252 EVOLUTION AND DISEASE. 
In man tubercle affects any organ and tissue of the 
body, and is due to the entrance and multiplication 
in the body of a micro-organism, the tubercle bacillus, 
identified by the genius of Koch. Until the discovery _ 
of this bacillus in tubercular lesions it was customary to | 
apply the term tubercle to almost any disease which was ~ 
characterized by the formation of nodules in the tissues, — | 
thus the term came to have a generic rather than a 
specific signification. Now that we have a pathological 
criterion of tubercle, the term tuberculosis has a definite 
meaning, and certain affections formerly included are 
now known to be due to other causes, and numerous af- 
fections formerly excluded now help to swell the list of 
troubles due to this omnipresent micro-organism. 
Among other mammals the disease has a_ peculiar 
distribution ; it is very common among cattle under the 
name of grape disease, or its German equivalent, Pez?- 
sucht. Monkeys living in confinement in this country 
are occasionally attacked by it, but not so frequently as 
was formerly supposed. Among grain-eating birds the 
disease is a perfect scourge ; the flesh-eating birds are 
not so liable to contract it, and are probably not in- 
frequently attacked in consequence of devouring tuber- 
cular grain-eating birds. In quadrumana and man the 
disease runs a similar course, whilst in cattle the lesions 
are so different, that it would be difficult to believe that 
it is in any way related to the tuberculosis of Primates, 
were it not for the existence of identical micro- 
organisms, and this again applies equally to birds in 
whom the lesions differ from those in man and cattle. 
It is also extremely difficult to understand the immunity 
ef horses, tuberculosis being rare among Hguzde. 
