Parasitic Theory.
—The parasitic theory of tumor formation has only within a few years taken definite form and shape, as a result of evolution from vague suggestions and scattered observations. It implies that tumors, and they are mainly of the malignant type, are due to irritation produced by extrinsic agencies, parasites of some kind, which, introduced from without, act as do bacteria in the now well-known infectious granulomas. While this theory, perhaps, does not afford an absolutely satisfactory explanation of all the phenomena of malignancy, it nevertheless comes nearer to it than does any other hypothesis now before the profession, the arguments in favor of it being scientific and positive, and those against consisting mainly of mere negations. Summed up these arguments may be stated as follows:
1. Comparative Pathology.
—The argument from comparative pathology begins with the lower forms of life. Tumors in trees and plants are well known to vegetable pathologists and botanists as of frequent occurrence. They vary in size from the most trifling galls to those large woody masses known as xylomas, which are essentially tree cancers, since they tend to the destruction of the tree. These are known to be invariably due to extrinsic agencies, such as insects, fungi, etc. As water freezing in the bark of a tree may crack it open and thus leave opportunity for subsequent infection, so may injuries upon the body surface make trifling lesions which predispose to subsequent infection and cancer in man and animals. Exclude parasites from such traumatic lesions on plants and there will be no xylomas.
PLATE XIV
Adenocarcinoma with Young Parasites. (Parasites blue. Plimmer’s method.)
This plate is introduced to illustrate the presence of parasites, whose minute and actual character is not yet positively determined, but whose existence is undeniable.
From Gaylord’s paper in the Third Annual Report of the New York State Pathological Laboratory of the University of Buffalo.
PLATE XV