_NATURAL IMMUNITY OF CERTAIN ANIMALS WITH RESPECT TO SNAKE-VENOMS._ It was long ago pointed out that certain warm-blooded animals, including the mongoose (_Herpestes ichneumon_, Family _Viverridæ_), hedgehog (_Erinaceus europæus_, Family _Erinaceidæ_), pig (_Sus scrofa_, Family _Suidæ_), and some herons (_Ajaja_, Subfamily _Plataleinæ_; _Cancroma_, Subfamily _Cancrominæ_; _Botaurus_, Subfamily _Ardeinæ_; _Mycteria_, Subfamily _Ciconiinæ_), known in Colombia under the names _Culebrero_ and _Guacabo_, exhibit a natural immunity with respect to snake-bites. Pigs devour vipers with great readiness, and in the region of North America which adjoins the Mississippi and its tributaries they are even trained to destroy the young rattle-snakes and other poisonous serpents with which the valleys of these watercourses are infested. During my stay in Indo-China I inoculated a young pig, beneath the skin of the back, with a dose of _Cobra_-venom (10 milligrammes) capable of killing a large-sized dog. The animal withstood the injection, but I am inclined to think that this is not a case of true immunity; it is probable that the pig owes its resistance to venom to the fact that its skin is lined with an enormous layer of adipose tissue, which is but very slightly vascular, and in which absorption takes place very slowly. This opinion is corroborated by my discovery that the serum of this animal is entirely destitute of any antitoxic substance. I mixed a dose of _Cobra_-venom, lethal for the rabbit, with 3·5 and 8 c.c. of pig-serum. These mixtures killed rabbits in the same time as the controls that received the venom diluted with equal quantities of rabbit-serum or physiological saline solution. The natural immunity of the mongoose and the hedgehog rests upon more scientifically established facts. My own experiments upon the immunity of the mongoose were made with six specimens of these little carnivores captured in Guadeloupe (French West Indies), an island in which no poisonous snakes exist; consequently their immunity could not have arisen from their having become accustomed to the bites of venomous reptiles.