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 SPONTANEOUS GENERATION 283 tion from putrefying meat, because they al- ways appeared at a certain stage of its decom- position, although no similar creatures existed in it beforehand, and because there was no other apparent cause for their production. A great change in opinion on this subject was introduced by the discoveries of Francesco Redi in 1668. He exposed fresh meat, during summer weather, in wide-mouthed bottles, protected by pieces of paper fastened over their necks. In the bottles thus secured no maggots were developed, notwithstanding that the putrefaction of the meat went on as usual ; while in other similar vessels, unprotected by paper covers, maggots swarmed in abundance at the customary time. It was evident there- fore that their origin was due to something introduced from without, and it soon appeared that they were really the progeny of flesh flies, which, attracted by the odor of the meat, hovered over it until they gained access to it, and deposited their eggs upon its surface. The eggs then hatched into maggots, which, after a certain period of growth, became trans- formed into perfect insects similar to their parents. The idea thus suggested was rapidly carried out by means of further observations on the reproduction and metamorphosis of in- sects in general. The investigations showed that in what had been supposed to be cases of spontaneous generation the animals were really produced from parents like themselves. The microscope soon brought into view many minute forms of life not previously known. The multiplicity of these forms, their endless variation, their small size, and their different conditions of life made it impossible at first to ascertain their complete physiological history or their mode of origin ; and in regard to many of them the idea of spontaneous genera- tion was again adopted. This was especially the case with the class known as infusoria ; that is, microscopic animals living in water or in wa- tery infusions of organic material, some of the smallest of which received the name of mo- nads. Investigations upon this point were con- sequently taken up afresh, with a view of deter- mining whether the infusoria in a watery liquid were produced by the ingredients of the solu- tion itself, or by germs derived from without. Experimenters boiled the watery infusions, to destroy the vitality of any animalcules or germs which they might already contain, and after- ward kept them, with a due supply of air, in hermetically sealed flasks. If, under these cir- cumstances, living forms still made their ap- pearance in the infusions, they must be attrib- uted to spontaneous generation ; if not, they must be regarded as dependent on the preex- istence or introduction of germs. These ex- periments were tried by different observers, with results which varied according to the nature and extent of the precautions adopted ; but the general conclusion, derived especially from the investigations of Spallanzani in 1775, was that a preliminary boiling in closed flasks, for a few minutes, effectually prevented the appearance of all the larger and more highly organized infusoria ; while, if the boiling were prolonged from half an hour to an hour, the infusion afterward remained absolutely desti- tute of all forms of life, even the smallest and simplest. Although at that time the real mode of generation of the infusoria had never been ascertained, nor their eggs detected by the microscope, it was considered certain that these animalcules must require for their pro- duction the existence of living germs, and con- sequently that they did not originate by spon- taneous generation. During the early and middle part of the present century the com- mon opinion of naturalists became gradually more decided in opposition to the doctrine of spontaneous generation, owing to the occa- sional repetition of experiments like Spallan- zani's, and also to important discoveries in regard to the sexless internal parasites, such as cy&ticercus and trichina. These creatures were found inhabiting the solid tissues of other, animals, and furthermore were seen to be in- capable of exercising the function of genera- tion. It was difficult therefore to account for their presence in the animal tissues unless by a growth upon the spot, and also to understand how the species could be reproduced by ordi- nary modes of generation. But continued in- vestigation removed both of these difficulties. It was shown by the researches of Siebold, Kuchenmeister, Leuckart, Pagenstecher, a*nd others, that the sexless parasites were in real- ity the embryonic or youthful progeny of per- fectly developed parents ; their mode of intro- duction into the internal cavities and tissues of the body was ascertained ; and they were found to acquire after a time sexual organs, and to produce a new progeny by sexual gen- eration. Thus, one by one, a great variety of obscure animal species became more perfectly known ; and a complete study of their physio- logical history revealed in every instance the regular mode of their origin and reproduction. But the class of infusoria still remained some- what refractory in this respect, and notwith- standing that the question had been for some years regarded as settled, it was reopened in 1858. M. Pouch et, an eminent naturalist and physiologist of Rouen, took the ground that the former experiments in regard to boiled in- fusions were incorrect, and that in point of fact a preliminary boiling did not prevent the appearance of infusorial life. Pouchet's views and assertions were supported by the testimony of several other experimenters, among the most distinguished of whom have been Mante- gazza and Bastian. They were opposed by many others, of equal reputation; and the weight of the discussion turned for a time upon the dissemination of germs in the atmos- phere, as the supposed source of life in organ- ic infusions. The most important experiments in this direction were those of Pasteur, from 1860 to 1865. This chemist had been espe-