The parasitic theory. This supposes that a living parasite invades the body, and by its presence excites the cells of certain tissues to grow in tumor form. It is known that active growth of the cells of the body can be excited in a number of ways, by chemical substances such as certain of the coal tar products, and that it often takes place under the influence of bacteria. It is further known that parasites can produce tumor-like growths in plants. The large, rough excrescences on the oaks are produced by a fly which lays its eggs in or beneath the bark, and the larva which develops from the egg secretes a substance which causes the cells about it to multiply, and a huge mass is formed which serves the developing insect for both food and protection. Large tumor-like masses are formed on the roots and stalk of cabbages as the result of the invasion of the cells by a minute organism: the tumors of olive trees are due to a bacterium; the peculiar growths on cedar trees, the so-called "witches' brooms," are produced by a fungus, and there are many other such examples. These have many analogies with tumors in animals. Under the stimulus of the parasite the cells seem to have unlimited growth capacity and a greater nutritive avidity than have the normal plant cells; the character of the mass produced differs as does the tumor, to a greater or less extent, from the normal growth; on the cedar, for instance, the "witches' broom" consists of a thick mass of foliage with small stems less green than the usual foliage, the leaves wider and not so closely applied to the stems. The entire plant suffers in its nutrition and a condition resembling tumor cachexia[1] is produced, and there are no fundamental differences between the plant and animal tumors. Support has also been given to the parasitic theory by the discovery within tumor cells of bodies which were supposed to be a peculiar sort of parasite. 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.