402 
He said to himself, if these infusorial animalcules come from 
erms, their germs must exist either in the substance infused, or 
in the water with which the infusion is made, or in the superja- 
cent air. Now the vilality of all germs is destroyed by heat. 
Therefore, if I boil the infusion, cork it up carefully, cementing 
the cork over with mastic, and then heat the whole vessel by 
heaping hot ashes oyer it, I must needs kill whateyer germs are 
present. Consequently, if Redi’s hypothesis hold good, when 
the infusion is taken away and allowed to cool, no animal- 
cules ought to be developed in it; whereas, if the animalcules 
are not dependent on pre-existing germs, but are generated 
from the infused substance, they ought, by-and-by, to make 
their appearance, Needham found that, under the circumstances 
in which he made his experiments, animalcules always did arise 
in the infusions, when a sufficient time had elapsed to allow for 
their development. 
In much of his work Needham was associated with Buffon, 
and the results of their experiments fitted in admirably with the 
great French naturalist’s hypothesis of “ organic molecules,” ac- 
cording to which, life is the indefeasible property of certain in- 
destructible molecules of matter, which exist in all living things, 
and have inherent activities by which they are distinguished from 
not living matter. Each individual living organism is formed by 
theirtemporary combination, They stand to it in the relation of the 
particles of water to acascade, ora whirlpool ; or to a mould, into 
which the water is poured. The form of the organism is thus deter- 
mined by the reaction between external conditions and the 
inherent activities of the organic molecules ot which it is com- 
posed ; and, as the stoppage of a whirlpool destroys nothing but 
a form, and leaves the molecules of the water, with all their in- 
herent activities intact, so what we call the death and putrefac- 
tion of an animal, or of a plant, is merely the breaking up of the 
form, or manner of association, of its constituent organic mole- 
cules, which are then set free as infusorial animalcules. 
It will be perceived that this doctrine is by no means identical 
with Adiogenes?s, with which it is often confounded. On this 
hypothesis, a piece of beef, or a handful of hay, is dead only in 
a limited sense. The beef is dead ox, and the hay is dead grass ; 
but the ‘‘organic molecules”’ of the beef or the hay are not dead, 
but are ready to manifest their vitality as soon as the bovine or 
herbaceous shrouds in which they are imprisoned are rent by the 
macerating action of water. The hypothesis therefore must be 
classified under Xenogenesis, rather than under Abiogenesis. 
Such as it was, I think it will appear, to those who will be just 
énough to remember that it was propounded before the birth of 
moder chemistry, and of the modern optical arts, to be a most 
ingenious and suggestive speculation. 
But the great tragedy of Science—the slaying of a beautiful 
hypothesis by an ugly fact—which is so constantly being enacted 
under the eyes of philosophers, was played, almost immediately, 
for the benefit of Buffon and Needham. 
Once more, an Italian, the Abbé Spallanzani, a worthy suc- 
cessor and representative of Redi in his acuteness, his ingenuity, 
and his learning, subjected the experiments and the conclusions 
of Needham to a searching criticism. It might be true that 
Needham’s experiments yielded results such as he had described, 
but did they aie out his arguments? Was it not possible, in 
the first place, that he had not completely excluded the air by 
his corks and mastic? And was it not possible, in the second 
place, that he had not sufficiently heated his infusions and the 
superjacent air? Spallanzani joined issue with the English 
naturalist on both these pleas, and he showed that if, in the first 
place, the glass vessels in which the infusions were contained 
were hermetically sealed by fusing their necks, and if, in the 
second place, they were exposed to the temperature of boiling 
water for three-quarters of an hour,* no animalcules ever made 
their appearance within them. It must be admitted that the 
experiments and arguments of Spallanzani furnish a complete 
and a crushing reply to those of Needham. But we all too often 
forget that it is one thing to refute a proposition, and another to 
prove the truth of a doctrine which, implicitly or explicitly, 
contradicts that proposition, and the advance of science soon 
showed that though Needham might be quite wrong, it did not 
follow that Spallanzani was quite right. 
