256 PARASITIC BACTERIA. 



than those kept in the stable, even though subsequently they are 

 put under similar conditions. In localities where the animals run 

 out of doors all the time the disease is rare. The more closely they 

 are housed the greater the tendency to this disease, and it is practi- 

 cally certain that this greater tendency is not because they are so 

 much more likely to be infected, but because of the depressing in- 

 fluence which such a restricted life has upon the vitality of the 

 animals, reducing their resisting powers. It is also a general be- 

 lief that highly bred cattle have a greater tendency to this disease 

 than less highly bred stock. Stated in this way the conception may 

 not be correct; but it is practically certain that animals which have 

 been bred for the purpose of producing great quantities of milk are 

 rather more likely to yield to the disease than those not so highly 

 specialized. Such a specialization of the vitality in the direction of 

 an abnormally high action of the milk glands cannot fail to be at the 

 expense of other vital functions. These breeds have been developed 

 in one direction until they have become abnormal. It is not to be 

 wondered at if such an abnormal development should have resulted 

 in the reduction of their general vitality, and of their resisting power 

 against disease. It is the active, vigorous cow, which produces, 

 perhaps, but little milk and is not carefully housed by the farmer 

 that has the power of resisting disease. In short, the prevalence and 

 the increase of some of the diseases of domestic animals must be 

 attributed, in no inconsiderable measure, to the introduction into 

 our herds of conditions of life which lessen their resisting power, 

 and not wholly to the increasing chances of contagion due to close 

 contact of animal with animal. That the latter phenomenon is 

 also a factor is, of course, evident. 



The conditions of life among domestic animals are, to a very 

 large degree, under immediate and perfect control. We can regu- 

 late the amount of outdoor life they have, their activity, their food, 

 their drink, and many other factors upon which their physical 

 vigor depends. We may keep the cow housed so that she has 

 little air; we may give her highly stimulating food with practically no 

 chance to use her muscles; or we can make quite a different animal 

 of her by changing her life and food. We can control the conditions 



