Part 12

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.

II.

The first time Pasteur went to Alais the silkworm epidemic was universally attributed to a single cause--_pébrine_. _Pébrine_ was called _the disease_. This word expressed everything. It indicated the existence of a mysterious scourge, the origin and nature of which could not be traced, but which was ready to fall upon all the establishments devoted to the nurture of the worms. Whatever might happen, or whatever might be the cause of ruin in a silkworm nursery, _the disease_ was held responsible. One of the most striking proofs that the evil was attributed to _pébrine_ alone is found in the fact that a prize of 5,000 florins was offered by the Austrian Government in 1868, as a reward for the discovery of the best remedy for the prevention and cure of _pébrine_--'the epidemic disease which devastates the silkworm.'

A rapid glance at the principles which have just been established suffices to show that _pébrine_ might now be regarded as vanquished. Pasteur had demonstrated that moths free from corpuscles never produced a single corpusculous egg; he had proved, moreover, that eggs brought up in a state of isolation, at a distance from contaminated eggs, produce no worms, chrysalides, or moths which are corpusculous. It was easy, therefore, to multiply cultivations free from _pébrine_. The production of silk and the production of eggs was thus secured. To make sure that the eggs were pure it was only necessary to have recourse to the microscopic examination of the moths which had produced them. These observations might be made by women, by young girls, even by children. It was sufficient to crush up a moth in a little water, and to put a drop of this mixture under the microscope, to see the corpuscles clearly, if they existed. It seemed, then, that the plague was got rid of. But Pasteur was not slow in recognising that the general belief in a single malady could not be justified. If the experiments of 1866 had demonstrated to him the full extent of the corpusculous malady, and had established the principles of a treatment proper for its prevention, the method he had adopted had also shown him that _pébrine_ was far from being the only cause from which the silk culture suffered.

It was in 1867 that this result was obtained. From an experimental point of view, that year counted double for Pasteur. Influenced by a profound sympathy for the misery which he had witnessed during two successive years, and, at the same time, impatient to find the cause of the scourge, Pasteur, in the months of February, March, and April, in advance of the great industrial cultivations, commenced a series of experiments on worms hatched by artificial heat, and fed with mulberry leaves from a hothouse.

During these forced experiments Pasteur observed that out of sixteen broods derived from non-corpusculous parents, fifteen succeeded, while the sixteenth perished almost entirely between the fourth moulting and the climbing on to the brambles. After having exhibited a most healthful appearance, the worms died suddenly. In a cultivation of 100 worms, ten, fifteen, twenty dead ones were picked up daily: these turned black, and became putrid with extraordinary rapidity, often within the space of twenty-four hours. Sometimes they were soft and flabby, like an empty, crumpled intestine. Consulting the authors who had written upon silkworms, Pasteur could not doubt that he had before his eyes a characteristic specimen of the disease called _morts-flats_, or _flacherie_. Not only were these worms free from all _pébrine_ spots, but no corpuscles were to be found in any part of their bodies. A still more significant fact was, that corpuscles were also absent from the chrysalides and the moths of those few worms which were able to spin their cocoons. Although this sample was confined to a single group of eggs derived from parents free from corpuscles, Pasteur continued to entertain doubts as to the existence of only a single disease, and also as to the necessary connection of _pébrine_ with _flacherie_.

These suspicions were confirmed by his cultivations of April and May. Numerous cases of _flacherie_ presented themselves. Uncertainty was no longer possible as to the mutual independence of the two maladies--_pébrine_ and _flacherie_. The cultivations most seriously invaded by the last-mentioned disease came from eggs produced by parents free from corpuscles, and led on to reproducers also free from this parasite. On visiting a multitude of industrial cultivations, Pasteur discerned that what had passed in his own laboratory was of very general occurrence, and that, contrary to the received opinion, two distinct maladies divided between them the cause of all the misfortunes. _Pébrine_ was evidently the most widely spread, but _flacherie_ had also its share, and a very large share, in the calamity.

Here, once more, the microscope came to Pasteur's assistance. If, at the period of the rearing of the silkworms, when the mean temperature is always rather high, some mulberry leaves are crushed in a mortar and mixed with a little water, the liquid being left to itself, in twenty-four hours it will be found filled with microscopic organisms; some motionless, resembling little rods or spores joined end to end, like strings of beads, others more or less active, flexible, endowed with a sinuous movement like that of the vibrios found in nearly all organic infusions in process of decomposition. Whence come these microscopic organisms? The facts relating to spontaneous generation indicate that the germs of these organisms were on the surface of the pounded leaf, spread in the form of dust over the instruments used to triturate the leaf, possibly on the mortar, the pestle, or in the water added to the pounded leaves.

