ORIGIN OF LIFE, 65 
There are only two possible modes of accounting 
for the fact that “certain of the most minute living 
things are known to appear in some fluids inde- 
pendently of pre-existing visible germs.” If there- 
fore it can be shown that living though invisible 
germs did not pre-exist in certain fluids in which 
such minute living things subsequently make their | 
appearance as usual, we thereby prove that in such 
instances they must have owed their appearance to 
the other process—viz., to Archebiosis. Nothing 
can be plainer than this; if a given event must be 
occasioned by one or other of two causes, and if in 
certain instances we can show that the event followed, 
notwithstanding the absence of one of these causes, 
then the event must have been occasioned by the 
other cause. An experiment of this nature is named 
a ‘crucial instance’ or experimentum cructs. 
‘Let us look then into the nature of the crucial 
instance which lies at our disposal in this emergency. 
If we wish to ascertain whether living matter 
exists, or rather if we wish to make sure that living 
matter does not exist in any given fluid, the only 
course open to us is to submit the fluid to the in- 
fluence of agencies which we have previously ascer- 
tained to be capable of ‘killing’ such matter—that 
is to say, of putting an end to the combination of 
properties the existence of which formerly gave us 
F 
