PREDISPOSITION. 21 



Heredity. 



Heredity is the appearance, in descendants, of " like " of parents or of 

 ancestors. This production of " like " is limited to parental or ancestral 

 function, and, consequently, cannot begin after conception. Heredity in 

 disease is manifested 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 contains those of its paternal and maternal 

 ancestors in proportions which, as a rule, decrease more or less according 

 to the remoteness of the relationship. These hereditary properties, even 

 when decreased to an extremely small fraction by the successive division 

 of generations, are eapable 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"). Un^er 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 tvrice 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 

 compatriots 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, like 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 foetus. In eleven cases of pregnant ewes which had died 

 from anthrax, the bacilli of this disease were' found in only two of the 

 fceti " (Oadiac). S'uch intra-uterine infection may occur in man as well 

 as in animals, but it is an infection, not an inheritance. As far as the 

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

 before or during conception. 



HEREDITARY IMMUNITY.— Immunity against certain diseases, such 

 as tetanus, may be conferred on the foetus through the mother ; but no 

 immunity is transmitted through the ovum of an immune mother or through 

 the spermatozoon of the sire. Thus, if the mother be rendered immune 

 before conception 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. 



