All the more so, inasmuch as experimental studies on tuberculosis have not only preserved men; they have also preserved animals. Thanks to Koch, there is now a very simple way of recognising if an animal is or is not tuberculous. Koch was able to extract from tubercle bacilli, a substance which he has called _tuberculin_. At first he thought tuberculin cured the disease; but this was an error. Subsequent experiments showed that tuberculin exercised quite a different action to that of healing. It has the property, when injected in small doses into a tuberculous animal, of provoking an intense fever, whilst it produces no reaction whatsoever in a normal animal. If, therefore, tuberculin is injected into every animal in the cattle shed, we can feel sure--and this is impossible otherwise--that such or such animals are tuberculous or healthy. All cows that show a rise in temperature after an injection of tuberculin are tuberculous; the others, on the contrary, are in good health. Thus the sanitary inspection of stables and cattle-sheds can be carried out thoroughly; and we are now able to protect not only men but also animals from the disease of tuberculosis. Such results could only have been obtained at the cost of many and methodical experiments. Whatever may be the genius of anti-vivisectionists, they would never have been able to imagine anything similar had they been left to their own intellectual powers. It is not in the study that we are able to discover this long series of unforeseen, extraordinary, almost miraculous facts which laboratory experimentation has been able to find out. Man, said Pascal, tires of conceiving sooner than Nature tires of providing; and experimentation is man's method of interrogating Nature. * * * * * The third discovery which I shall take as an example demonstrating the value of experimentation, is the history of _Serotherapy_. And I may be permitted to dwell somewhat on this subject as I had the good fortune, in 1888, of making the decisive experiment which was the beginning of serotherapy.