PRESIDENTIAL ADDRESS. 59 
and #. Todtiana—rather resembling each other in super- 
ficial appearance, and often growing near each other. £. 
Todtiana has frequently very large and peculiar Brachyse- 
lid galls; the jarrah is remarkably free from disease of any 
kind. This gall, specific of the insect causing it, is equally 
a specific characteristic of its host. Measleg and scarlet 
fever, as manifested by their typical rashes, are specific re- 
actions in Homo, as characteristic of the host as of the un- 
known parasites causing the diseases. Man might be ex- 
elusively defined as an animal capable of reacting to the 
viruses of measles or scarlet fever by diseases characterised 
by particular rashes. Similarly the manifestations of dis- 
ease in rats or mice may be in some cases—as for instance 
rat leprosy—almost or quite specific features. 
Plague. 
It is now established conclusively that the usual means of 
dissemination amongst man, of ordinary bubonic and septi- 
cxmie plague, is by means of the rat through the intermedia- 
tion of rat fleas. Pneumonic plague, on the other hand, 
spreads directly from man to man, owing to the vast num- 
bers of plague bacilli expectorated by the sick and distribut- 
ed as fomites. 
THE ROMANCE OF PLAGUE. 
Of all diseases, plague is perhaps the most dramatic, 
tragical and historically interesting. Its spectacular 
clinical symptoms—the so-called ‘‘tokens’’ referred to 
by De Foe, and occurring in some_ epidemics—its 
rapidity of onset, its intense mortality, its decimation 
of populations, its effect on trade and commerce, 
the part it has played in history, the foundation it has laid 
for legends, arid, in modern times, the stimulus it has given 
to the cleansing of the foul slums of our great cities—all 
contribute to these characters, impress the popular mind, 
