40 THE EVOLUTION OF LIFE 
These investigations convinced M. Pasteur that 
ordinary air contains a considerable, though still a 
variable, number of spherical or ovoidal corpuscles, 
whose form and structure made him think they were 
organised. ‘They varied from the smallest appreci- 
able size up to bodies about zso0 inch in diameter. 
Some of them, it was supposed, might be spores of 
fungi. ‘‘ Mais quant a affirmer,’ Pasteur says, ‘“‘ que 
ceci est un spore, bien plus la spore de telle espéce 
déterminée, et que cela est un ceuf et l’ceuf de tel 
microzoaire, je crois que cela n’est pas possible.” 
He produced no direct evidence that the bodies 
were really germs capable of developing into one or 
other kind of Mould, though he says, in a note (oc. 
cit. p. 34), he had “ originally intended to attempt this 
kind of proof.” At this date also Pasteur did not 
pretend to have recognised Bacteria among the 
corpuscles and other débris filtered from the 
atmosphere, though on indirect evidence he assumed 
them to be present. 
If this was the state of things at the time of, and 
for full ten years after, the publication of Pasteur’s 
memoir, a wholly different state of knowledge now 
exists as a result of the labours, during the last thirty 
years, of an army of bacteriologists. 
Common or saprophytic bacteria of different kinds 
undoubtedly exist in the atmosphere, though not in 
such numbers as were at one time supposed when 
surgeons performed all their operations in a mist of 
carbolised spray. They vary also enormously in 
different localities—being less abundant in country 
districts than in towns; diminishing again rapidly 

