If the truth of the parasitic theory could be proved, there would be justifiable expectation that the tumor disease might be controlled as are many of the parasitic diseases, but the hypothesis awaits the demonstration of its correctness.

Despite the study of tumors which is being actively pursued in many places and by the most skilled investigators, no parasites have been found in animal tumors; the objects previously described as parasites have been found not to be such.

It is difficult to bring in accord with the parasitic theory the great variation in tumor structure, the relation of certain tumors, as the malignant tumors of the breast and uterus, with the age of the bearer, the congenital tumors which develop in intra-uterine life, and there are many other conditions which oppose the theory.
The traumatic[2] theory.

There is much in favor of this.

In a certain number of cases tumors do develop at the site of injuries.