sms, and other analogous though apparently motionless
ones, which by decomposing the milk render it sour and putrid. They
are the lactic and putrid ferments, as the yeast-plant is the
alcoholic ferment of sugar. Keep them and their germs out of your
milk and it will continue sweet. But milk may become putrid without
becoming sour. Examine such putrid milk microscopically, and you find
it swarming with shorter organisms, sometimes associated with the
vibrios, sometimes alone, and often manifesting a wonderful alacrity
of motion. Keep these organisms and their germs out of your milk and
it will never putrify. Expose a mutton-chop to the air and keep it
moist; in summer weather it soon stinks. Place a drop of the juice of
the fetid chop under a powerful microscope; it is seen swarming with
organisms resembling those in the putrid milk. These organisms, which
receive the common name of bacteria, [Footnote: Doubtless organisms
exhibiting grave specific differences are grouped together under this
common name.] are the agents of all putrefaction. Keep them and their
germs from your meat and it will remain for ever sweet. Thus we begin
to see that within the world of life to which we ourselves belong,
there is another living world requiring the microscope for its
discernment, but which, nevertheless, has the most important bearing
on the welfare of the higher life-world.

And now let us reason together as regards the origin of these
bacteria. A granular powder is placed in your hands, and you are
asked to state what it is. You examine it, and have, or have not,
reason to suspect that seeds of some kind are mixed up in it. To
determine this point you prepare a bed in your garden, sow in it the
powder, and soon after find a mixed crop of docks and thistles
sprouting from your bed. Until this powder was sown neither docks nor
thistles ever made their appearance in your garden. You repeat the
experiment once, twice, ten times, fifty times. From fifty different
beds after the sowing of the powder, you obtain the same crop. What
will be your response to the question proposed to you? 'I am not in a
condition,' you would say, 'to affirm that every grain of the powder
is a dock-seed, or a thistle-seed; but I am in a condition to affirm
that both dock and thistle-seeds form, at all events, part of the
powder.' Supposing a succession of such powders to be placed in your
hands with grains becoming gradually smaller, until