he trees, but
their parasitic enemies have developed with them and check their
increase. These pests were brought to this country in which there were
no conditions retarding their increase and have produced great damage.

It is very difficult to estimate the degree of racial susceptibility.
The negro race seems to be more susceptible to certain diseases, such
as tuberculosis and smallpox, less so to others, as yellow fever,
malaria and uncinariasis. What are apparently differences in
susceptibility may be explained by racial customs. A statistical
inquiry into death in India from poisonous snakes might be interpreted
as showing a marked resistance on the part of the white to the action
of the venom, but it is merely a question of the boots of the whites
and the naked feet and legs of the natives. The relatively greater
frequency of smallpox in the blacks is due to the greater difficulties
in carrying out vaccination measures among them and the greater
opportunity for infection which results from their less hygienic life.
It has always been noted that when plague prevails in Oriental cities,
the natives are more frequently attacked than are Europeans. This does
not depend upon differences in susceptibility, but on the better
hygienic conditions of the whites which prevent the close relation to
rats and vermin by which infection is extended. There would be but
little extension of the hookworm disease in a community where shoes
were worn and the habits were cleanly.

It is by no means improbable that the formation of the habits of
civilization was influenced by infection. Most of these habits, such
as personal cleanliness, the avoidance of close contact, the demand
for individual utensils for eating and drinking, are all of distinct
advantage in opposing infection. Certain habits, on the other hand,
such as kissing, which probably represents the extension of a habit of
sexual origin, are disadvantageous and infection is often transmitted
in this way. In syphilitic infection the mouth forms one of the most
common localizations of the disease and may contain the causal
organisms in great numbers. This, the _spirochaeta pallida_, is an
organism of great virulence, and man is the most susceptible animal.
The disease, like gonorrhoea, is essentially a sexual disease, the
primary location is in the sexual organs, and it is transmitted
chiefly by sexual contact. Of all the infectious diseases, it is the
one most frequently tran