SEXUAL GENERATION. 855 



For a time after the researches to which we have just alluded had taken their place 

 in the history of science, there was little written about spontaneous generation. Redi 

 had satisfactorily described the mode of generation of many of the entozoa, the origin 

 of which had been obscure ; Harvey had enunciated, in substance, his famous axiom, 

 " omne animal ex ovo ; " Regnerus de Graaf had described, in the ovaries, the vesicles 

 which have since borne his name ; and the knowledge of ovulation and development 

 began to make definite progress, the important fact having been ascertained, that vivipa- 

 rous, as well as oviparous animals, are produced from ova. 



With the discovery, by Leeuwenhoek, of living beings in water, called by him ani- 

 malcules, but since known as infusoria, a new problem was presented to students of 

 natural history. Here were animal organisms, so small as to be invisible to the naked 

 eye, existing in great variety and in infinite numbers, the mode of generation of which 

 was not understood. As these organisms were studied more closely, their multiplication 

 by segmentation and by budding became known, and these have since been described as 

 processes of generation peculiar to some of the lower orders of beings; but, at the same 

 time, some writers revived the theory of spontaneous generation, to account for the 

 original appearance of animalcules in water, and this idea has its advocates at the pres- 

 ent day. If, however, we follow out the history of the spontaneous-generation theory, 

 we find that the different epochs have repeated themselves ; that the theory took its 

 origin from an ignorance of the mode of generation of organisms quite high in the scale 

 of being ; that the progress of exact knowledge gradually restricted the theory to lower and 

 lower organisms, until, by this rigid process, it became extinct, simply from want of ma- 

 terial ; that its application to entozoa was eliminated in the same way ; that it was revived 

 by the discovery of infusoria ; and that now its limits have been restricted by positive 

 advances in knowledge, it being demonstrated, by Balbiani and others, that many varie- 

 ties of infusoria present the phenomena of sexual generation. 



Of the advocates of spontaneous generation within a comparatively recent period, 

 perhaps the most prominent has been Pouchet ; but modern researches have shown 

 pretty clearly that the infusoria produced in organic infusions are due, in all probability, 

 to the introduction of ova or spores floating in the air, which are developed when they 

 meet with proper conditions of heat and moisture. In numerous experiments by differ- 

 ent observers, which it is not necessary to cite in detail, it appeared that, when organic 

 infusions had been exposed to a degree of heat sufficient to destroy germs, and the intro- 

 duction of new germs from the air was prevented, no infusoria were developed ; and this 

 was the case when air was admitted to the infusions, care being taken to pass the air 

 through heated tubes or sulphuric acid, so as to destroy all organic matter. The present 

 aspect of the question of spontaneous generation is the following : 



First, it is reduced to the very lowest orders of infusoria, such as vibriones and bac- 

 teria, which simply present movement, have no distinguishable internal structure, and 

 are exceedingly minute. 



Second, the question is discussed as to what degree of temperature and length of 

 exposure to heat are necessary in order to destroy preexisting germs in organic infu- 

 sions ; for the idea that living organisms ever result from an aggregation of inorganic 

 particles has been generally abandoned, and the so-called spontaneous production of 

 animals has been reduced to a coming together of organic molecules. 



It is at once apparent to the rigidly scientific mind that the second division of the 

 question presents great difficulties in the way of its positive solution. It is granted, for 

 example, that vibriones and bacteria are living, animal organisms. It is proposed by the 

 advocates of the theory of spontaneous generation, that these beings arise without pre- 

 existing germs, by an aggregation of organic particles. The opponents of this view assert 

 that, when the air admitted to organic infusions is freed from germs or organic particles, 

 and when the organic infusions are subjected to a high temperature for a time sufficient 

 to destroy all possible preexisting germs, no generation of infusoria can take place. 



