Snake poisons is a subject which has always attracted much attention, and which has made great progress within the last quarter of a century, especially as regards the defensive reaction by which the blood may be rendered proof against their effect by processes similar to vaccination--antipoisonous serotherapy. The studies to which we allude have not only conduced to a method of treatment against snake-bites, but have thrown a new light on the great problem of immunity. They have shown that the antitoxic serums do not act as chemical antidotes in destroying the venom, but as physiological antidotes; that, in addition to the poison glands, snakes possess other glands supplying their blood with substances antagonistic to the poison, such as also exist in various animals refractory to snake poison, the hedgehog and the mungoose for instance. Unfortunately, the specificity of the different snake poisons is such that, even when the physiological action appears identical, serum injections or graduated direct inoculations confer immunity towards one species or a few allied species only. Thus, a European in Australia who had become immune to the poison of the deadly _Notechis scutatus_, manipulating these snakes with impunity, and was under the impression that his immunity extended also to other species, when bitten by a _Denisonia superba_, an allied Elapine, died the following day. In India, the serum prepared with the venom of _Naia tripudians_ has been found to be without effect on the poison of _Naia bungarus_, the two species of _Bungarus_, and the Vipers _Vipera russelli_, _Echis carinatus_, and _Lachesis gramineus_. _Vipera russelli_ serum is without effect on Colubrine venoms, and on those of _Echis_ and _Lachesis_. In Brazil, serum prepared with the venom of _Lachesis lanceolatus_ has proved to be without action on _Crotalus_ poison. These examples, and others which could be given, show that the hopes which were at first entertained as to the benefits to be conferred on mankind by the serum treatment were somewhat over-sanguine--at least as regards countries like India, where, different kinds of poisonous snakes occurring together, it is sometimes impossible to know by which the bite has been inflicted. Chemistry teaches that snake venoms consist for the most part of solutions of modified proteids, and all attempts to separate the toxic principles from such proteids have hitherto been unsuccessful. Accordingly, at the present time we must regard such toxic principles as residing in some special grouping of a portion of the atoms in the complex venom proteid molecule. The analysis of their physiological actions has proved them to be made up of a great many more constituents than would be imagined from their chemical composition.