Intellectual error, as well as practical danger, arises from the attempt to transfer to man results supposed to be gained by fallacious experimentation on the lower animals. The fallacy consists in noting general resemblance of structure, but not the far more remarkable differences of function. If, for instance, the life habits of two dogs of good breed are closely studied, it will be seen that, although certain individual differences are observed between the dogs, yet they are as nothing when compared with the enormous variation of function between the dog and the human being. The bones and garbage swallowed without injury, and the licking of its body, show the different type of digestion and assimilation, the action of the kidneys, of the various senses, and the possession of senses which we are unable to appreciate; in short, its distinctive type of existence proves the impossibility of drawing safe inferences for man from the digestive or other canine functions. Again, observation and rational experiment, solely for the benefit of one species of animal, may incidentally lead to the benefit of other races of animals, but direct experiment on one type for the supposed benefit of another kind is unscientific. It is this error that vitiates the famous postulates of Professor Koch, through the system of ‘controls,’ the latest exemplification of this fallacy being the attempt to prove the existence of cholera in man by cultivating the bacilli in animals. The same error also produces the failure of M. Pasteur to prevent hydrophobia in man. It is well known how the influence of what we term ‘mind’ governs the action of the bodily functions, either promoting or disturbing their normal condition. This is a fact of growing importance in practical medicine. Similar influence is exerted in varying degrees on all living creatures. Destructive or non-natural experimentation on living animals is always subject to the fallacy of morbid condition. The established law of research stated above exposes the error of pursuing biological investigation (or the study of vital action) by the process of mutilating or diseasing living animals. In research the radical difference between inorganic and organic Nature cannot be too clearly insisted on. Whilst in the former we can resolve compounds into their elements and recombine them, such process is impossible in organic Nature. We can take a steam-engine or a watch to pieces, examine their parts, repair them, and put them together again, thus proving our knowledge in this realm of Nature. But a living thing cannot be treated in the same way. Not only the difference of animal type forbids destructive method of investigation, but as the type rises in the scale of creation the growing fact of individual idiosyncrasy increases the uncertainty of erroneous method. Therefore the law of scientific research, which forbids the destruction of phenomena to be studied, is profoundly true.