Oct. lo, 1889] 



NATURE 



581 



important French irdustries seemed inevitable. Under these 

 circumstances the great chemist Dumas, who was born at Alais, 

 in the centre of one of the districts most striously aTected, urged 

 his friend Pasteur to undertake an investigation of the subject. 

 Pasteur, who at this time had never seen a silkworm, naturally 

 felt diffident about attempting so difficult a task, but at last, at 

 Dumas' renewed entreaty, he consented, and in lune 1865 

 betook himself to the south for the purj ose of studying the 

 disease on the spot. His previous training here again stood him 

 in good stead, and in September 1865 he was able to communi- 

 cate to the Academy of Sciences results of observation and 

 experiment which, striking at the root of the evil, pointed the 

 way to the means of securing immunity from the dreaded plague. 

 This paper was freely criticized. Here, it was said, was a 

 chemist who, quitting his proper sphere, had the hardihood to 

 lay down rules for the guidance of the physician and biologist in 

 fields specially their own. Why should his proposals be more 

 successful than all the other nostrums which had already so 

 egregiously failed ? 



In order to appreciate the difficulties which met Pasteur in 

 this inquiry, and to understand how wonderfully he overcame 

 them, I must very shortly describe the nature of this disease, 

 which is termed pebrhie, from the black spots which cover the 

 silkworm. It declares itself by the stunted and unequal growth 

 of the worms, by their torpidity, and by their fastidiousness as to 

 food, and by their premature death. 



Before Pasteur went to Alais the presence of certain micro- 

 scopic corpuscles had been noticed in the blood and in all the 

 tissues of the diseased caterpillar, and even in the eggs from 

 which such worms were hatched. These micro-organisms often 

 fill the whole of the silk organs of the insect, which in a healthy 

 condition contain the clear viscous liquid from which the silk is 

 made. Such worms are of course valueless. Still this know- 

 ledge did not suffice, for eggs apparently healthy gave rise to 

 stricken worms incapable of producing silk, whilst again other 

 worms distinctly diseased yielded normal cocoons. These diffi- 

 culties, which had proved too much for previous observers, were 

 fully explained by Pasteur. '• The germs of these organisms," 

 said he, " which are so minute, may be present in the egg and 

 even in the young worms, and yet baffle the most careful search. 

 They develop with the growth of the worm, and in the chrysalis 

 they are more easily seen. The moth derived from a diseased 

 worm invariably contains these corpuscles, and is incapable of 

 breeding healthy progeny." 



This moth-test is the one adopted by Pasteur, and it is an 

 infallible one. If the female moth is stricken, then her eggs — 

 even though they show no visible sign of disease — will produce 

 sick worms. If in the moth no micrococci are seen, then her 

 immediate progeny at any rate will be sound and free from 

 inherited taint, and will always produce the normal quantity of 

 silk. But this is not all. Pasteur found that healthy worms 

 can be readily infected by contact with diseased ones, or through 

 germs contained in the dust of the rooms in which the worms 

 are fed. Worms thus infected, but free from inherited taint, can, 

 however, as slated, spin normal cocoons, but — and this is the 

 important point — the moths which such chrysalides yield invari- 

 ably produce diseased eggs. This explains the anomalies pre- 

 viously noticed. The silkworms which die without spinning are 

 those in which the disease is hereditary, viz. those born from a 

 diseased mother. Worms from sound eggs which contract the 

 disease during their life-time always spin their silk, but they 

 give rise to a stricken moth, the worms from which do not reach 

 maturity and furnish no silk. 



As I have said, these results were but coldly received. It was 

 hard to make those engaged in rearing the worms believe in the 

 efficacy of the proposed cure. Then, seeing this state of things, 

 Pasteur determined to take upon himself the role of a prophet. 

 Having in 1866 carefully examined a considerable number of 

 the moths which had laid eggs intended for incubation, he wrote 

 down a prediction of what would happen in the following year 

 with respect to the worms hatched from these eggs. In due 

 course, after the worms from a mixed batch of healthy and un- 

 healthy eggs had spun, the sealed letter was opened and read, 

 and the prediction compared with the actual result, when it was 

 found that in twelve out of fourteen cases there was absolute 

 conformity between the prediction and the observation, for 

 twelve hatchings were predicted to turn out diseased, and this 

 proved to be the case. Now all these "educations" were 

 believed to be healthy by the cultivators, but Pasteur foretold 

 that they would turn out to be diseased by the application of the 



moth-test in the previous year. The other parcels of eggs were 

 pronounced by Pasteur to be scund, because they were laid by 

 healthy moths containing none of the micrococci, and both these 

 yielded a healthy crop. So successful a prophecy could not but 

 gain the belief of the most obtuse of cultivators, and we are not 

 surprised to learn that Pasteur's test was soon generally applied, 

 and that the consequence has been a return of prosperity to^ 

 districts in which thousands of homes had been desolated by a 

 terrible scourge. 



