544 TRANSACTIONS OF THE CANADIAN INSTITUTE. [Vor. VIII. 
could lead to the evolution of such a disease after man took to dwelling 
in caves. 
One could multiply similar instances, but probably these two will 
suffice to illustrate the influences which must have led to the evolution 
of human disease. 
The effect of human disease upon human evolution has been equally 
important. It has been pointed out that one of the results of the evolution 
of man, was, that he rose superior to the ordinary workings of the law of 
natural selection. This, however, is not altogether true, especially of 
primeval man. Among his progenitors the struggle for existence meant 
chiefly the struggle for food, and the struggle against the disabilities of his 
environment, but with the greater incidence of disease, and especially 
infectious disease, in the human race, disease itself became a selective 
factor. 
In regard to parasitic diseases we may distinguish several types— 
ist, the type in which recovery means that the body is fortified against a 
second attack, viz., a definite immunity exists after one attack. Such 
diseases, for instance, as the eruptive fevers like smallpox, scarlet fever 
and measles, illustrate this. Exposure of a large human community to 
such diseases means, first, the weakest are killed off; second, those which 
survive may be permanently weakened and probably rapidly cease to 
leave descendants; third, recovery is complete and individual immunity 
is acquired ; fourth, a certain number show a natural immunity and do not 
contract the disease and probably leave descendants who, more or less, 
inherit this immunity. Thus with each recurring epidemic there may 
be established by the selective influence of the disease a resisting popula- 
tion. This type of natural selection is probably responsible for a good 
many of the phenomena which we witness to-day among the civilized 
and uncivilized races. Measles is not the serious disease for the civilized 
races of mankind in Europe and America, where they have been exposed 
to it for centuries, as it is for the aboriginal races of the Pacific where 
it has been recently introduced; perhaps we should say re-introduced 
because probably the progenitors of these races in the Asiatic Continent 
suffered with the rest of mankind from the various human epidemics in 
the infancy of the race, but they became naturally isolated and excluded 
from the selective influences. The same is true of a number of other 
diseases. 
In another type of infectious disease, the spread in the body is slow 
and chronic; the results are always fatal; no immunity is established. 
An example of this is Leprosy. Its effect, if given free scope, is to grad- 
