56 J: B. CLELAND. 
genic microbe a better chance to spread, but also tends to 
increase the pathogenicity of feebly pathogenic types. We 
are not quite clear as to what this increased pathogenicity 
really means. It may mean that the tendency of each spe- 
cies to vary round a mean, here gets its opportunity to 
perpetuate mutants of an aggressive type. Or it may mean 
that the strain as a whole accommodates itself better to 
its environment, becomes more acclimatised, with results 
disadvantageous to the host. 
As an offset to the increased virulence of the invader, we 
have an increase of protective bodies in the host, whilst if 
the host’s response is poor, it may die, and a weakling— 
‘from this point of view—be thereby eliminated with advan- 
tage to the species. It may be briefly stated here that it is 
rarely of any advantage to the invader to destroy its host. 
It is, in fact, seldom of any advantage to it to incapacitate 
the host, or make it ill, or even to cause any reaction to its 
presence. Reaction—and these reactions constitute and 
cause the signs and symptoms of disease—is an effort on the 
part of the host to nullify the invasion of the parasite and 
to repair the damage done. In object, if not always in ef- 
fect, such reaction is purely protective. It may almost be 
stated as an aphorism that the first sign of getting well is 
getting ill! The invader’s aim is clearly to avoid any re- 
action at all if possible. 
The community animals, rats and mice, therefore, like the 
community animal man, might be expected to offer better 
facilities for the development of new races of patho- 
genic organisms than non-social animals. As a matter of 
fact, we do find that, just as man has a number of diseases, 
and is infested by a moderate number of parasitic animals, 
so are rats and mice affected by a considerable number- 
of animal parasites and several epizootic diseases. 
The Mantfestations of Disease may be Specific Attributes 
both of the Parasite and the Host.—In the early days of the 
