HEREDITARY DISEASES OF HORSES. 
527 
hereditary diseases develop themselves only some considera- 
ble period after birth, and the inherent tendency may even 
remain later during many years. Thus, in man, gout and 
gravel do not usually develop themselves until after the 
meridian of life, and in horses and cattle the tendency to con- 
sumption, scrofula, and rheumatism may remain dormant for 
many years. Nay, more; diseases of an undoubtedly here- 
ditary nature may remain latent even for a generation or two, 
and afterwards re-appear with all their wonted severity: 
“Silente saepe morbo in genitore, dum ex aevo derivatur in 
nepotem ; w * and such cases are not of infrequent occurrence, 
and are certainly not at all incompatible with the hereditary 
nature of a disease. They may be satisfactorily explained in 
various w r ays. The morbid tendency may be so slight as not 
to interfere with health, or the animal may have been reared 
in circumstances where the exciting causes of the disease 
have been avoided. But in these cases, where a hereditary 
disease disappears for a generation or two, the tendency to 
the disease and the conditions in which that tendency con- 
sists are still transmitted, as is obvious from the fact, that the 
disease develops itself in the descendants with all the cha- 
racters 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 con- 
siderable time after it has been got rid of in the majority of 
the progeny, isolated individuals appear, which, in the phrase- 
ology of breeders, “call back” to their more remote pro- 
genitors, 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 
hereditary are sometimes produced accidentally, and without 
the intervention of any hereditary tendency. Rheumatism, 
which often owes its existence to an inherent rheumatic dia- 
thesis, may be developed in most animals by continued ex- 
posure to the ordinary exciting causes of the malady. Spe- 
cific or deep-seated ophthalmia, although generally dependent 
on a constitutional 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 produc- 
tion of any disease by extraneous causes is not at all incom- 
* Boerhaave, ‘Aphor. de Curandis Morbis/ 1075. 