It is a curious fact, that if the intestinal canal of a worm in full process of digestion be opened, the pounded leaf which fills it from one end to the other will not show microscopic organisms of any kind, but only cells of parenchyma, green granules of the chlorophyl of the leaf, and remains of the air-vessels of the plant. Through the action of the liquids secreted by the glands which line the integuments of the intestinal canal, the germs of organisms are themselves digested or hindered in their development. The digestive functions of silkworms are so active that everything is carried away, destroyed in the same manner as the leaves themselves.

But if from any cause the digestion of the worms be impeded or suspended, then the germs introduced with the food into the intestinal canal will give rise to the multiplication of microscopic organisms which are always found in the artificially bruised leaf when mixed with a little water. How numerous are the causes which may check this digestive function of the worm--a function of such importance to a creature which in the space of one month passes from the weight of half a milligramme to that of five, six, seven, or even eight grammes! Pasteur proved that whenever a worm was attacked with _flacherie_, it always had, associated with the food in its intestinal canal, one or other of the microscopic organisms which are invariably to be met with among crushed mulberry leaves. Summing up in a kind of aphorism a series of observations, Pasteur observes: 'Every _ver flat_ is one which digests badly, and, conversely, every worm which digests badly is doomed to perish of _flacherie_, or to furnish a chrysalis and a moth the life of which, through the injury produced by organised ferments, is not normally perfected.'

Thus, as in the case of _pébrine_, the morbid symptoms of _flacherie_ are very variable. All depends on the intensity of the evil--that is to say, on the abundance and the nature of the parasites developed in the intestinal canal, and also on the period in the life of the worm when this fermentation begins to show itself. The most dangerous of all these ferments are those of the family of vibrios. If they exist in the first phases of the life of the worm, it dies quickly and very soon becomes putrid, sometimes resolving itself into an infected pus. The disease often manifests itself in a manner particularly distressing and disastrous to the cultivator. The worms have presented the most beautiful appearance up to the time of climbing the heather. The mortality has scarcely been two or three per cent., which is nothing; the moultings have been effected in a perfect manner, when suddenly, some days after the fourth moulting, the worms become languid, crawling with difficulty, and hesitating to take the leaves which are thrown upon their hurdles. If some few have mounted on to the heather, they stretch themselves on the twigs, their bodies swollen with food which they cannot digest. Sometimes they remain there motionless till they die, or, falling, remain suspended only by their false feet. The few moths which have succeeded in piercing their cocoons do not show any corpuscles. They can produce eggs, but these eggs, coming from parents weakened by disease, give rise the following year to a generation threatened with _flacherie_. It is in this sense that the disease may be regarded as hereditary, although the parasites of the intestinal canal to which _flacherie_ is due do not transmit themselves to the eggs or to the worms which issue from them. The worms inherit weakly constitutions, and, being without power of resistance against anything that can derange their digestive functions, they are at the mercy of the accidents of their culture.

Too large an assemblage of worms in one nursery; too high a temperature at the time of moulting; a thunderous atmosphere, which predisposes organic matter to fermentation; the use of heated or wet leaves, especially if the wetting be caused by a fog or by the morning or evening dew, which deposits on the leaf the germs suspended in a great mass of air;--these are so many causes calculated to diminish the activity of the digestive functions of the worms, and to produce in consequence a fermentation of the leaf in the intestinal canal--the malady now under consideration. Often also _flacherie_ depends upon mistakes committed by the husbandman while tending his precious 'kine,' to use an expression of the sixteenth century.

A Chinese book published on the rearing of silkworms contains a series of little practical counsels. 'The person who takes care of the silkworms,' says this guide to the perfect cultivator, 'ought to wear a simple garment, not lined. He must regulate the temperature of the spinning-house according to the sensation of heat or cold which he experiences; if he feels cold, he may conclude that the worms are cold, and he will increase the fire; if he feels hot, he will conclude that the worms are hot, and he will suitably diminish the fire.'

