Experimental study of the rabbit papilloma of Shope, a growth caused by a virus, has shown that it possesses the immediate characters whereby tumors are recognized. Often it looks and acts like a malignant neoplasm. It differs from the tumors as a group, however, in its incidence which is that of an infectious process, and from other mammalian tumors in that its cause has been demonstrated. The possible bearing of the findings upon the problem of tumor causation is discussed. The morphology and behavior of the generality of tumors can no longer be taken to exclude the possibility that these are produced by extraneous, living entities. The incidence of some of the tumors at least, and the failure to demonstrate their cause can both be explained on the assumption that they are due to such entities, widely distributed in or upon the animal population but effective only under special circumstances. Present knowledge makes this assumption reasonable as a basis for further work.

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