Experiments with apomorphin likewise show that the reaction to this substance varies in different species of animals. The resistance of the cat to this drug is, according to Guinard(31), ten times greater than that of the dog, and the latter is more sensitive than the rabbit to the crystalline form of apomorphin when given intravenously. According to Kobert(45) amygdalin is without effect on dogs, but it is poisonous to rabbits. Lapicque(49) found that the toxicity of curara varies in different species of frogs, the dose required to produce paralysis in _Rana esculenta_ being three times greater than in _Bufo vulgaris_. Weir Mitchell(59) pointed out long ago that turtles stand enormous doses of curara. Schmiedeberg's experiments with caffein on _Rana temporaria_ and _Rana esculenta_ (and more recently those of Jacobi and Golowinski(42) with caffein, theobromin, and theophyllin) are also of interest in this connection. These experimenters observed well-marked differences of reaction to methyl-xanthins in these closely allied forms. Experiments with quinin have shown that the action of this substance differs in some animals. It causes a fall of temperature immediately after its administration in the guinea pig, but frequently produces, at first, a rise of temperature, followed by an unimportant fall, in rabbits, dogs, and man. The numerous investigations which have been carried out on the effect of atoxyl within recent years have contributed much to the comparative pharmacology of this substance. Although the symptoms and organic changes produced by this substance in a variety of animals present no great differences, the resistance of some has been found to vary; according to Köster(46) it is more toxic for dogs than for rabbits. A number of other substances have been found by various experimenters to vary in toxicity for different species of animals. Cantharadin, phenol, atropin, and strychnin may be mentioned as illustrations. Pharmacological studies on lower forms of life have also revealed marked variations in the effect of some poisons. Observations made by Danilewski(18) with hydrochinone indicate that solutions of 1 to 100 or 200 are toxic to Celentrates, causing paralysis in these organisms. Echinoderms are killed within one or two hours in 1 to 1,000 or 2,000 solution, while in Vermes even weaker solutions cause tetanus and finally paralysis. The experiments of Drzewina(19) with potassium cyanid are also interesting in this connection. Teleosts placed in 100 cc of sea water containing twentieth-normal potassium cyanid showed signs of asphyxia and died in 10 to 20 minutes. Actinia placed in a solution of sea water containing five times as much potassium cyanid were active on the thirteenth day of the experiment. Similar results were obtained with other marine organisms.