What does excrescence (from The Call of the Wild) mean?

An excrescence is a normal outgrowth or natural appendage, such as a cockscomb, rose thorn, or fingernail. More commonly, however, an excrescence refers to an abnormal or disfiguring outgrowth or addition — for example, a person's bunion or wart, a burl (burr) on a tree, or even an ultra-modern glass and steel house in a neighborhood of Victorian-style homes.

In The Call of the Wild, Buck's search for John Thornton leads him through a deeply quiet forest:

The bird life had flitted. The squirrels were in hiding. One only he saw, — a sleek gray fellow, flattened against a gray dead limb so that he seemed a part of it, a woody excrescence upon the wood itself.