INHERITANCE OF ACQUIRED CHARACTERS 181 



time held to be inherited, it is probable that the develop- 

 ment of such unsoundness is due largely to the action of 

 external factors. In other words, they are acquired. It 

 is true, however, that certain individual animals or 

 families are much more subject to bone disease than other 

 families. In such case we must recognize a predisposition 

 to disease. 



168. Congenital disease. The fact that a disease 

 exists at birth is not always adequate evidence that 

 disease has been inherited. It is possible for certain germ 

 diseases to infect the foetus in utero. It is also probable 

 that the ova or spermatozoa may under certain conditions 

 carry the infection and this infection may be present and 

 active during the prenatal life of the animal. 



169. Predisposition to disease. Many diseases ap- 

 pear to " run in families." For this reason we have recog- 

 nized the fact that the members of certain families are 

 subject to tuberculosis, gout, rheumatism, imbecility, 

 insanity or other diseases. In all these cases there exists 

 a predisposition on the part of the members of a given 

 family to acquire the diseased condition. Through weak- 

 ness of certain organs or general lack of constitutional 

 vigor, the infective germs of many diseases may overcome 

 the resistance of the animal organism to disease. This 

 predisposition is certainly and often strongly inherited. 

 From the standpoint of the practical animal breeder, 

 therefore, a predisposition to disease may be quite as 

 significant as the actual transmission of the disease. 



170. Immunity. An interesting correlated fact is 

 the probable inheritance of immunity from certain 

 diseases. Certain families seem to possess immunity 

 from certain diseases, such as smallpox or diphtheria. It 

 is not possible at this time to enter into a discussion as 



