PEEDISPOSITION. 21 



disease is mauife.sted by parental or ancestral tendency or by direct trans- 

 mission from a parent. 



HEREDITARY TENDENCY.— The ovum of the female on being 

 fecundated by a spermatozoon of the male, becomes a living being gifted with 

 the mental and physical properties of both its parents, in varying proportions, 

 and consequently contams those of its paternal and maternal ancestors in 

 proportions which, as a rule, decrease more or less according to the remote- 

 ness of the relationship. These hereditary properties, even when decreased 

 to an extremely small fraction by the successive division of generations, are 

 capable of becoming stimulated into development by surroundings or by forces 

 unknown to us. Thus, after a period of possibly 500,000 generations, a 

 single-toed mare gives birth to a foal which has one or more feet resembling 

 those of its three-toed ancestor (see " Points of the Horse"). Under ordinary 

 circumstances, the nearer the ancestor, the stronger is the influence of 

 heredity ; although in no case (speaking within reasonable limits) can such 

 influence be entirely effaced. The more nearly related dam and sire are, the 

 larger is the proportion, in the offspring, of properties which they both derive 

 from a common parent or ancestor. Hence, consanguinity is a fruitful cause 

 of excessively developed hereditary defects and hereditary tendencies to 

 disease. The union of a brother and sister would be twice as close a case 

 of incest, as one between a parent and its offspring. 



Acquired characteristics, as for instance, roaring, navicular disease, side- 

 bones, spavin, and ringbone, are not hereditary ; because these diseases could 

 not be produced in a descendant without an exciting cause. Although the 

 offspring of roarers, for example, are far more apt to become " musical " in 

 England and other damp and comparatively cold climates, than their com- 

 patriots which are descended from sound- winded dams and sires ; they hardly 

 ever go wrong in their wind, if bred and reared in a dry hot climate. Idee that 

 of India and South Africa. Here, the effect of heredity is confined to the 

 transmission of predisposition. 



DIRECT TRANSMISSION.— In some cases, the blood of the dam carries 

 the microbes of infectious diseases, like anthrax, to the foetus. " The 

 researches made by M. Chauveau have shown that bacilli rarely pass from the 

 mother to the fcetus. In eleven cases of pregnant ewes which had died from 

 anthrax, the bacilli of this disease were found in only two of the fceti " 

 [Gadeac). Such intra-uterine infection " mSij occur in man as well as in 

 animals, but it is an infection, not an inheritance" (Kanthack). This author 

 tells us in Gibson's " Text Book of Medicine " that " as far as the mammalia 

 are concerned, there is no evidence that an ovum is ever infected before or 

 during conception." 



HEREDITARY IMMUNITY.— It appears from the researches of Ehrlich, 

 Vaillard, and others, that immunity against certain diseases, such as tetanus, 

 may be conferred on the foetus through the mother ; but that no immunity is 

 transmitted through the o\iim of an immune mother or through the sper- 

 matozoon of the sire. Thus, if the mother be rendered immune before con- 

 ception or before delivery, the offspring may, through the blood of the mother, 

 acquire immunity, but only for a brief period. Here we have apparently the 

 action of an antitoxin. 



