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 
