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