THE INHERITANCE OF DISEASE 109 



before birth. In the latter instance, the cause of the 

 disease may be either the unhealthy condition of the 

 mother reacting on the child, or it may be ante-natal 

 infection by a specific microbe. A congenital disease, 

 because it is in existence already at the moment of birth, 

 is for that reason by no means inherited in all cases. It is 

 especially necessary to guard against this mistake in the 

 cases of certain contagious diseases. In these cases the 

 infection of the child may either take place through the 

 mother in utero, or the germ itself — either the ovum or 

 spermatozoon — may have been carrying the infective 

 agent, which shows its effects only during the development 

 of the embryo. Thus, congenital syphilis is not an in- 

 herited disease, but is merely due to ante-natal infection 

 from either parent. Congenital tuberculosis is very rare, 

 though both mother or father may suffer from this disease. 



(6) Inborn Diseases. 



Inborn diseases are those which have " their physical 

 basis in the germ- plasm of the parental sex-ceUs." Ger- 

 minal infection is, as we have already pointed out, a mere 

 infection, and has, as such, nothing to do with intrinsic 

 changes of the germ. Abnormal peculiarities and defects 

 of the germ may arise spontaneously — i.e., without our 

 being able to assign a cause for them. Certain malforma- 

 tions — idiocy, colour-blindness, etc. — are such instances of 

 pathological germinal variations, and are, as such, of course 

 inheritable. (They need not necessarily be inherited in 

 every case ; that depends, as we shall see later, on the dis- 

 tribution of diseased determinants among the progeny.) 

 The difference in the inheritability of acquired and inborn 

 diseases comes out strikingly in Deafness, as quoted by 

 Professor J . A. Thomson. There is a distinction to be made 

 between congenital (inborn) deafness and accidentally 

 acquired deafness due to diseases during childhood. The 



