I.
On June 6, 1865, Pasteur started for Alais. The emotion he felt on the actual spot where the plague raged in all its force, in the presence of a problem requiring solution, caused him at once to forget the sacrifices he had made in quitting his laboratory at the École Normale. He determined not to return to Paris until he had exhausted all the subjects requiring study, and had triumphed over the plague.
In a few hours after his arrival he had already proved the presence of corpuscles in certain worms, and was able to show them to the President and several members of the Agricultural Committee, who had never seen them. The following day he installed himself in a little house three kilometers from Alais. Two small cultures were there going on; they were nearly the last, the cocoons had all been spun. One of these cultures, proceeding from eggs imported that very year from Japan, had succeeded very well. The other, proceeding also from Japanese eggs which had been reproduced in the country, had arrived at their fourth moulting and had a very bad appearance. But, strange to say, on examining with the microscope a number of chrysalides and moths of the group which had so delighted its proprietor, Pasteur found corpuscles almost always present, whereas the examination of the worms of the bad group only exhibited them occasionally. This double result struck Pasteur as very strange. He at once sent messengers into all the neighbourhood of Alais to seek for the remains of backward cultivations. He attached extreme importance to ascertaining whether the presence of corpuscles in the chrysalides or moths of the good groups, and the absence of the same corpuscles in the worms of the bad groups, was an accidental or a general fact. He soon recognised that these results did very frequently occur. But what would happen when the worms of the bad group spun their cocoons? Pasteur found that in the chrysalides, especially in the old ones, the corpuscles were numerous. As regards the moths proceeding from these cocoons, not one was free from them, and they existed in profusion.
Following up the idea that a connection between the disease and the corpuscles might possibly exist, as other observers had previously imagined, Pasteur declared, in a Note presented to the Agricultural Committee of Alais on June 26, 1865, twenty days after his arrival, that it was a mistake to seek for the corpuscle in the eggs or in the worms. Both the one and the other could carry in them the germ of the disease, without exhibiting distinct corpuscles, visible under the microscope. The evil developed itself especially in the chrysalides and in the moths, and it was in them that search should be made. Finally, Pasteur came to the conclusion that the only infallible method of procuring healthy eggs must be by having recourse to moths free from corpuscles.
Pasteur hastened to apply this new method of obtaining pure eggs. Notwithstanding that the malady was universally prevalent, he succeeded, after several days of assiduous microscopic observations, in finding some moths free from corpuscles. He carefully preserved their eggs, as well as other eggs which had proceeded from very corpusculous couples, intending to wait for what these eggs would produce the following year; the first would be probably free from corpuscles, while the latter would contain them. He would thus have in future, though on a small scale, samples of originally healthy and of originally unhealthy cultivations, by the comparison of which with the cultivations of the trade—all more or less smitten with the evil—totally new views might be expected to emerge. Who can tell, thought Pasteur, whether the prosperity of the silk cultivation may not depend on the practical application of this production of pure eggs by means of moths free from corpuscles?
Scarcely had Pasteur made known, first to the Committee of Alais, and then to the Academy of Sciences, the results of his earliest observations and the inductions to which they pointed, when critics without number arose on all sides. It was objected that the labours of several Italian savants had established beyond all doubt that the corpuscles were a normal element of certain worms, and especially of all the moths when old; that other authors had affirmed it to be sufficient to starve certain worms to make these famous corpuscles appear in all their tissues; and that Dr. Gaetano Cantoni had already tried some cultivations with eggs coming from moths without corpuscles, and that he had totally failed.
'Your efforts will be vain,' wrote the celebrated Italian entomologist Cornalia; 'your selected eggs will produce healthy worms, but these worms will become sickly through the influence of the epidemic demon which reigns everywhere.'
Anyone but Pasteur would have been staggered, but he was not the man to allow himself to be discouraged by à priori opinions, and by assertions which were more or less guesswork. He was resolved not to abandon his preconceived idea until experiment had pronounced upon it with precision. All scientific research, in order to be undertaken and followed up with success, should have, as point of departure, a preconceived idea, an hypothesis which we must seek to verify by experiment. To judge of the value of the facts which Pasteur had just announced, it was necessary to know if there existed the relation of cause and effect between the corpuscles and the disease. This was the great point to be elucidated.
But if, without preliminary groping, he had discovered the way to be pursued, Pasteur subsequently brought to bear his rare prudence as an experimentalist. For five years he returned annually for some months to Alais. The little house nestling among the trees called Pont-Guisquet became at the same time his habitation and his silkworm nursery. It is hemmed in by mountains, up the sides of which terraces rise, one above the other, planted with mulberry trees. The solitude was profound. Madame Pasteur and her daughter constituted themselves silkworm-rearers—performing their part in earnest, not only gathering the leaves of the mulberry trees, but also taking part in all the experiments. The assistants of the École Normale, Duclaux, Maillot, Gernez, and Raulin, grouped themselves around their master. Thus, in an out-of-the-way corner of the Cévennes was formed a colony seeking with ardour the solution of an obscure problem, and the means of curing or preventing a disease which had for so long a time blighted one of the great sources of the national wealth.
