82 



\'ETERINAR\- PATJIOLOGY. 



that it was transmitted to tlieir offspring; and was then a nat- 

 ural immunity. 



The resistance possessed by dogs to most diseases is ex- 

 plained in a similar way tu the Cubans' resistance to yellow 

 fever. Thus the dog has descended from the jackal and the 

 wolf, two types of animals that have lived largely upon the 

 carcasses of animals dead of various diseases. As the animals 

 fed (in carcasses they fought, thus inoculating each other, so in 

 the beginniniT the least resistant individuals died, the more resis- 



Flg CO.- 



I'Horoptew C'oniiminis Ovis, ma.tcnificd 150 diameters. 



tant animals survived. Tims the constant fighting and inocu- 

 lating has established in them a firm resistance that is trans- 

 mitted to their progeny as a natural immunity. This immunity 

 has become so fixed that it does not var_v even in the domestic 

 dog. The above is a plausible explanation of race or species 

 immunitx". Tlie exact origin of indi\-idual immunit-\' is considered 

 l)y some to Ije an ac(|uired tolerance, i. e., an actpiired immunity, 

 and 1)\- (itliers, as simjily an individual resistance not developed 

 by liaving tlie disease to which the given individual is immune. 

 Acquired immunity is an artficially produced condition bv 

 virtue of which the animal is capable of resisting disease, and 