One point which had been ignored before the experiments of Pasteur was the contagious character of _flacherie_. This contagion may surpass that of _pébrine_ itself as regards duration. In _pébrine_ the dried corpusculous matter loses all virulence after the lapse of some weeks. The disease cannot, therefore, communicate itself from one year to another by the corpusculous dust of a rearing establishment. The germs, on the contrary, of the microscopic organisms which provoke fermentation in the mulberry leaves, especially the vibrios, retain their vitality for several years. The dust of a silkworm nursery infected by _flacherie_ appears under a microscope quite full of cysts or spores of vibrios. These spores or cysts rest, like the sleeping beauty in the forest, until a drop of water falls upon them and awakens them into life. Deposited on the leaves which are to serve as nourishment, these germs of vibrios are carried into the intestinal canal of the worm, develop and multiply themselves, and completely disturb the digestive functions, unless the digestion is so strong that the germs are immediately arrested, and disposed of like the food itself. This is what happens when the worms are in full vigour. It is a struggle for life, in which the worms often gain the victory.

Giving to some very healthy worms a meal of leaves covered with the dry dust of a silkworm nursery, infected the year before by _pébrine_ and _flacherie_, Pasteur reproduced _flacherie_, and not _pébrine_. Still more readily did he produce the first of these maladies, when he gave, as food, leaves polluted by the contents of the intestinal canal of worms which had died of the disease. As in the case of _pébrine_, the excreta of the worms attacked by _flacherie_, defiling the leaves, carry the mischief to the healthy worms, or add to the dangerous fermentation in the intestines of those which are already in part attacked.

To preserve silkworms from accidental _flacherie_, hygienic precautions are sufficient. As regards hereditary _flacherie_, or, to speak more correctly, that which develops itself easily on any diminution of vigour in the eggs and in the embryo, Pasteur again found a remedy by having recourse to the microscope. By means of the microscope it is possible to obtain information as to the health of the worms, the chrysalides, and the moths destined to produce the eggs. Every attention should be directed to the complete exclusion of ferments from the intestinal canal of the worms, and from the stomach-pouch of the chrysalides--a little pouch to which the intestinal canal of the worm is reduced, with its contents more or less transformed. But if there is not time to make this examination for parasitic ferments with the microscope, a simple inspection of the worms in their last stage will suffice. Pasteur laid great stress upon the observation of the worms when they climbed on to the heather.

'If I were a cultivator of silkworms,' he wrote in his beautiful work on the diseases of silkworms, 'I would never hatch an egg produced from worms that I had not observed many times during the last days of their life, so as to make sure of their vigour at the moment when they spin their silk. If you use eggs produced by moths the worms of which have mounted the heather with agility, have shown no signs of _flacherie_ between the fourth moulting and mounting time, and do not contain the least corpuscle of _pébrine_, then you will succeed in all your cultivations.'

III.

We have now arrived at the end of this long investigation. All the obscurity which enveloped the origin of the diseases of silkworms had now been dispelled. Pasteur had arrived at such accurate knowledge both of the causes of the evil and their different manifestations, that he was able to produce at will either _pébrine_ or _flacherie_. He could so regulate the intensity of the disease as to cause it to appear on a given day, almost at a given hour. He had now to carry into practice the results of his laboratory labours.

Since the beginning of the plague, and after some doubts which were soon dispelled, it was clearly seen that all the mischief was to be attributed to the bad condition of the eggs. The remedy of distant explorations for procuring non-infected eggs was both insufficient and precarious. It simply amounted to going very far to seek, and paying very dear, for seed which could not be relied on with certainty. The prosperity of the silkworm culture could only be secured by measures capable of restoring to the native eggs their pristine qualities.

The results obtained by Pasteur were sufficient to solve this problem. The struggle against _flacherie_ was easy, but there remained the struggle against _pébrine_. To triumph over this disease, which was so threatening, Pasteur devised a series of observations as simple as they were ingenious.

Here is a crop which has perfectly succeeded. The moultings, and the climbing upon the heather, are all that could be desired. The cocoons are finished, and the appearance of the moths alone is waited for. They arrive, and they pair. Then begins the work of the cultivator, who is careful about the production of his eggs. He separates the couples at the end of the day; laying each female moth by itself on a little linen cloth suspended horizontally. The females lay their eggs. After the laying, he takes each female in turn and secures her by a pin passed through the wings to a folded corner of the little cloth, where are grouped some hundreds of eggs which she has laid. The male moth also might be pinned in another corner of the cloth, but the examination of the male is useless, as it has been found that he does not communicate the _pébrine_. The female moth, after having been desiccated by free contact with the air, is examined at leisure, it may be even in the autumn or winter. Nothing is easier than to ascertain whether there are any corpuscles in its dead body. The moth is crushed in a mortar and mixed with a little water, and then a drop of the mixture is examined by the microscope. If corpuscles be found, the bit of cloth corresponding to the examined moth is known, and it is burnt with all the eggs it contains.