One of the first cares of Pasteur was to settle the question as to the contagion of the disease. Many hypotheses had been formed regarding this contagion, but few experiments had been made, and none of them were decisive. Opinions were also very much divided. Some considered that contagion was certain; the majority, however, either doubted or denied its existence; some considered it as accidental. It was said, for example, that the evil was not contagious by itself, but that it became so through the presence and complication of other diseases which were themselves contagious. This hypothesis was convenient, and it enabled contradictory facts to be explained. If some persons had seen healthy worms, which had been mixed up either by mistake or intention with sickly ones, perish, and if they insisted on contagion, others forthwith replied by diametrically opposite observations.
But whatever the divergences of opinion might be, everyone at all events believed in the existence of a poisonous medium rendered epidemic by some occult influence. Pasteur soon succeeded, by accurate experiments, in proving absolutely that the evil was contagious.
One of the first experiments was as follows. After their first moulting, he took some very sound worms free from corpuscles, and fed them with corpusculous matter, which he prepared in the following simple manner. He pounded up a silkworm in a little water, and passed a paint-brush dipped in this liquid over the whole surface of the leaves. During several days there was not the least appearance of disease in the worms fed on those leaves; they reached their second moulting at the same time as the standard worms which had not been infected. The second moulting was accomplished without any drawback. This was a proof that all the worms, those infected as well as the standard lot, had taken the same amount of nourishment. The parasite was apparently not present. Matters remained in this state for some days longer. Even the third moulting was got through without any marked difference between the two groups of worms. But soon important changes set in. The corpuscles, which had hitherto only showed themselves in the integuments of the intestines, began to appear in the other organs. From the second day following the third moulting—that is to say, the twelfth after the infection—a visible inequality distinguished the infected from the non-infected worms. Those of the standard lot were clearly in much the best health. On examining the infected worms through a magnifying glass, a multitude of little spots were discovered on their heads, and on the rings of their bodies, which had not before shown themselves. These spots appeared on the exterior skin when the interior skin of the intestinal canal contained a considerable number of corpuscles. It was these corpuscles that impeded the digestive functions, and interfered with the assimilation of the food. Hence arose the inequality of size of the worms. After the fourth moulting, the same type of disease was noticed as that which was breaking out everywhere in the silkworm nurseries, especially the symptom of spots on the skin, which had led to the disease being called pébrine. The peasants said that the worms were peppered. The majority of the worms were full of corpuscles. Those which spun their cocoons produced chrysalides which were nothing but corpusculous pulp, if such a term be allowed.
It was thus proved that the corpuscles, introduced into the intestinal canal at the same time as the food of the worms, convey the infection into the intestinal canal, and progressively into all the tissues. The malady had in certain cases a long period of incubation, since it was only on the twelfth day that it became perceptible. Finally, the spots of pébrine on the skin, far from being the disease itself, were but the effect of the corpuscles developed in the interior; they were but a sign, already removed from the true seat of the evil. 'If these spots of pébrine,' thought Pasteur, 'were considered in conjunction with certain human maladies in which spots and irruptions appear on the body, what interesting inductions might present themselves to minds prepared to receive them!'
Pasteur was never tired of repeating this curious experiment, or of varying its conditions. Sometimes he introduced the corpusculous food into healthy worms at their birth, sometimes at the second or third moulting. Occasionally, when the worms were about to spin their cocoons, the corpusculous food was given them. All the disasters that were known to have happened in the silkworm nurseries, their extent and their varied forms, were faithfully reproduced. Pasteur created at will any required manifestation of pébrine. When he infected quite healthy worms, after their fourth moulting, with fresh corpusculous matter, these worms, even after several meals of corpusculous leaves alternated with meals of wholesome leaves, made their cocoons. It might have been supposed that in this case the contagion had not taken effect. This was but a deceptive appearance. The communication of the disease exhibited itself in a marked degree in the chrysalides and in the moths. Many of the chrysalides died before they turned into moths, and their bodies might be said to be entirely composed of corpuscles. Such moths as were formed, and which emerged from their cocoons, had a most miserable appearance. The disease sometimes went so far as to render breeding and the laying of eggs impossible.
Faithful to the rules prescribed by the experimental method, Pasteur was careful to reproduce these same experiments with the worms of the standard lot, from which all infected worms had been selected. He fed these healthy worms on leaves over which a clear infusion made from the remains of moths or worms exempt from corpuscles had been spread with a paint-brush, instead of leaves contaminated with corpusculous remains. This food kept the worms in their usual health. Could there be a better proof that the corpuscles alone were the real cause of the pébrine disease?
