The various theories as to cause may be divided into three heads.
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.