'.' OF 

 ORGANIC DEPENDENCE AND DTS(Ei4SE 105 



is that of a universal struggle to maintain existence at the 

 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 tentudinaria 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 



