404 
NATURE 
[ Sez. 15, 1870 
that I intend to suggest that no such thing as Abiogenesis ever 
has taken place in the past or ever will take place in the future. 
With organic chemistry, molecular physics, and physiology yet 
in their infancy, and every day making prodigious strides, I 
think it would be the height of presumption for any man to say 
that the conditions under which matter assumes the properties 
we call ‘‘ vital” may not, some day, be artificially brought 
together. All I feel justified in affirming is that I see no reason 
for believing that the feat has been performed yet. 
And looking back through the prodigious vista of the past, I 
find no record of the commencement of life, and therefore I am 
devoid of any means of forming a definite conclusion as to the 
conditions ofits appearance. Belief, in the scientific sense of the 
word, is a serious matter, and needs strong foundations, To 
say, therefore, in the admitted absence of evidence, that I have 
any belief as to the mode in which the existing forms of life have 
originated, would be using words in a wrong sense. But expec- 
tation is permissible where belief is not ; and if it were given me 
to look beyond the abyss of geologically recorded time to the 
still more remote period when the earth was passing through 
physical and chemical conditions, which it can no more see again 
than a man can recall his infancy, I should expect to be a witness 
of the evolution of living protoplasm from not living matter. I 
should expect to see it appear under forms of great simplicity, 
endowed, like existing fungi, with the power of determining the 
formation of new protoplasm from such matters as ammonium 
carbonates, oxalates and tartrates, alkaline and earthy phos- 
phates, and water, without the aid of light. ‘That is the expec- 
tation to which analogical reasoning leads me; but I beg you 
once more to recollect that I have no right to call my opinion 
anything but an act of philosophical faith. 
So much for the history of the progress of Redi’s great doctrine 
of Biogenesis, which appears to me, with the limitations I 
have expressed, to be victorious along the whole line at the 
present day. 
As regards the second problem offered to us by Redi, whether 
Xenogenesis obtains, side by side with Homogenesis ; whether, 
that is, there exist not only the ordinary living things, giving rise 
to offspring which run through the same cycle as themselves, 
but also others, producing offspring which are of a totally different 
character from themselves, the researches of two centuries have 
led to a different result. That the grubs found in galls are no 
product of the plants on which the galls grow, but are the result 
of the introduction of the eggs of insects into the substance of 
these plants, was made out by Vallisnieri, Reaumur, and others, 
before the end of the first half of the eighteenth century, The 
tapeworms, bladderworms, and flukes continued to be a strong- 
hold of the advocates of Xenogenesis for a much longer period. 
Indeed, it is only within the last thirty years that the splendid 
patience of Von Siebold, Van Beneden, Leuckart, Kiichen- 
meister, and other helminthologists, has succeeded in tracing 
every such parasite, often through the strangest wanderings and 
metamorphoses, to an egg derived from a parent, actually or 
potentially like itself; and the tendency of inquiries elsewhere 
has all been in the same direction. A plant may throw off bulbs, 
but these, sooner or later, give rise to seeds or spores, which 
develop into the original form. A polype may give rise to 
Medusz, or a pluteus to an Echinoderm, but the Medusa and 
the Echinoderm give rise to eggs which produce polypes or 
plutei, and they are therefore only stages in the cycle of life of 
the species. 
But if we turn to pathology it offers us some remarkable 
approximations to true Xenogenesis. 
As I have already mentioned, it has been known since the time 
of Vallisnieri and of Reaumur, that galls in plants, and tumours 
in cattle, are caused by insects, which lay their ergs in those parts 
of the animal or vegetable frame of which these morbid struc- 
tures are outgrowths. Again, it is a matter of familiar experience 
to everybody that mere pressure on the skin will give rise to a 
corn. Now the gall, the tumour, and the corn are parts of the 
living body, which have become, to a certain degree, independent 
and distinct organisms. Under the influence of certain external 
conditions, elements of the body, which should have developed 
in due subordination to its general plan, set up for themselves 
and apply the nourishment which they receive to their own 
purposes, 
From such innocent productions as corns and warts, there are 
all gradations to the serious tumours which, by their mere size 
and the mechanical obstruction they cause, destroy the organism 
out of which they are developed ; while, finally, in those terrible 
structures known as cancers, the abnormal growth has acquired 
powers of reproduction and multiplication, and is only morpho- 
logically distinguishable from the parasite worm, the life of which 
is neither more nor less closely bound up with that of the 
infested organism. 
