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 



