One of the most recent and the most comprehensive memoirs upon the terrible epidemic had been presented to the Academy of Sciences by M. de Quatrefages. One paragraph of this paper had forcibly struck Pasteur. M. de Quatrefages related that some Italian naturalists, especially Filippi and Cornalia, had discovered in the worms and moths of the silkworm minute corpuscles visible only with the microscope. The naturalist Lebert affirmed that they might always be detected in diseased silkworms. Dr. Osimo, of Padua, had even perceived corpuscles in some of the silkworms' eggs, and Dr. Vittadini had proposed to examine the eggs with a microscope in order to secure having sound ones. M. de Quatrefages only mentioned this matter of the corpuscles as a passing remark, being doubtful of its importance, and perhaps of its accuracy. This doubt might have removed from Pasteur's mind the thought of examining the significance of these little corpuscles, but, amid the general confusion of opinions, Pasteur was attracted to the study of these little bodies all the more readily because it related to an organic element which was visible only with the microscope. This instrument had already rendered such services to Pasteur in his delicate experiments on ferments, that he was fascinated by the thought of resuming it again as a means of research.
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.'