be 

 operative only if it increase the chance that both parents 

 are tainted. If the transmissible condition be one that is 

 very common there may be as much chance of its presence 

 in both of an unrelated pair as in both of a pair of 

 cousins; but any comparatively rare disease is more likely 

 to be present in two cousins than in two unrelated persons. 



Accordingly we find a general belief in the medical 

 profession that in diseases so relatively infrequent as 

 retinitis pigmentosa and deaf-mutism consanguinity of 

 the parents plays an important part. And the same is 

 true of some other conditions where, as in the diseases 

 just named, both sexes are liable to suffer from, and both 

 liable to transmit, the disease. 



But in sex-limited conditions, such as Leber's disease 

 and congenital colour-blindness, where only the males 

 suffer, though the disease is carried down by (apparently) 

 normal females, consanguinity of parents is known to be 

 infrequent. If we start with a colour-blind male we know 



