which composes wood； and of an interior semifluid mass which contains a
substance； identical in its composition； in a broad sense； with that
which constitutes the flesh of animals。 Subsequently； after the
structure of the yeast plant had been carefully observed； it was
discovered that all plants； high and low； are made up of separate bags
or 〃cells；〃 as they are called； these bags or cells having the
composition of the pure matter of wood； having the same composition；
broadly speaking； as the sac of the yeast plant； and having in their
interior a more or less fluid substance containing a matter of the same
nature as the protein substance of the yeast plant。 And therefore this
remarkable result came outthat however much a plant may differ from an
animal； yet that the essential constituent of the contents of these
various cells or sacs of which the plant is made up； the nitrogenous
protein matter； is the same in the animal as in the plant。 And not only
was this gradually discovered； but it was found that these semifluid
contents of the plant cell had； in many cases； a remarkable power of
contractility quite like that of the substance of animals。 And about 24
or 25 years ago； namely； about the year 1846； to the best of my
recollection； a very eminent German botanist； Hugo Von Mohl； conferred
upon this substance which is found in the interior of the plant cell；
and which is identical with the matter found in the inside of the yeast
cell； and which again contains an animal substance similar to that of
which we ourselves are made uphe conferred upon this that title of
〃protoplasm；〃 which has brought other people a great deal of trouble
since！ I beg particularly to say that； because I find many people
suppose that I was the inventor of that term； whereas it has been in
existence for at least twenty…five years。 And then other observers；
taking the question up； came to this astonishing conclusion （working
from this basis of the yeast）； that the differences between animals and
plants are not so much in the fundamental substances which compose them；
not in the protoplasm； but in the manner in which the cells of which
their bodies are built up have become modified。 There is a sense in
which it is trueand the analogy was pointed out very many years ago
by some French botanists and chemiststhere is a sense in which it is
true that every plant is substantially an enormous aggregation of
bodies similar to yeast cells； each having to a certain extent its own
independent life。 And there is a sense in which it is also perfectly
truealthough it would be impossible for me to give the statement to
you with proper qualifications and limitations on an occasion like
thisbut there is also a sense in which it is true that every animal
body is made up of an aggregation of minute particles of protoplasm；
comparable each of them to the individual separate yeast plant。 And
those who are acquainted with the history of the wonderful revolution
which has been worked in our whole conception of these matters in the
last thirty years； will bear me out in saying that the first germ of
them； to a very great extent； was made to grow and fructify by the study
of the yeast plant； which presents us with living matter in almost its
simplest condition。
Then there is yet one last and most important bearing of this yeast
question。 There is one direction probably in which the effects of the
careful study of the nature of fermentation will yield results more
practically valuable to mankind than any other。 Let me recall to your
minds the fact which I stated at the beginning of this lecture。 Suppose
that I had here a solution of pure sugar with a little mineral matter
in it； and suppose it were possible for me to take upon the point of a
needle one single； solitary yeast cell； measuring no more perhaps than
the three…thousandth of an inch in diameternot bigger than one of
those little coloured specks of matter in my own blood at this moment；
the weight of which it would be difficult to express in the fraction of
a grainand put it into this solution。 From that single one； if the
solution were kept at a fair temperature in a warm summer's day； there
would be generated； in the course of a week； enough torulae to form a
scum at the top and to form lees at the bottom； and to change the
perfectly tasteless and entirely harmless fluid； syrup； into a solution
impregnated with the poisonous gas carbonic acid； impregnated with the
poisonous substance alcohol； and that； in virtue of the changes worked
upon the sugar by the vital activity of these infinitesimally small
plants。 Now you see that this is a case of infection。 And from the
time that the phenomenon of fermentation were first carefully studied；
it has constantly been suggested to the minds of thoughtful physicians
that there was a something astoundingly similar between this phenomena
of the propagation of fermentation by infection and contagion； and the
phenomena of the propagation of diseases by infection and contagion。
Out of this suggestion has grown that remarkable theory of many
diseases which has been called the 〃germ theory of disease；〃 the idea；
in fact； that we owe a great many diseases to particles having a
certain life of their own； and which are capable of being transmitted
from one living being to another； exactly as the yeast plant is capable
of being transmitted from one tumbler of saccharine substance to
another。 And that is a perfectly tenable hypothesis； one which in the
present state of medicine ought to be absolutely exhausted and shown not
to be true； until we take to others which have less analogy in their
favour。 And there are some diseases most assuredly in which it turns
out to be perfectly correct。 There are some forms of what are called
malignant carbuncle which have been shown to be actually effected by a
sort of fermentation； if I may use the phrase； by a sort of disturbance
and destruction of the fluids of the animal body； set up by minute
organisms which are the cause of this destruction and of this
disturbance； and only recently the study of the phenomena which
accompany vaccination has thrown an immense light in this direction；
tending to show by experiments of the same general character as that to
which I referred as performed by Helmholz； that there is a most
astonishing analogy between the contagion of that healing disease and
the contagion of destructive diseases。 For it has been made out quite
clearly； by investigations carried on in France and in this country；
that the only part of the vaccine matter which is contagious； which is
capable of carrying on its influence in the organism of the child who is
vaccinated； is the solid particles and not the fluid。 By experiments
of the most ingenious kind； the solid parts have been separated from
the fluid parts； and it has then been discovered that you may vaccinate
a child as much as you like with the fluid parts； but no effect takes
place； though an excessively small portion of the solid particles； the
most minute that can be separated； is amply sufficient to give rise to
all the phenomena of the cow pock； by a process which we can compare to
nothing but the transmission of fermentation from one vessel into
another； by the transport to the one of the torula particles which
exist in the other。 And it has been shown to be true of some of the
most destructive diseases which infect animals； such diseases as the
sheep pox； such diseases as that most terrible and destructive disorder
of horses； glanders； that in these； also； the active power is the
living solid particle； and that the inert part is the fluid。 However；
do not suppose that I am pushing the analogy too far。 I do not mean to
say that the active； solid parts in these diseased matters are of the
same nature as living yeast plants； but； so far as it goes； there is a
most surprising analogy between the two； and the value of the analogy
is this； that by following it out we may some time or other come to
understand how these diseases are propagated； just as we understand；
now； about fermentation； and that； in this way； some of the greatest
scourges which afflict the human race may be； if not prevented； at
least largely alleviated。
This is the conclusion of the statements which I wished to put before
you。 You see we have not been able to have any accessories。 If you
will come in such numbers to hear a lecture of this kind； all I can say
is； that diagrams cannot be made big enough for you； and that it is not
possible to show any experiments illustrative of a lecture on such a
subject as I have to deal with。 Of course my friends the chemists and
physicists are very much better off； because they can not only show you
experiments； but you can smell them and hear them！ But in my case such
aids are not attainable； and therefore I have taken a simple subject and
have dealt with it in such a way that I hope you all understand it； at
least so far as I have been able to put it before you in words； and
having once apprehended such of the ideas and simple facts of the case
as it was possible to put before you； you can see for yourselves the
great and wonderful iss