2i8 THE BCIENTIFIO MONTHLY
lus working through an agent external to the cancer cell could only be supplied by a microorganism constantly at work within the tumor. By microorganism we mean a very small living organism of either plant or animal character which can be seen only microscopically and which is sometimes so small as to escape detection even under the microscope. The smallest organisms of this kind are able to pass through a filter which retains ordinary bacteria. We know, in fact, that certain micro- organisms may cause tumor-like cell proliferations, especially in plants. Certain plant tumors have been shown to be definitely caused by bac- teria. In the case of animal tumors the proof that microorganisms are present has so far not been definitely established. We know, however, of very interesting experiments in which tumors have been produced artificially without a transfer of the tumor cells themselves. In ordi- nary experiments we can readily transfer tumor cells from one indi- vidual to another, of the same species, and thus cause in the second individual the growth of the same kind of tumor. Above, in speakiog of these artificially produced cancers, I had not in mind the sort that are produced by transmission of cancerous cells, but I referred especially to experiments carried out by Peyton Bous with certain hird cancers in which filtrates of the bird cancer free from living cells appeared to call forth the production of new similar tumors. There are other experi- ments on record in which the introduction of one kind of tumor stimu- lated the cells of the animal to the production of another kind. These results can be explained only in one of two ways : either extremely small microorganisms were transferred with the tumor filtrate, or a certain chemical substance which stimulated the tumor cells to proliferate and which was constantly produced within the tumor cells was transferred with the filtrates, or, in other cases, with the living tumor cells, into the other individual and there produced the new formation of tumors. It is very probable that it will be possible to determine experimentally which of these two alternatives is correct, and thus arrive at a definite con- clusion of the problem which we are seeking to solve. This will be the last link in the chain of factors determining the origin of cancer.
It is therefore not justifiable to state, as has been constantly afiSrmed only a few years ago, that the cause of cancer is unknown to us. In fact, we do know perhaps the principal factors with the result that in animals at least we can decrease or increase at will the cancer rate of certain strains by regulating the factors which we have analyzed so far. The next step will be to determine whether extremely small organisms or chemical substances are present in addition to the known factors. But even if we should have answered this last question, new problems will arise as to the manner in which chemical agencies (through which probably also microorganisms would ultimately act on the cells) can cause cell proliferation. And with this problem cancer research will
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