In autumn it is not uncommon to see flies motionless upon a window-pane,
with a sort of magic circle, in white, drawn round them. On microscopic
examination, the magic circle is found to consist of innumerable spores,
which have been thrown off in all directions by a minute fungus called
_Empusa museae_ the spore-forming filaments of which stand out like a
pile of velvet from the body of the fly. These spore-forming filaments
are connected with others which fill the interior of the fly's body
like so much fine wool, having eaten away and destroyed the creature's
viscera. This is the full-grown condition of the _Empusa_. If traced
back to its earliest stages, in flies which are still active, and to
all appearance healthy, it is found to exist in the form of minute
corpuscles which float in the blood of the fly. These multiply and
lengthen into filaments, at the expense of the fly's substance; and when
they have at last killed the patient, they grow out of its body and give
off spores. Healthy flies shut up with diseased ones catch this mortal
disease, and perish like the others. A most competent observer, M. Cohn,
who studied the development of the _Empusa_ very carefully, was utterly
unable to discover in what manner the smallest germs of the _Empusa_ got
into the fly. The spores could not be made to give rise to such germs by
cultivation; nor were such germs discoverable in the air, or in the food
of the fly. It looked exceedingly like a case of Abiogenesis, or, at any
rate, of Xenogenesis; and it is only quite recently that the real course
of events has been made out. It has been ascertained that when one of
the spores falls upon the body of a fly, it begins to germinate, and
sends out a process which bores its way through the fly's skin; this,
having reached the interior cavities of its body, gives off the minute
floating corpuscles which are the earliest stage of the _Empusa_. The
disease is "contagious", because a healthy fly coming in contact with a
diseased one, from which the spore-bearing filaments protrude, is pretty
sure to carry off a spore or two. It is "infectious" because the spores
become scattered about all sorts of matter m the neighbourhood of the
slain flies. Silkworms are liable to many diseases; and, even before
1853, a peculiar epizootic, frequently accompanied by the appearance
of dark spots upon the skin (whence the name of "Pebrine" which it has
received), had been noted for its mortality. But in the years following
1853 this malady broke out with such extreme violence, that, in 1858,
the silk-crop was reduced to a third of the amount which it had reached
in 1853; and, up till within the last year or two, it has never attained
half the yield of 1853. This means not only that the great number of
people engaged in silk growing are some thirty millions sterling poorer
than they might have been; it means not only that high prices have had
to be paid for imported silkworm eggs, and that, after investing his
money in them, in paying for mulberry-leaves and for attendance, the
cultivator has constantly seen his silkworms perish and himself plunged
in ruin; but it means that the looms of Lyons have lacked employment,
and that, for years, enforced idleness and misery have been the
portion of a vast population which, in former days, was industrious and
well-to-do.
In reading the Report made by M. de Quatrefages in 1859, it is
exceedingly interesting to observe that his elaborate study of the
Pebrine forced the conviction upon his mind that, in its mode of
occurrence and propagation, the disease of the silkworm is, in every
respect, comparable to the cholera among mankind. But it differs
from the cholera, and so far is a more formidable malady, in being
hereditary, and in being, under some circumstances, contagious as well
as infectious.
The Italian naturalist, Filippi, discovered in the blood of the
silkworms affected by this strange disorder a multitude of cylindrical
corpuscles, each about 1/6000th of an inch long. These have been
carefully studied by Lebert, and named by him _Panhistophyton_; for the
reason that in subjects in which the disease is strongly developed, the
corpuscles swarm in every tissue and organ of the body, and even pass
into the undeveloped eggs of the female moth. But are these corpuscles
causes, or mere concomitants, of the disease? Some naturalists took
one view and some another; and it was not until the French Government,
alarmed by the continued ravages of the malady, and the inefficiency of
the remedies which had been suggested, despatched M. Pasteur to
study it, that the question received its final settlement; at a great
sacrifice, not only of the time and peace of mind of that eminent
philosopher, but, I regret to have to add, of his health.
But the sacrifice has not been in vain. It is now certain that this
devastating, cholera-like Pebrine is the effect of the growth and
multiplication of the _Panhistophyton_ in the silkworm. It is contagious
and infectious, because the corpuscles of the _Panhistophyton_ pass away
from the bodies of the diseased caterpillars, directly or indirectly, to
the alimentary canal of healthy silkworms in their neighbourhood; it
is hereditary because the corpuscles enter into the eggs while they are
being formed, and consequently are carried within them when they
are laid; and for this reason, also? it presents the very singular
peculiarity of being inherited only on the mother's side. There is not a
single one of all the apparently capricious and unaccountable phenomena
presented by the Pebrine, but has received its explanation from the
fact that the disease is the result of the presence of the microscopic
organism, _Panhistophyton_.