These experiments, I repeat, threw a strong light on the nature of the disease, and exactly accounted for what took place in the industrial cultivations. From the malady which attacked the worms at their birth, decimating a whole cultivation, down to the invisible disease that may be said to inclose itself in the cocoon, all was now explained. One of the effects of the plague which had most excited the surprise and thwarted the efforts of cultivators was the impossibility of finding productive eggs, even when they tried to obtain them from the cocoons of groups which had succeeded perfectly well as far as the production of cocoons was concerned. It was proved that almost invariably the following year the eggs of these fine-looking groups were unproductive. Numbers of the agricultural boards, and practitioners, not being able to believe in the existence of the disease in collections that were so satisfactory as regards the abundance and beauty of the cocoons, persisted in thinking that the failures had an origin not connected with the seed itself. This resulted in deception after deception, often even in mistakes that were much to be regretted. Frequently the best husbandmen were known to reserve for the production of eggs some very fine cultivations, not having observed in the worms either spots of pébrine or corpuscles even up to the time when the mounting of the brambles had been accomplished; and the year following they had the pain of seeing all the cultivations from these eggs perish. These circumstances, so well calculated to produce discouragement and to give the disease a mysterious character, met with their natural explanation in the facts proved by experimental infection.
Still, as it never occurs to the cultivator to infect the worms directly by giving them food charged with corpusculous débris, it might be asked how, in the industrial establishments, such results can be produced. Pasteur lost no time in solving this difficulty.
In a cultivation containing corpusculous worms these worms perpetually furnish contagious matter, which falls upon the leaf and fouls it. This is the excreta of the worms, which the microscope shows to be more or less filled with corpuscles drawn from the lining of the intestinal canal. It is there that they swarm. It is easy to understand that these excreta, falling on the leaves, contaminate them all the more easily because the worms, by the weight of their bodies in crawling, press the excreta against the leaves. This is one cause of natural contagion. By the excreta of corpusculous worms which he crushed, mixed with water, and spread with a paint-brush over the mulberry leaves intended for a single meal, Pasteur was able to communicate the contagion to as many worms as he liked.
He also indicated another natural and direct cause of contagion. The six fore-feet of the worm have sharp hooks at their ends, by means of which the worms prick each other's skins. Let any one imagine a healthy worm passing over the body of the corpusculous worm. The hooks of the first worm, by penetrating the skin of the second, are liable to be soiled by the corpuscles immediately below that skin; and these hooks are capable of carrying the seeds of disease to other healthy worms, which may be pricked in their turn. To demonstrate experimentally, as Pasteur did, the existence of this cause of contagion, it was only necessary to take some worms and allow them to wound each other. Lastly, infection at a distance, through the medium of the air and the dust it carries, is a fact equally well established. It is sufficient, by sweeping the breeding-houses, or by shaking the hurdles, to stir up the dust of corpusculous excretions and the dried remains of dead worms, and to allow them to be spread over the hurdles of the healthy worms, to cause, after a certain time, contagion to appear among these worms. When very healthy worms were placed in a breeding nursery at a considerable distance from unhealthy worms, they, in their turn, became infected.
After so many decisive experiments it was no longer possible not to see in pébrine an essentially contagious disease. Nevertheless, among facts invoked in favour of non-contagion, there was one which it was difficult to explain. There existed several examples of successful cultivations conducted in nurseries which had totally failed from the effects of pébrine the year before. The explanation is, as shown by Pasteur, that the dust can only act as a contagion when it is fresh. Corpusculous matter, when thoroughly dried, loses its virulence. A few weeks suffice to render such matter inoffensive: hence the dust of one year is not injurious to the cultivations of the next year. The corpuscles contained in the eggs intended for future cultivation alone cause the transmission of the disease to future generations.
And what can be more easily understood than the presence of corpusculous parasites in the egg? The egg comes into existence during that marvellous phase of the life of a silkworm when, after having spun its cocoon, it sleeps within it as a chrysalis, resolving itself, so to speak, into those kinds of albumen and yelk from which the fully-formed moth will emerge, as a chick emerges from its egg. Let anyone imagine this origin of an approaching life, no longer in its normal purity, but associated with a parasite which will find in the materials surrounding it, so adapted to life and transformation, the elements of its own nourishment and multiplication. This parasite will be present when the eggs of the female moth, tender and soft as albumen, begin to define their outlines. Woe betide those eggs if they then enclose any particles of corpuscle, or of its original matrix. In vain will the envelope of those eggs become by degrees hard and horny; the enemy is within, and later on he will be discovered in the embryo of the silkworm.
Thus this terrible plague is at the same time contagious and hereditary, helping us to understand the evolution of this double character in certain maladies both of men and animals.