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.
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.'