president’s address. 
297 
larger types of beings ; whilst, of course, wo all recognize that 
apparent size is as nothing, that it is a mere accident, a question of 
the construction of our enquiring eyes, a condition that is at once 
altered and rectified by a magnifying glass. 
If it bo true that the invasion and presence of various small 
organisms in the bloo<l or tissues is the cause of the various specific 
diseases to which 1 have alluded, then the application of such 
knowledge as naturalists can obtain, as to the food and other 
conditions necessary to their existence; their mode of ingress to 
the body ; their development and multii)lication, becomes clear 
and obvious. It oi)ens uj) to our minds possibilities both of 
prevention, and of either mitigation or cure. For it is evident 
that if we can starve these Germs of their necessary nutriment, or 
make their new habitation unsuitable for their healthy and vigorous 
development, their career as invaders will necessarily either bo cut 
short, or bo rendered feeble and impotent; and therefore the 
disease-changes which they can produce less violent and less lethal. 
Something of this kind appears naturally to have taken place in 
those pereons in whom some of the zymotic diseases (of which 
^Measles, Scarlet Fever, Whooping Cough, &c., may bo taken as 
familiar types) have once run their course ; in those, that is, who 
are 2 iopularly said to have already had these diseases. And 
although the exact abiding change which is produced has not been 
ascertained, yet it is well known, and quite understood, to be one 
Avhich renders the fluids or tissues jiartially or wholly unsuitable 
for their future healthy growth. 
This theory, too, is the well-known explanation of the protective 
power of the Cow Pox, which once having permeated a human 
system, has rendered it unsuitable for the future healthy and 
vigorous development of its greater relation — the Small Pox. 
In default of available means of destroying the Germs of other 
malignant diseases, prolonged efforts have been made (and notably 
by the great French pathologist, Pasteur) so to diminish the 
intensity of the destructive force of some of these specific Germs, 
that they may be safely inoculated into human bodies Avithout 
danger to life, and yet be potent enough in their effects to anticipate 
