78 J. B. CLELAND. 
As regards tuberculosis, however, the infection of 
both animals and man is unquestionably more or less direct 
by contamination from the surroundings of previous cases, 
the tubercle bacilli not leading a saprophytic multiplicative 
existence in the interval. 
In this group of diseases, then, we see two different types 
of infection, one, illustrated by tuberculosis, showing more 
or less direct infection from one victim to another; the 
other being apparently due to the establishment of an or- 
ganism which usually leads a saprophytic existence, but 
which, given the opportunity, can multiply and produce dis- 
ease in higher animals. Under which of these two ecate- 
gories do leprosy in rats and leprosy in man come? 
It is recognised that leprosy in human beings is rarely 
conveyed to those in contact with lepers. Occasionally, af- 
ter lone and intimate association, such cases occur. The 
danger of a leper to others is greatly less than that of a 
tubercular person to his associates. Cases occur in fami- 
lies, but here it is hard to say whether one had been infect- 
ed from another or whether all had had a common origin. 
Leprosy is a relatively rare disease, both the human form 
and the rat one. Actinomycosis, a disease apparently not 
spread by direct association with previous cases, is common 
in our cattle, and even in man not rarer than leprosy. It 
seems to me quite possible, therefore, that leprosy, both in 
rats and men, is due to the accidental establishment in 
the tissues of an organism which is normally a saprophyte 
of their surroundings. Undoubtedly the phylogenetic his- 
tory of the tubercle bacillus would show that it was original- 
ly such a saprophyte purely confined to growth in our sur- 
roundings, as for instance the acid-fast Timothy-grass ba- 
eillus is still; that its next stage was its accidental introduc- 
tion by the alimentary canal or through wounds into the 
tissues of vertebrates, where it found itself capable of living 
