The bearing which the subject of the following communication has upon the much-vexed question of the unity of the human species, and the inferences which may be drawn from it in relation thereto, invest it with an importance which would not otherwise belong to it.

The position which the question I allude to at present occupies is this. On the one hand, the advocates of the view that all men are not of the same species, but that they compose a genus consisting of many species, maintain that the parasites which infest the different races of man are distinct; and because we usually find that distinct species of parasites are allotted respectively to the different species of the lower animals, they infer that the same rule must hold with man, and that therefore each different race possessing a distinct parasite must be a distinct species.