MR. A. R. WALLACE has been so good as to forward me the extract from the Japan Mail above referred to, together with his reply. The article in question bears the title, “Protection by Mimicry—a Problem in Mathematical Zoology.” The authors, while admitting the broad principles involved in Dr. Fritz Muller's theory, fail to see why the advantage derived by the mimicking species, in cases where the latter is less numerous than the model, should be as the square of the relative numbers. They admit that “the ingenious explanation seems perfectly satisfactory,” but the proportional benefit appeared to them exaggerated. Mr. Wallace has now, I think, cleared up the misunderstanding with reference to this part of the question, but it may be of use in assisting towards the further discussion of the problem if I here give the simple algebraical treatment adopted in the original paper.