other hand, a number, especially of experimental investigators, without being able to accept as valid any of the claims as to the discovery of a microorganism as the cause of human cancer, always pointed out the possibility that microorganisms might at least in a certain number, perhaps even in a majority of cases, be a factor in the production of cancer and tried to find new experimental means to approach these problems; accordingly on various occasions we pointed out the possibility that ultra-microscopic, perhaps intracellular, microorganisms might induce body cells, under certain conditions, to proliferate in such a manner that cancer resulted. However, in no case of human cancer has the causative significance of a microorganism so far been proven. We shall see later on that in a certain kind of animal cancer this proof has recently been supplied by Peyton Rous.
