DISEASE AND RELATED PHENOMENA IN ANIMAL BREEDING 523 



for instance, for the fetus to be infected with disease at any time from the 

 moment of conception to that of birth, and ante-natal infections are as 

 truly acquired as are post-natal infections. It is possible for the fetus 

 to be infected directly and independently of the mother, as occurs at 

 times in cases of venereal disease; it is also possible for the fetus to become 

 infected from the mother through the placenta, a much less common 

 phenomenon. In either case, however, it is confusion of thought to speak 

 of such instances as examples of the inheritance of disease. The many 

 ramifications of the subject cannot be dealt with fully here; suffice it to 

 say that as with acquired characters so with disease transmission the 

 affirmative case has not been proven. 



The Inheritance of Predisposition to Disease. Predisposition to 

 disease may be either specific or general. In specific predisposition 

 the individual is liable, should prejudicial conditions occur, to contract 

 a particular disease, as for example gout, whereas general predisposition 

 gives room for infection with a series of similar diseases. This latter 

 type of predisposition is very commonly expressed in general consti- 

 tutional debility rather than in any specific fashion. It is particularly 

 shown in nervous diseases in man, where a predisposition to nervous 

 derangement is expressed in a variety of ways in a given line of descent, 

 but even here the predisposition is often specific to a remarkable degree. 



That predispositions are inherited cannot well be questioned, but in 

 tracing their transmission it is necessary to guard against factors which 

 confuse cases. Thus it does not necessarily follow that an individual 

 predisposed to a disease should invariably become a victim to that disease. 

 Transportation in early life to a new environment or particular attention 

 throughout life to matters making for health with respect to a known 

 inborn tendency may entirely overcome the predisposition. The degree of 

 predisposition also varies; in some cases it is so strong as to amount 

 practically to transmission of disease; in other cases it is relatively 

 weak, and requires either that the inciting cause be impressed frequently 

 upon the individual before resistance is broken down and the individual 

 yields to attack or that some other violently unfavorable condition 

 should enter into the life of the individual. 



It is worth while remarking that predisposition to disease like many 

 defects of structure often skips generations in inheritance. In general it 

 appears that the normal state of health is the dominant condition and 

 that predispositions are for the most part dependent upon the action of 

 recessive factors or sets of factors. Accordingly predisposition to disease 

 is more likely to crop out as a result of consanguineous matings, because 

 related individuals are more likely to bear the same recessive factors in 

 their germinal material. In this fashion it is not difficult to account for 

 the persistent atavistic appearance of disease in some lines of descent. 



