ORGANIC DEPENDENCE AND DISEASE 
105 
is tliat of a universal struggle to maintain existence at tlie 
cost of some other’s normal physiology. It is the unremit- 
ting battle of the invisible. The fight is not hidden away 
from eyes that are trained to see, and its victims lie dead 
on every field. It is a war whose harvest, surpassing com- 
putation, is the only salvation of a crowding world. 
But it is easy to forget that the picture has another side 
and that the attacks of the protozoan army and all the race 
of unicellular parasites are not by any means malign in 
their results. It is vice rather than virtue that ever chal- 
lenges widest attention. Probably more of these adjust- 
ments are benign and helpful to the host than are harmful, 
and there are as many “good” germs as “bad” ones. It is 
reasonable that the student of germ pathology should be so 
profoundly impressed with the mischievous invasions as to 
understate the helpful activities, and thus be led to such a 
conviction as that expressed by Eccles, “The Path of Evo- 
lution is the Path of Past Disease.” The history of such 
protozoan adjustments is not properly within the present 
scope of our subject, though we must look at certain paleon- 
tologic records which have been interpreted as devastating 
results of widespread germ infection. 
Fig. 105. Valves of the brachiopod DalmaneUa testudinaria from the Lorraine 
shale (Upper Ordovician), in which disease in, or injury to, the mantle of the 
shell has produced irregular growth. 
It is not to be wondered that several writers, Metchni- 
koff and Eccles among them, who have had under their eyes 
the universality of parasitism in existing nature, should be 
very positive in their expressed inferences that destructive 
pathologic parasitism has continued for vast ages. The 
statement is undoubtedly deductively correct within certain 
