Venom of _Lachesis gramineus_ (Green Pit-Viper, India):-- Rabbit (by intravenous injection, according to G. Lamb) 0·002 gramme per kilogramme Venom of _Crotalus adamanteus_ (Californian Rattle-Snake):-- Rabbit (by intravenous injection, according to McFarland, G. Lamb, and Flexner and Noguchi) 0·00025 gramme per kilogramme It will have been seen from the foregoing figures, that the respective sensitiveness of the dog, cat, rabbit, guinea-pig, rat, mouse, and frog, with regard to the same venom, is in no way proportional to the weight of these animals. The species mentioned are, per unit of weight, more or less resistant to intoxication; and, on experimenting with other animals, as for instance the monkey, pig, ass, and horse, we find that the monkey is much more susceptible to intoxication than the dog, and that the ass is extremely sensitive (0·010 gramme of Cobra-venom is sufficient to kill it), while the horse is less so, and the pig is by far the most resistant. The same weight of dry Cobra-venom, let us say 1 gramme to be precise, will enable us to kill 1,250 kilogrammes of dog, 2,000 kilogrammes of rabbit, 2,500 kilogrammes of guinea-pig, 1,430 kilogrammes of rat, or 8,333 kilogrammes of mouse. The lethal dose for a horse being, as I have ascertained by my own experiments, about 0·025 gramme, 1 gramme of dry Cobra-venom will therefore suffice to kill 20,000 kilogrammes of horse. Assuming that man, in proportion to his weight, possesses a resistance intermediate between that of the dog and that of the horse, we may consider that the lethal dose for a human being is about 0·015 gramme. It follows, therefore, that 1 gramme of venom would kill 10,000 kilogrammes of man, or, let us say, 165 persons of an average weight of 60 kilogrammes. Another extremely important fact, which must not be lost sight of, is that differences of toxicity, which are often considerable, are exhibited by the venoms of different specimens of the same species of snake, or by the venom of the same snake collected at different times. I have found, for instance, in the case of the specimens of _Naja_ and _Lachesis_ reared in my laboratory, that, according to the length of time that the animals had been without food, and to the nearness or otherwise of the moulting period, the venom was more or less active, and that on evaporation it left behind a more or less considerable quantity of dry extract. In certain cases, immediately after the moult and after a prolonged fast, the venom was _ten times_ more active than after a plentiful meal or before the moult. The figures given above must therefore not be regarded as determining the minimal lethal doses of the different venoms, except in a purely comparative way, and they must be considered only as data useful to know when it is desired to experiment upon animals with these substances.