THE COMMUNICABLE DISEASES 517 
flying cage the disease has occurred in varieties with high 
general susceptibility. There are at the present writing 
ten orders on exhibition in this large enclosure and there 
have been more. An occasional case of the disease occurs, 
but only in the orders which show it elsewhere. The 
Herodiones, of which we have had nearly one hundred 
autopsies and many now are on exhibition, are always 
well represented in this cage and yet show no tubercu- 
losis. In the ten orders mentioned above three show no 
cases of the disease. 
These observations illustrate the spread of 
tuberculosis, especially to the most susceptible varieties, 
and how non-susceptil)les under good hygienic conditions 
fail to become infected even when infected animals are 
near them. The freedom of activity in the large 
enclosure is doubtless an important factor. 
The history of the past three years with regard to the 
control of tuberculosis in the small cages shows that 
twenty-nine were infected, but by the measures employed 
nineteen have remained free of the disease for one year ; 
three of the remaining ten are known to have received 
newly arrived and possibly infected specimens. 
The accredited method of transmission in birds, the 
swallowing of material soiled with the feces richly laden 
with germs, is the principal reason why infected en- 
closures and their immediate environment are the prin- 
cipal breeding places for tuberculosis. To be sure air 
currents may blow the virus around, allowing it to light 
upon food in other cages but this cannot be a great menace 
if for no other reason than that we have had no epizootic 
outbreak of the disease, when there were groups of deaths 
in doves and guans. 
Evidences with which to trace transmissions are much 
clearer in the birds than in the mammals with the excep- 
tion of monkeys and some ungulates. Of course cases 
are perhaps too few in the carnivores and rodents to 
permit correct deductions but it is very rare that more 
