THE WONDERS OF LIFE 



tury. It was not until 1 840-1 860 that it was shown 

 by the experiments of Siebold, Leuckart, Van Beneden, 

 Virchow, and other famous biologists, that all these in- 

 testinal animals have come from without into the ani- 

 mals they live in, and propagate there by eggs. Of 

 late years the proof has been applied all round. 



On the other hand, the hypothesis of saprobiosis re- 

 tained its position until quite recently for one section 

 of the smallest and lowest organisms, the microscopic 

 forms of life, invisible to the naked eye, which were 

 formerly called infusoria, and which we now call by the 

 wider name of protists or unicellulars. When Leeuwen- 

 hoek discovered the infusoria in 1675 with the newly 

 invented microscope, and showed that they arise in 

 great quantities in infusions of hay, moss, flesh, and 

 other putrid organic substances, it was generally be- 

 lieved that they were spontaneously generated there. 

 The Abb^ Spallanzani showed in 1687 that no infusoria 

 appear in these infusions if they are well boiled and 

 the vessel is carefully closed ; the boiling kills the germs 

 in them, and the exclusion of air prevents the entrance 

 of fresh germs. In spite of this, many microscopists 

 still believed that certain infusoria, particularly the very 

 small and simple bacteria, could be born directly from 

 putrid or diseased tissues of organisms, or from decom- 

 posing organic fluids; the opinion was maintained by 

 Pouchet at Paris in 1858, and afterwards by Charlton 

 Bastian. The controversy about the subject moved the 

 Paris Academy in 1858 to offer a prize for "careful 

 research that would throw new light on the question of 

 spontaneous generation." It fell to the famous Louis 

 Pasteur, who proved, by a series of ingenious experi- 

 ments, that there are everywhere in the atmosphere 

 numbers of germs of microbes or microscopic organisms 

 floating among the dust particles, and that these grow 

 and reproduce when they reach water. Not only in- 



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