Hereditary Disease. ' 33 



and ovum are likely to be imnuine and the new cellular structure 

 resulting from the copulation of these two (the embryo) will 

 possess the same characteristic and transmit it again, because 

 its own sexual cells are clearly the descendants of the rest of 

 the group. In the same way a predisposition may be established 

 and transmitted, because the chemical substance in some way 

 weakens the germ cell and diminishes its resistive ability, and by 

 the fusion of two similarly enfeebled sexual cells the progeny, as 

 the product of both, partakes of their equalities. Similarly to 

 such chemical influence as Ribbert suggests, any exaggeration 

 or diminution of the body temperature must affect the germ 

 cells. 



Inherited pathological conditions and properties of immunity 

 need not necessarily manifest themselves in each successive gen- 

 eration of posterity indefinitely; for the most part they are 

 limited to a few generations and then disappear. But they may 

 recur in later generations. If but one of the progenitors possess 

 a certain predisposition and come in sexual union with a non- 

 predisposed individual, the pathological condition will progress- 

 ively diminish by half (Ribbert) because of its distribution in 

 the two sexual cells ; is proportionately corrected by the healthy 

 cell of the copulation and by germinal variation ; and grows 

 weaker and weaker until it is no longer a factor in the posterity. 

 Should the peculiarity reappear after several intervening genera- 

 tions, it may be assumed that this occurs as a result of the union 

 of two germ cells, each possessed beforehand of a disposition 

 which was hitherto latent, but which from the summation of the 

 characteristics of both parent cells has again become sufficientlv 

 intensified to reappear as a pathological fault (Ribbert). Hence we 

 distinguish: a direct li credit y from father and mother to the oiT- 

 spring; a latent heredity if the offspring of affected parents are 

 spared but in the next or later generations the disease should appear, 

 arising from a transmitted tendency ; a collateral heredity, should 

 the disease manifest itself in the side lines of the family group ; 

 an atavistic heredity, when the origin of the disease suggests a 

 reversion to the family ancestry. 



In human medical practice the following are considered as 

 diseases transmissible by heredity: Hsemophilia or hsemorrhagic 

 diathesis, color blindness or Daltonism (so named after the 

 Englishman, Dalton, who was himself color-blind), lenticular 

 opacity or "gray cataract" (or cataract), near-sightedness or 