I must now ask you to accompany me to another and a new 

 field of Pasteur's labours, which, perhaps more than his others, 

 claims your sympathy and will enlist your admiration, because 

 they have opened out to us the confident hope of at least obtain- 

 ing an insight into some of the hidden causes and therefore to 

 the possible prevention of disease. 



In the first place, I must recall to your remembrance that most 

 infectious diseases seldom if ever recur, and that even a slight 

 attack renders t he subject of it proof against a second one. Hence 

 inoculation from a mild case of small-pox was for a time prac- 

 ticed, but this too often brought about a serious if not fatal attack 

 of the malady, and the step taken by Jenner of vaccinating, 

 that is of replacing for the serious disease a slight one which; 

 nevertheless is sufficient protection against small-pox infection, 

 was one of the highest importance. But Jcnner's great dis- 

 covery has up to recent years remained an isolated one, for it 

 led to no general method for the preventive treatment of other 

 maladies, nor had any explanation been offered of its mode of 

 action. It is to Pasteur that science is indebted for the 

 generalization of Jenner's method, and for an explanation which 

 bids fair to render possible the preventive treatment of many — 

 if not of all — infectious diseases. It was his experience, based 

 upon his researches on fermentation, that led to a knowledge of 

 the nature of the poison of such diseases, and showed the possi- 

 bility of so attenuating or weakening the virus as to furnish a 

 general method of protective or preventive inoculation. 



I have already pointed out how a pure cultivation of a microbe 

 can be effected. Just as the production of pure alcohol depends 

 on the presence of the pure yeast, so special diseases are de- 

 pendent on the presence of certain definite organisms which can 

 be artificially cultivated, and which give rise to the special 

 malady. Can we now by any system of artificial cultivation so 

 modify or weaken the virus of a given microbe as to render it 

 possible to inoculate a modified virus which, whilst it is without 

 danger to life, is still capable of acting as a preventive to further 

 attack ? This is the question which Pasteur set himself to solve, 

 nor was the task by any means an apparently hopeless one. He 

 had not only the case of Jennerian vaccination before him, but 

 also the well-known modifications which cultivation can bring 

 about in plants. The first instance in which Pasteur succeeded 

 in effecting this weakening of the poison was in that of a fatal 

 dise ase to which poultry in France are very liable, called chicken 

 cholera. Like many other maladies, this is caused by the presence 

 of a micro-organism f ound in the blood and tissues of the si ricken 

 fowl. Onedropof this blood brought under the skin of a healthy 

 chicken kills it, and the same microbe is found throughout its 

 body. And if a pure culture of these microbes be made, that 

 culture — even after a series of generations — is as deadly a poison 

 as the original blood. Now comes the discovery. If these 

 cultures be kept at a suitable temperature for some weeks exposed 

 to pure air, and the poisonous properties tested from time to time,, 

 the poison is found gradually to become less powerful, so that 

 after the lapse of two months a dose which had formerly proved 

 fatal now does not disturb in the slighte t the apparent health of 

 the fowl. But now let us inoculate a chicken with this weakened 

 virus. It suffers a slight illness, but soon recovers. Next let us 

 give it a dose of the undiluted poison, and, as a control, let 1:1s 

 try the action of the same on an unprotected bird. What is the 

 result ? Why, that the first chicken remains unaffected, whilst 

 the second bird dies. The inoculation has rendered it exempt 

 from the disease, and this has been proved by Pasteur to be true 

 in thousands of cases, so that, whereas the death-rate in certain 

 districts amongst fowls before the adoption of Pa'^teur's inocula- 

 tion method was 10 per cent., after its general adoption it has 

 diminished to less than I percent. 



We can scarcely value too highly this discovery, for it proves 

 that the poisonous nature of the microbe is not unalterable, but 

 that it can be artificially modified and reduced, and thus an 

 explanation is given of the fact that in an epidemic the virus, 

 may either be preserved or become exhausted according lo the 

 conditions to which it is subjected. We have here to do with a 