If there were a kind of diseased structure, the histological 
elements of which were capable of maintaining a separate and inde- 
pendent existence out of the body, it seems to me that the shadowy 
boundary between morbid growth and Xenogenesis would be 
effaced. And I am inclined to think that the progress of dis- 
covery has almost brought us to this point already. I haye been 
fayoured by Mr. Simon with an early copy of the last published 
of the valuable ‘‘ Reports on the Public Health,” which, in his 
capacity of their medical officer, he annually presents to the 
Lords of the Privy Council. The appeadix to this report con- 
tains an introductory essay ‘‘On the Intimate Pathology of 
Contagion,” by Dr. Burdon Sanderson, which is one of the 
clearest, most comprehensive, and well-reasoned discussions of a 
great question which has come under my notice for a long time. 
I refer you to it for details and for the authorities for the state- 
ments I am about to make. 
You are familiar with what happens in vaccination. A minute 
cutis made in the skin, and an infinitesimal quantity of vaccine 
matter is inserted into the wound. Within acertain timea vesicle 
appears in the place of the wound, and the fluid which distends 
this vesicle is vaccine matter, in quantity a hundred or a thou- 
sandfold that which was originally inserted. Now what has 
taken place in the course of this operation? Has the vaccine 
-maiter, by its irritative property, produced a mere blister, the 
fluid of which has the same irritative property? Or does the 
vaccine matter contain living particles, which have grown and 
multipiied where they have been planted? The observations ot 
M. Chauveau, extended and confirmed by Dr. Sanderson him- 
self, appear to leave no doubt upon this head. Experiments, 
similar in principle to those of Helmholtz on fermentation and 
putrefaction, have proved that the active element in the vaccine 
lymph is non-diffusible, and consists of minute particles not 
exceeding sya of an inch in diameter, which are made visible 
in the lymph by the microscope. Similar experiments have 
proved that two of the most destructive of epizootic diseases, 
sheep-pox and glanders, are also dependent for their existence 
and their propagation upon extremely small living solid particles, 
to which the title of scrozymes is applied. An animal suffering 
under either of these terrible diseases is a source of infection and 
contagion to others, for precisely the same reason as a tub of 
fermenting beer is capable of propagating its fermentation by 
‘‘infection,” or “‘ contagion,” to fresh wort. In both cases it is 
the solid living particles which are efficient ; the liquid in which 
they float, and at the expense of which they live, being altogether 
passive. 
Now arises the question, are these microzymes the results of 
Flomogenesis, or of Xenogenesis ; are they capable, like the 
Torule of yeast, of arising only by the development of pre- 
existing germs; or may they be, like the constituents of a nut- 
gall, the results of a modification and individualisation of the 
tissues of the body in which they are found, resulting from the 
operation of certain conditions? Are they parasites in the 
zoological sense, or are they merely what Virchow has called 
“‘heterologous growths”? It is obvious that this question 
has the most profound importance, whether we look at it 
from a practical or from a theoretical point of view. A 
parasite may be stamped out by destroying its germs, buta patho- 
logical product can only be annihilated by removing the condi- 
tions which give rise to it. 
It appears to me that this great problem will have to be solved 
for each zymotic clisease separately, for analogy cuts two ways. 
I have dwelt upon the analogy of pathological modification, 
which is in favour of the xenogenetic origin of microzymes ; 
but I must now speak of the equally strong analogies in favour 
of the origin of such pestiferous particles by the ordinary process 
of the generation of like from like. 
It is, at present, a well-established fact that certain diseases, 
both of plants and of animals, which have all the characters of 
contagious and infectious epidemics, are caused by minute 
organisms, The smut of wheat is a well-known instance of such 
a disease, and it cannot be doubted that the grape-disease and 
the potato-disease fall under the same category. Among animals, 
insects are wonderfully liable to the ravages of contagious and 
infectious diseases caused by microscopic /wzzg7. ‘ 
In autumn, it is not uncommon to see flies, motionless upon a 
