96 MICRO-ORGANISMS IN RELATION TO MAN, 
question as to whence these minute forms came, was debated 
with great vigour. Even at the end of the seventeenth cen- 
tury there were those who considered that bacteria were the 
progeny of minute organisms which were present in myriads 
in the air, from which they were deposited on putrescible 
substances. This theory, however, met with much opposit- 
ion, notably by an Englishman, Dr. Medham, who wrote in 
1749, and who was really the first to bring forward the theory 
’ 
of abiogenesis or ‘‘spontaneous generation,” a theory which 
attracted much attention, and which was only finally routed 
within our own times—to a large extent by the masterly and 
beautiful experiments of our countryman, Tyndall. 
Medham’s theory was, in brief, that the bacteria or ‘‘plant 
animals” as he called them, were developed from dead organic 
matter by some special vegetative power, and that from these 
plant-animals, by a process of evolution, other organisms 
again arose. 
The experiments on which he based his theory were that 
when liquids like beef-infusions are boiled and then set aside 
in well-stoppered bottles, a vigorous growth of micro-organ- 
isms always takes place. 
The objection taken to these experiments was two-fold : 
firstly, that air containing germs found entrance to the vessels 
after boiling; and secondly, that possibly the germs were so 
far resistant to increase of temperature that they might sur- 
vive a short period of boiling. Both these objections were 
shewn to be well founded: when putrescible substances were 
heated under certain conditions, and air was rigorously exclu- 
ded no putrefaction took place, and on this fact the methods 
of preservation of food-stuffs in vogue at the present time are 
founded; and on the other hand it was shown that when 
a growth of organisms did take place after boiling, even 
when the air-borne germs were excluded, it was due to the 
fact that the spores of some organisms are able to survive 
a boiling temperature for some time, although the ordinary 
