M. Pasteur had triumphed over splenic fever with as much rigour as precision. But he considered that he had still to make one further investigation. He had established the effects of the pest; he had discovered a preventive method with which to combat it: he now wished to know the origin of the evil. Whence comes splenic fever? Why is it endemic in certain departments of France, in certain parts of Russia, Germany, Austria, Italy, Spain, and America? How is it sustained? It was for a long time believed that splenic fever was born spontaneously under the influence of various accidental causes. M. Bouley has related, in his learned work on the 'Progress of Medicine by Experimentation,' that in 1842 the Minister of Agriculture, M. Cunin-Gridaine, at the request of the deputies of the departments that were ravaged by the epidemic, entrusted to M. Delafond, a professor of the school at Alfort, the task of investigating this malady, commonly called the 'blood disease,' in the districts in which it was raging. He was to search out its causes, and ascertain whether they did not result from the system of cultivation prevalent in the country.

M. Delafond arrived in Beauce. One fact struck him—namely, that almost all the animals attacked by the disease were young, fine, and vigorous: those, in short, that gave the best promise. Viewing the richness of the soil and the abundance and quality of the crops in conjunction with this observation, Delafond at once elaborated a speculative theory. 'The blood disease,' said he, 'is nothing but an overfulness—an excess of blood circulating in the vessels, and especially the predominance in that liquid of red globules.'

Starting from this idea, his one object was, by means of logical deduction, to trace everything to this fundamental error. He analysed the soil, and demonstrated to what extent it was fitted to furnish crops that were rich and abounding in nutritive properties. He analysed the plants. He then complacently referred the richness of blood of the Beauce sheep to the richness in nitrogenous principles of the substances on which they were fed. He examined the lesions of the diseased animals, and concluded that they were the consequence of the blood containing too large a proportion of the organic elements, called globules, fibrine, albumen, and too small a proportion of water.

'Reduce the proportion of nutritious elements,' he wrote as his advice to the agriculturists, 'mix roots with all that is too rich in nitrogenous principles, and you will reduce in proportion the losses caused by the excess of the ultra-nutritious substances with which you supersaturate your cattle.'

'Such is the very logical conclusion to which Delafond was led,' adds M. Bouley, ridiculing these observations, based on a method of reasoning, instead of on the experimental method. 'And as a fresh proof of his theory he mentions the fact that the disease decreases as you descend the country towards the Loire. On the right bank of that river—in Sologne, for instance, which is a low, sandy, damp district—blood disease is unknown. In the arrondissements of Gien and Montargis and in parts of those of Orleans and Pithiviers it prevails but little. There, Delafond imperturbably remarks, the soil is sandy and the herbage not nearly so rich as in the Beauce plateau; and there the blood disease is consequently less common.'

When we consider that such opinions could be written unchallenged only forty years ago, that they could even borrow a scientific character from the inspiration that gave them birth, we can see the progress that has since been made, and can realise how great were the obscurity and uncertainty which have been dispelled by the experimental method.

The presence of a parasite having been brought to public notice in the blood of animals suffering from splenic fever, at the very time when Pasteur had shaken the belief in spontaneous generation, people grew accustomed to the idea that the stricken animals might have contracted the germs of the malady from the outer world, without any spontaneous birth, strictly so called, of the disease. This opinion was strengthened by a knowledge of the spores of the splenic bacillus. Pasteur, aided by Messieurs Chamberland and Roux, commenced experiments with a view to solving this difficult etiological question. The first experiments took place in the fields of a farm at the village of St. Germain, near Chartres. Several groups of sheep were fed on lucern grass which had been sprinkled with artificially-reared splenic fever bacteria, or with their germs or spores. Although all the sheep of the same group absorbed an immense number of the spores of the parasite, many survived, even after being visibly affected. Those that died showed all the symptoms of what is called spontaneous splenic fever. The period of incubation lasted as long as eight or ten days, although, in its latter stages, the disease exhibited those startling features which have caused a belief that the incubating period is a very short one—short, that is to say, for those conditions of contagion where the parasite is not deposited in its pure state under the animal's skin.

