112 



Hereditary Diseases of Horses. 



causes of the disease have been avoided. But in these cases, 

 whei;e a hereditary disease disappears for a generation or two, 

 the tendency to the disease and the conditions in which that 

 tendency consists are still transmitted, as is obvious from the 

 fact, that the disease develops itself in the descendants with all 

 the characters of a hereditary nature. It requires, indeed, many 

 generations, and a careful selection of parents, to eradicate from 

 a stock a hereditary tendency to disease, and, for a considerable 

 time after it has been got rid of in the majority of the progeny, 

 isolated individuals appear, which, in the phraseology of 

 breeders, " call back" to their more remote progenitors, and 

 possess, like them, an unusual tendency to disease. 



There are few diseases which invariably owe their develop- 

 ment to hereditary causes. Diseases usually regarded as here- 

 ditary are sometimes produced accidentally, and without the 

 intervention of any hereditary tendency. Rheumatism, which 

 often owes its existence to an inherent rheumatic diathesis, may 

 be developed in most animals by continued exposure to the 

 ordinary exciting causes of the malady. Specific or deep- 

 seated ophthalmia, although generally dependent on a constitu- 

 tional predisposition, sometimes destroys the eyesight of 

 animals in whose pedigree no such disease has been known ; 

 and even consumption and scrofula — diseases easily traceable 

 in the vast majority of cases to hereditary predisposition — are 

 occasionally developed in previously healthy constitutions by 

 the conjoined agency of bad feeding, cold, and neglect. From 

 this it is obvious that the production of any disease by extraneous 

 causes is not at all incompatible with its being in other cases 

 decidedly hereditary. Such cases as we have just adduced only 

 serve to show that the same disease is not always referable to 

 the same causes, and that causes very different in their nature 

 occasionally produce the same effects. 



Diseases accidentally produced during the lifetime of an 

 individual occasionally become hereditary, but not usually so. 

 Blindness produced by injury or ordinary external causes, and 

 roaring produced by phlebitis or even by bronchitis, are seldom 

 hereditary ; and it appears as a general rule, admitting, however, 

 of some exceptions, that a local injury or disease produced by 

 accidental causes is not likely to be hereditary, although a 

 generally deteriorated state of health, however produced, is very 

 apt to be so. 



There are various maladies which, from their simulating some 

 of the characters of hereditary diseases, have been thought by 

 many to be truly hereditary. Abortion affords an apt illustra- 

 tion of such a mistake. This disorder frequently prevails in a 

 stock for a long series of years, and sometimes even during 



