540 TRANSACTIONS OF THE CANADIAN INSTITUTE. [Vov. VIII. 
The tuberculous man lives for a varying period of time, protected and 
prevented from dying. A tuberculous animal in the wild state either 
recovers or is destroyed by ordinary selective forces, as soon as it becomes 
too weak to continue the struggle, and this probably takes place very 
early. That most fatal form of epidemic human disease, bubonic plague, 
is now traced to certain endemic areas where it persists in a chronic form. 
To-day, we have just begun to realize that Typhoid Fever persists in a 
community largely in individuals chronically harbouring the infection, 
so called typhoid carriers. Again, in malaria, we know from the researches 
of Koch and others that the majority of the native population in malarial 
districts is infected with the malaria organism. In this case, those who 
could not resist the infection have been killed off. The disease has itself 
acted as a selective factor and a chronic infection persists, which does not 
materially affect the individual suffering from it, but renders him a menace 
to all persons coming into his neighbourhood who do not possess a 
similar resistance. ‘Thus, the more the human race became domesticated, 
the more it became a prey to chronic and subacute diseases. 
Finally, an undoubted influence in the evolution of human disease 
was the relationship which developed between man and the rest of the 
animal kingdom, when he took to domesticating animals. This brought 
about a close association between the human form and other groups, 
which could not have existed in pre-human times. It is _ possible 
that tuberculosis was introduced into the human race under such cir- 
cumstances. The domestication of the animals themselves was an in- 
fluence in the propagation of epidemic disease among them, as Von Hanse- 
mann has shown. ‘They also suffer because their human master was able 
to interfere with the rigid natural selection of the wild state. How many 
of our human infectiousdiseases are to be traced todomestic animals it is hard 
to say. Certainly many of the parasitic worms infected man from this 
source. Many skin diseases probably had the same origin and in the 
case of one infectious disease at least, viz., smallpox, we may possibly 
look for its origin among the domestic animals. It has now been proved 
beyond a doubt that the so-called cowpox, or vaccinia, is simply a milder 
form of the same disease as human smallpox. It must be admitted, 
however, that there is probably as much evidence for the view that cowpox 
originated among cattle through their association with man as the con- 
verse. Brinckerhoff and Tyzzer have shown in the Philippines that 
in the ordinary monkey, inoculation with smallpox virus develops a 
variolous infection, which, however, histologically is less like human 
smallpox than theinfection in Anthropoids, whichisthe true vartolainoculata, 
but the wide-spread occurrence of vaccinia-like eruptions among domestic 
animals at least suggests the possibility of an origin from an animal source. 