Modern chemistry, the birth of the latter half of the eighteenth 
century, grew apace, and soon found herself face to face with 
the great problems which biology had vainly tried to attack with- 
out her help. The discovery of oxygen led to the laying of the 
foundations ofa scientific theory of respiration, and to an exami- 
nation of the maryellous interactions of organic substances with 
* See Spallanzani, ‘‘ Opeté,” vi. pp. 42 and 51, 
NATURE 
[ Sepé. 15, 1870 
oxygen. The presence of free oxygen appeared to be one of the 
conditions of the existence of life, andof those singular changes 
in organic matters which are known as fermentation and putre- 
faction. The question of the generation of the infusory animal- 
cules thus passed into a new phase. For what might not have 
happened to the organic matter of the infusions, or to the oxy- 
gen of the air, in Spallanzani’s experiments? What security was 
there that the development of life which ought to haye taken 
place had not been checked or prevented by these changes ? 
The battle had to be fought again. It was needful to repeat 
the experiments under conditions which would make sure that 
neither the oxygen of the air, nor the composition of the organic 
matter, was altered in sucha matter as to interfere with the exist- 
ence of life. 
Schulze and Schwann took up the question from this point of 
view in 1836 and 1837. The passage of air through red-hot 
glass tubes, or through strong sulphuric acid, does not alter the 
proportion of its oxygen, while it must needs arrest or destroy 
any organic matter which may be contained in the air. These 
experimenters, therefore, contrived arrangements by which the 
only air which should come into contact with a builed infusion 
should be such as had either passed through red-hot tubes or 
through strong sulphuric acid. The result which they obtained 
was that an infusion so treated developed no living things, while 
if the same infusion was afterwards exposed to the air such 
things appeared rapidly and abundantly. The accuracy of these 
experiments has been alternately denied and affirmed. Suppos- 
ing them to be accepted, however, all that they really proved 
was that the treatment to which the air was subjected destroyed 
something that was essential to the development of life in the 
infusion. This ‘‘something” might be gaseous, fluid, or solid ; 
that it consisted of germs remained only an hypothesis of greater 
or less probability. 
Contemporaneously with these investigations a remarkable 
discovery was made by Cagniard de la Tour. He found that 
common yeast is composed of a vast accumulation of minute 
plants. The fermentation of must or of wort in the fabrication 
of wine and of beer is always accompanied by the rapid growth 
and multiplication of these Zovw/@. Thus fermentation, in so 
far as it was accompanied by the development of microscopical 
organisms in enormous numbers, became assimilated to the de- 
composition of an inlusion of ordinary animal or yegetable 
matter ; and it was an obvious suggestion that the organisms 
were, in some way or other, the causes both of fermentation and 
of putrefaction, The chemists, with Berzelius and Liebig at 
their-head, at first laughed this idea to scorn ; but in 1843, a 
man then very young, who has since performed the unexampled 
feat of attaining to high eminence alike in Mathematics, Physics, 
and Physiology—I speak of the illustrious Helmholtz—reduced 
the matter to the test of experiment by a method alike elegant 
and conclusive. Helmholtz separated a putrefying or a fer- 
menting liquid from one which wes simply putrescible or fer- 
mentable by a membrane which allowed the fluids to pass through 
and become intermixed, but stopped the passage of solids. The 
result was, that while the putrescible or the fermentable liquids 
became impregnated with the results of the putrescence or fer- 
mentation which was going on on the other side of the membrane, 
they neither putrefied (in the ordinary way) nor fermented ; nor 
were any of the organisms which abounded in the fermenting or 
putrefying liquid generated inthem. Therefore the cause of the 
development of these organisms must lie in something which 
cannot pass through membranes ; and as Helmholtz’s investiga- 
tions were long antecedent to Graham’s researches upon col- 
loids, his natural conclusion was that the agent thus intercepted 
must be a solid material. In point of fact, Helmholtz’s experi- 
ments narrowed the issue to this: that which excites fermenta- 
tion and putrefaction, and at the same time gives rise to living 
forms ina fermentable or putrescible fluid, is not a gas and is not 
a diffusible fluid; therefore it is either a colloid, or it is matter 
divided into very minute solid particles. 
The researches of Schroeder and Dusch in 1854, and of 
Schroeder alone, in 1859, cleared up this point by experiments 
which are simply refinements upon those of Redi. A lump of 
cotton-wool is, physically speaking, a pile of many thicknesses 
of avery fine gauze, the fineness of the meshes of which de- 
pends upon the closeness of the compression of the wool, 
Now, Schroeder and Dusch found, that, in the case of all the 
putrefiable materials which they used (except milk and yolk of 
egg), an infusion boiled, and then allowed to come into contact 
with no air but such as had been filtered through cotton-wool 
neither putrefied nor fermented, nor developed living forms. Tt 