But if prickly plants (notably the pointed ends of dried thistle leaves, or beards of barley blades cut into little bits about a centimeter in length) were added to this infected food, the mortality increased to a striking extent. On examination after death, the lesions of these animals were found to be similar to those observed in sheep which were attacked by splenic fever in sheds, or which died of the disease in the open fields.

From that time forward, the idea which had been predominant in the minds of Pasteur and his fellow-workers during all their inquiries, was materially strengthened. They were convinced that the animals which died of blood disease in the department of Eure et Loire had been infected by germs or spores of the splenic microbe contained in their food; but the question remained, Whence came these germs?

From the moment when all belief in the spontaneous generation of the parasite is rejected, attention is naturally drawn to the possible consequences which may arise from burying in the earth animals which have died of splenic fever. In the greater number of cases, when the knacker's establishment is too far off and the dead animal is of little value, a trench is dug on the spot, at a depth varying from half a meter to a meter. If the animal dies in a field, it is buried where it falls; if it dies in a shed the body is carried into a neighbouring field. There it is buried, and putrefaction sets in; and since all the splenic fever filaments of the blood are destroyed by putrefaction, it was thought that no dissemination of the germs of splenic fever, after the animal had been buried, could occur. Pasteur showed that this opinion rested on a superficial observation. Even when the animal is not cut up, blood spreads itself outside of the body in more or less abundance. Is it not an habitual characteristic of the disease, that at the time of death blood issues from the nostrils and the mouth, and that the urine is often bloody? All around the corpse, therefore, the earth is polluted with blood. Moreover it takes several days for the splenic fever microbe to resolve itself into harmless granulations by the action of gases, other than oxygen, which putrefaction generates. During this time, the excessive inflation of the dead body causes the liquids of the interior to issue from all the natural apertures. How often also, a rent in the skin or the tissues increases this flow. The blood and other matters, mixed with the surrounding aerated soil, are no longer in the conditions of putrefaction, but rather in those which form a suitable medium of cultivation for the microbe. Experiments confirmed these views. Adding some splenic fever blood to earth sprinkled with the water of yeast, or with urine, at summer temperature, or at the temperature which the fermentation of a dead body keeps up around it, as in a dung heap, in less than twenty-four hours the splenic fever filaments deposited with the blood had multiplied and resolved themselves into spores. These spores were afterwards found in their state of latent life, ready to germinate and to communicate splenic fever, after remaining in the earth for months, and even years.

These experiments, curious as they were, were only, so to speak, laboratory experiments. It was necessary to investigate what happened in the open country with all the variations of dryness, of damp, and of cultivation. A happy inspiration came to Pasteur and his assistants. They had buried in the midst of summer, in an isolated corner of the farm of St. Germain, near Chartres, a sheep which had died of natural splenic fever, and of which they had made the autopsy. Ten months afterwards, and again fourteen months afterwards, the idea occurred to them of collecting some of the earth from this grave. After having examined it, and established the presence of the spores of the microbe, they produced, by the inoculation of guinea-pigs, the splenic disease and death. But the circumstance which deserves the greatest attention, is that the same experiment was successfully made with the earth on the surface of the grave, though this earth had not been disturbed during the interval. Some experiments were afterwards made on the earth of some trenches dug in a meadow of the Jura, where some cows which had died of splenic fever had been buried at a depth of two meters. Two years afterwards, by successive washings of the earth on the surface of the graves, deposits were extracted which at once produced splenic disease. At three trials within these two years the same surface earths produced splenic fever, while, away from the graves, the earth exhibited nothing of the kind. Finally, Pasteur and his assistants proved that on the surface of the earth which covered the buried animals, the germs were again found, after all the operations of ploughing, sowing, and reaping.

But how, it will be asked, can the earth, which is so powerful a filter, allow the germs of microscopic organisms to rise again to its surface? Is one not tempted here to quote Pasteur against himself, since, in his joint researches with M. Joubert, Pasteur had proved that the waters of springs issuing from the earth, even at a shallow depth, are entirely free from germs? Such waters, nevertheless, being supplied from the earth's surface, which is constantly washed by rain, the effect must be to carry down the finest particles to the springs. But these latter, notwithstanding conditions so conducive to their pollution, remain perfectly pure. Can there be a better proof that earth of a certain thickness will arrest all solid particles, even the most minute? Nevertheless, in these experiments on splenic fever, we hear of microscopic germs, starting from the depths and coming up to the surface—that is to say, in a direction contrary to the flow of the rain. This is an enigma.

