LAW OF SIMILARITY. 29 



6. They show a striking tendency to modify and 

 absorb into themselves all extraneous diseases ; for 

 example, in an animal of consumptive constitution, 

 pneumonia seldom runs its ordinary course, and when 

 arrested, often passes into consumption. 



7. Hereditary diseases are less effectually treated by 

 ordinary remedies than other diseases. Thus, although 

 an attack of phthisis, rheumatism or opthalmia may be 

 subdued, and the patient put out of pain and danger, 

 the tendency to the disease will still remain and be 

 greatly aggravated by each attack. 



In horses and neat cattle, hereditary diseases do not 

 usually show themselves at birth, and sometimes the ten- 

 dency remains latent for many years, perhaps through 

 one or two generations and afterwards breaks out with 

 all its former severity." 



The diseases which are found to be hereditary in 

 horses are scrofula, rheumatism, rickets, chronic cough, 

 roaring, ophthalmia or inflammation of the eye, — grease 

 or scratches, bone spavin, curb, &c. Indeed, Youatt 

 says, " there is scarcely a malady to which the horse 

 is subject, that is not hereditary. Contracted feet, curb, 

 spavin, roaring, thick wind, blindness, notoriously de- 

 scend from the sire or dam to the foal." 



The diseases which are found hereditary in neat 

 cattle are scrofula, consumption, dysentery, diarrhea, 

 rheumatism and malignant tumors. Neat cattle being 

 less exposed to the exciting causes of disease, and less 



