38 THE EVOLUTION OF LIFE 
unfit for the occurrence of future processes that may 
lead to life-origination: this is the major -difficulty, 
especially as preconceptions are so strong with many 
people that they regard the actual problem as fore- 
closed. If, after the purifying process, living things 
appear and multiply within the experimental vessels, 
such persons are apt invariably to attribute the fact 
to a survival of pre-existing germs rather than to 
a birth of new germs. 
We must be sure also that there has been no 
contamination with atmospheric germs after the 
preliminary heating, and during the time that the 
experimental vessels are kept previous to the 
examination of their contents. But in all the funda- 
mental experiments to be recorded by me in this 
work the fluids have been guarded in hermetically 
sealed vessels—a process which must be absolutely 
safe, and far more stringent than trusting to bent 
tubes or plugs of cotton wool as Pasteur and others 
have often done. 
After a few words concerning the now fully 
admitted distribution of germs in air and water, we 
must address ourselves specially, and at length, to 
the all-important problem of the limits of vital 
resistance to heat, in order to obtain the necessary 
data as to the degree of heat really needed to 
sterilise the experimental vessels and their contents. 
If we can arrive at definite conclusions on this point, 
those conclusions must not be departed from except 
upon the basis of fresh independent evidence; an 
obvious rule which has been only too much and too 
frequently ignored in the past. 
