28-i SPONTANEOUS GENERATION cially interested in the study of fermentation, which was shown to be a change dependent on the presence and growth of microscopic vege- table cells. lie boiled a suitable organic infu- sion in glass flasks, the necks of which were drawn out and sealed while ebullition was go- ing on, thus excluding completely the atmos- pheric air. Afterward, when the flask and its contents had become cooled, the air was read- mitted through the neck of the flask, by break- ing off its narrow end. But this operation was done, with different sets of flasks, in dif- ferent localities, in order to determine whether the difference of locality had any influence upon the subsequent appearance or non-ap- pearance of life within the flask. The bearing of these experiments upon the question at issue was as follows. If it were the constituent gases of the atmosphere alone which excited the spontaneous growth of living forms by the necessary supply of oxygen, then the produc- tion of these forms should follow with the same readiness in all localities, because the gaseous constitution of the atmosphere is every- where the same. But if, in order to produce life, the atmosphere must also bring with it certain organic germs, then the locality might make a difference in the result, because these floating particles would naturally vary in abun- dance in different regions. Investigation showed a manifest difference, according to the place wh^re the air was admitted to the flask. In one of the most significant of Pasteur's experi- ments, a flask containing an organic infusion was boiled, sealed, allowed to cool, and after- ward carried to the valley of Chamouni in Sa- voy, where its neck was opened and the air ad- mitted on the Montanvert, 6,000 ft. above the sea level. It was immediately resealed, brought back to Paris, and kept for four years perfect- ly unchanged. It was then reopened and again exposed to the air, and in three days after- ward contained a growth of microscopic vege- tation. These experiments were considered by Pasteur and his associates as demonstrating the existence in the atmosphere of extraneous particles, the introduction of which into an infusion was the necessary condition of infu- sorial life. A further difficulty now began to bo appreciated in this method of investigation. It had at first been taken for granted that a boiling temperature would necessarily destroy the vitality of both the infusoria and their germs. But this gradually became a matter of doubt, especially as the length of time du- ring which the boiling was continued evident- ly had an influence on the subsequent appear- ance of life in the infusion. It was found necessary to determine more exactly the limits of this influence; and among the most valua- ble experiments in that respect were those of Jeffries Wyman in 1867. He showed that, in infusions of a certain constitution, the minute forms known as bacteria might appear in closed flasks after boiling ; that the longer the boil- ing was continued, the fewer the instances in which bacteria were afterward developed ; and that they never made their appearance in infu- sions which had been boiled continuously for five or six hours. These results were variously interpreted by different observers; a certain number still maintaining that bacteria might often be developed after the application of a heat sufficient to destroy their previous vital- ity. In the modern renewal of the question of spontaneous generation, dating from Pouchet in 1858, another element has had its influence upon this discussion ; that is, the idea of evolu- tion, as accounting for the present existence of organic life upon the earth. It is assumed that there was once a period in the history of the earth when,' from its elevated temperature and the different combination of its chemical ele- ments, life could not possibly exist upon it; that, as living beings subsequently made their appearance, they must necessarily have origi- nated by the spontaneous organization of in- animate materials ; and that these primitive and imperfect structures have gradually, by modification and descent, given rise to all the forms of animal and vegetable life now inhab- iting the globe. Some of those who accept the evolution doctrines believe that the conditions necessary for a spontaneous production of life have long since passed away, with the earlier stages of the world's history ; others maintain that these conditions still exist, and are effec- tive for the continued creation of bacteria and their allied forms. It is common to meet with expressions, among writers of this class, which declare that spontaneous generation is not so much a matter of question or experiment as a logical sequence of the doctrine of evolution. The stricter school of physiologists maintain, on the contrary, that it is a subject to be inves- tigated on its own merits, by means of observa- tion and experiment, like any other question re- lating to the phenomena of life. Of late years the experimental evidence bearing on this topic has received an important addition from the independent researches of naturalists in regard to the infusoria. Some of the forms originally included in this group have been found, on more extended examination, to possess a higher organization, and have been by common con- sent transferred to the class of worms. Like others of this class, they reproduce their spe- cies by sexual generation, and often contain living embryos in the interior of their bodies. The infusoria proper are now known to be mostly ciliated animalcules ; that is. they are provided with minute, vibrating, hair-like ap- pendages, by which they perform rapid move- ments of locomotion. They have also been shown, principally by the labors of Stein, Bal- biani, Engelmann, and Claparede and Lach- mann, to perform the act of sexual generation, and to produce their young by means of fertile eggs, from which embryos are developed. The more minute and lowly forms, on the other hand, usually included under the general term bacteria, do not belong to the animal kingdom,
