Moreover, however imperfect our defence against tuberculosis may still be, it is by no means _nil_; great progress has been made; the mortality has decreased in a considerable proportion. During the last twenty-five years, it has decreased by about 25 per cent., and notably in England, where the laws of public hygiene, energetically upheld by the good sense of the people, are strictly applied, the mortality has diminished by 50 per cent. This is only a beginning, and the near future will bring about the complete extermination of the disease.
Now, honestly, I ask if the rabbits which Villemin sacrificed weigh more in the scales of universal progress, and even in public morality, than the three millions of individuals who, by progress in hygiene, have been preserved from an early and painful death. I estimate at a high price the life and the sufferings of fifty rabbits, but, at the risk of appearing a barbarian, I prefer, to these fifty rabbits, the three millions of young people who have been saved by Villemin's discovery, and the millions which it will still save.
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.