This method of procuring pure eggs is, in fact, only the rational development of the first inductions which Pasteur had submitted to the Agricultural Committee of Alais in June 1865. At that time he hardly ventured to hope that he should be able to find the means of preparing more than very small quantities of healthy eggs for his experiments; but events were so ordered that the starting-point, which seemed to be purely scientific, unfolded a method susceptible of a widespread practical application. This process of procuring sound eggs is now universally adopted. In the Basses-Alpes, in Ardèche, in Gard, in the Drôme, and in other countries, may be met with everywhere, at the time of the cultivation, workshops where hundreds of women and young girls are occupied, with a remarkable division of labour and under the strictest supervision of skilful overseers, in pounding the moths, in examining them microscopically, and in sorting and classifying the little cloths upon which the eggs are deposited.

* * * * *

But if Pasteur had brought back wealth to ruined countries, if he had returned to Paris happy in the victory he had gained, he had also undergone such fatigues, and had so overstrained himself in the use of the microscope while absorbed in his daily and varied experiments, that in October 1868 he was struck with paralysis of one side. Seeing, as he thought, death approaching, he dictated to his wife a last note on the studies which he had so much at heart. This note was communicated to the Academy of Sciences eight days after this terrible trial.

A soul like his, possessing so great a mastery over the body, ended by triumphing over the affliction. Paralysed on the left side, Pasteur never recovered the use of his limbs. To this day, sixteen years after the attack, he limps like a wounded man. But what stages had this wounded man yet to travel over, what triumphs were yet in store for him!

_DECISIVE EXPERIMENTS._

After having dictated this scientific note, which he thought would have been his last, his courage forsook him for a time. 'I regret to die,' he said to his friend, Sainte-Claire Deville, who had hastened to his bedside; 'I should have wished to render more service to my country.' His life was spared, but for several months Pasteur remained entirely paralysed, incapable of the slightest movement. Smitten thus in his full strength at the age of forty-five, he took a sad review of his own state. Even at the height of his attack his mind had always retained its clearness. He had pointed out to the doctor without any faltering of voice the progressive symptoms of the paralysis. Then reproaching himself for having added to the grief of his wife by thus dwelling on the details of his illness, he never allowed another word to escape him about his infirm condition. Sometimes, even when he heard his two assistants, M. Gernez and M. Duclaux, whose devotion to him during those sad days could only be compared to that of his wife, talking to him of future labours, he entered into these thoughts and appeared to add faith to their hopes. He finished by sharing them.

In January 1869, although it was still impossible for him to drag himself about his room, he was so much excited by the contradictions that his system of culture had aroused that he wished to start again for Alais. 'Aided by the method of artificial cultivation,' he remarked, 'we shall soon annihilate these latest oppositions. There is here both a scientific principle and an element of national wealth.'

His wish could not be opposed, but a terrible and anxious journey it was! At some leagues from Alais, at a place called Saint Hippolyte-du-Fort, where the earliest experiments were made, Pasteur stopped. He installed himself--we might rather say he encamped--with his family and his assistants, in a more than humble lodging, one of those miserable, cold, paved houses of the rural districts. Seated in his arm-chair, Pasteur directed the experiments, and verified the observations which he had made the year before. Each of his predictions as to the destiny of the different groups of worms was fulfilled to the smallest detail. In the following spring he left for Alais, where he followed in all their phases, from the egg up to the cocoon, the cultivations there undertaken, and he had the happiness of proving once more the certainty of his method.

But opposition still continued. The French Government, shaken by the violence and tenacity of the opponents, hesitated to decide upon this process of culture. The Emperor interposed; he instructed Marshal Vaillant to propose to Pasteur to go into Austria to the Villa Vicentina, which belonged to the Prince Imperial. For ten years the silk harvest at this place had not sufficed to pay the cost of eggs. Pasteur accepted with joy the prospect of a great decisive experiment. He traversed France and Italy, reclining in a railway carriage or in an arm-chair, and at last arrived at the Imperial villa near Trieste. Pasteur succeeded in a marvellous manner. The sale of the cocoons gave to the villa a net profit of twenty-six million francs. The Emperor, impressed with the practical value of the system, nominated Pasteur a Senator, in the month of July 1870. But this nomination, like so many other things, was swept away before it had time to appear in the 'Journal Officiel.' Pasteur, however, cared little for the title of Senator. He returned to France on the eve of the declaration of war.