nvasion 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.

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. The
coincidence of injury and tumor is apt to be overestimated because of
the strong tendency to connect succeeding events. Tumors are not most
common on those parts of the body which are most exposed to injury.
They are rare, for instance, on the hands and feet, and very rarely do
they appear at the site of wounds caused by surgical operations. For
those tumors which develop in intra-uterine life it is difficult to
assign injury as a cause. There do