52 ANIMAL LIFE 
of a few kinds of animals, like parasites and the animalcules 
of stagnant water. It was maintained that parasites arose 
spontaneously from the matter of the living animal in 
which they lay. Many parasites have so complicated and 
extraordinary a life history that it was only after long and 
careful study that the truth regarding their origin was dis- 
covered. But in the case of every parasite whose life his- 
tory is known the young are offspring of parents, of other 
individuals of their kind. No case of spontaneous genera- 
tion among parasites is known. The same is true of the 
animalcules of stagnant water. If some water in which 
there are apparently no living organisms, however minute, 
be allowed to stand for a few days, it will come to be 
swarming with microscopic plants and animals, Any or- 
ganic liquid, as a broth or a vegetable infusion exposed for 
a short time, becomes foul through the presence of innumer- 
able bacteria, infusoria, and other one-celled animals and 
plants, or rather through the changes produced by their 
life processes. But it has been certainly proved that these 
organisms are not spontaneously produced by the water or 
organic liquid. A few of them enter the water from the 
air, in which there are always greater or less numbers of 
spores of microscopic organisms. These spores (embryo or- 
ganisms in the resting stage) germinate quickly when they 
fall into water or some organic liquid, and the rapid suc- 
cession of generations soon gives rise to the hosts of bacteria 
and Protozoa which infest all standing water. If all the 
active organisms and inactive spores in a glass of water are 
killed by boiling the water, “ sterilizing ” it, as it is called, 
and this sterilized water or organic liquid be put into a 
sterilized glass, and this glass be so well closed that germs 
or spores can not pass from the air without into the steril- 
ized liquid, no living animals will ever appear in it. It is 
now known that flesh will not decay or liquids ferment 
except through the presence of living animals or plants. 
To sum up, we may say that we know of no instance of the 