The explanation will cause surprise. The earth-worms transport the germs, and bring up, from the depths where they lie buried, the terrible microbes. In the tiny cylinders of earth which the worms deposit on the surface of the soil, after the dews of the morning or after rain, the splenic germs are to be found. It is easy to prove this directly. If in earth, with which spores of the microbe have been previously mingled, we place some worms, and at the end of several days open the bodies of these worms, with all necessary precautions, so as to extract from them the earthy matter which fills their intestinal canals, we find in them large numbers of splenic fever spores. It is, then, absolutely proved, that if splenic fever germs exist, as they often do, in the light earth which covers the pits in which animals dead of that disease lie buried, these germs result from the disintegration by rain of the little excremental cylinders deposited by the earth-worms. The dust of this disintegrated earth spreads itself over the grasses on a level with the soil, and thus it is that animals come to find on the pasture-field, and in particular kinds of forage, the germs of splenic fever by which they are infected.

'In these results,' said Pasteur a short time ago at the Academy of Medicine, 'what outlooks are opened to the mind in regard to the possible influence of earths in the etiology of diseases, and the possible danger of the earth of cemeteries!'

The earth-worms also bring to the surface other germs, which, while they are as harmless to the worms as the splenic germs, are nevertheless bearers of diseases to which animals are liable. All sorts of germs are found in them, and the germs of splenic fever are in fact always associated with those of putrefaction and septicÆmia.

'And now,' concluded Pasteur, when laying before the Academy a rapid survey of the etiology of splenic fever, 'is not the remedy naturally indicated? We should never bury animals in fields destined either for cultivation, for forage, or for sheep pasture. When it is possible a sandy soil should be chosen for the purpose, or any poor calcareous soil, dry, and easily desiccated—in a word, soil not suited to the existence of earth-worms.' M. Tisserand, Director of Agriculture, has remarked that splenic fever is unknown in the region of the Savarts of Champagne, although it is surrounded by countries invaded by the disease. If the conditions of commerce introduce splenic fever, it is but a passing accident. Must not this be attributed to the fact that in these poor soils, such as that of the camp at ChÂlons, where the thickness of arable soil is only from 4 to 5 inches, superposed upon chalk, the worms cannot live? In such a soil the burial of a splenic fever animal will give rise to great quantities of germs, which, owing to the absence of earth-worms, will abide in the depths of the soil and remain harmless. Finally, it has been proved that the countries subject to splenic fever have an argillaceous-calcareous soil, and that the disease is unknown in schistose and granitic soils. The contrast of the results, in relation to such differences of soil, is seen sometimes in the Department of the Aveyron, between the right and left side of one and the same road or watercourse.

May we not now in all confidence assert that, if the cultivators choose, splenic fever may soon be a thing of the past among their animals, their shepherds, and among the butchers and the tanners of the towns, because splenic fever and malignant pustule are never spontaneous? The disease exists only where it has been sown, or where it has been diffused by the unconscious instrumentality of the earth-worm.

The progress of vaccination will also contribute to the disappearance of splenic fever; for this preventive, if extensively used, as there is no doubt it will be, must end by establishing a race of domestic animals which, having all sprung from vaccinated parents, will in consequence be more resistant to the disease in its worst form. It will be with them in relation to splenic fever as it is with ourselves in relation to small-pox. It is a well-known fact that the ravages of small-pox are much less considerable in our days than when it first appeared in Europe. It is difficult not to attribute this, at least in part, to the prevalence of vaccination.

In the populations where small-pox is introduced for the first time it has an exceptional intensity. Some months ago a significant fact of this nature occurred in Paris. A whole family of Esquimaux perished from small-pox in the 'Jardin d'Acclimatation.' They had never been vaccinated, nor had their ancestors. They were new to the attacks of small-pox, which did not spread beyond them.