276 PARASITIC BACTERIA 
some of them escape with no signs of the disease, showing a 
superior resisting power. Undoubtedly the resisting power of 
animals is due to a proper physical vigor, little understood, but 
plainly dependent upon proper conditions of life. Let the con- 
ditions be normal, and the animal may resist the attack of para- 
sitic bacteria; but let them become abnormal, so as to reduce 
vitality, and the animal is much more likely to succumb. 
Tuberculosis, for example, is much more prevalent among 
cattle that are kept stabled most of the time, than among those 
that spend a considerable portion of the time in the open air. 
This may be due, in part, to the fact that stabled cattle have a 
greater chance of acquiring the contagion, since they are kept 
so close together. But this is certainly not the whole reason. 
Young cattle that are kept in the open for a year or two are less 
liable to take the disease 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 practically certain that this 
greater tendency is not because they are so much more likely to 
be infected, but because of the depressing influence which such a 
restricted life has upon the vitality of the animals, reducing their 
resisting powers. It is also a general belief 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 spe- 
clalized. Such a specialization of the vitality in the direction of 
an abnormally high action 6f 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, 
