While most naturalists readily admit the correctness of the above-mentioned general law, some are inclined to hold that it is not universally applicable, and that there are excep tions to it both in the vegetable and animal kingdoms of organized nature. It is among the simplest kinds of plants and animals that these exceptions are conceived to exist, and more particularly among cryptogamic plants of the nature of mould, small microscopic animalcules formed in infusions of decaying organic matters, and the Entozoa which live in the bodies of other animals. These living productions are supposed by some to arise in dependently of others of the same kind, nearly in the manner of minerals, by the aggregation of their component molecules, with this diffe rence, that these molecules are of an organic kind. This sort of production without parents has been termed Spontaneous Generation. It has also received at different times various other appellations, such as equivocal, doubtful, primitive, original, and heterogeneous gene ration.
At one time it was a common belief among scientific men as well as the vulgar that many animals might be produced by sponta neous generation, as for example, the numerous insects or their larvw infesting putrid sub stances, various kinds of worms (Annelida), and Molluscous animals, as well as even some fishes and reptiles ; but the increased knowledge of the structure and habits of these animals, and in particular the observations of Redi* and others, demonstrated the error of this opinion, and shewed it to have arisen merely from the circumstance of their real mode of generation not having been observed. After this many felt inclined to reject entirely the occurrence of spontaneous generation iu any class of organized beings, and at the present day the question cannot be regarded as by any means entirely set at rest. From the nature of the observations and experiments required in an investigation of this nature, there is almost an impossibility of arriving at a perfectly satis factory conclusion ; but so far as the facts at present entitle us to form an opinion, it may be stated that spontaneous generation, if it occurs, takes place in the simplest kinds of organized beings only ; that in most of them it is only occasional ; and that therefore this form of ge neration is to be looked upon as a rare excep tion to the usual and almost universal mode of reproduction by the separation of a living por tion from a parent body.
Minute animalcules, the greater number of which are so small as to be visible only with the microscope, are formed in the infusions of almost all kinds of organic matter, such as starch, sugar, gum, seeds, and different animal substances, when these infusions enter into pu trefaction. The kinds of these animalcules are
very numerous, and the circumstances which seem to determine the formation of one or other sort are infinitely varied. Thus the nature of the substance suspended in the infusion ; and, in the same infusions, the degree of heat, the extent of the decomposition, the quantity and nature of the air admitted, the rapidity with which it is renewed, and the strength of the in fusion or the relative proportion of water and organic matter in it, all appear to exert a certain influence in determining the formation of one or other of the kinds of animalcule.
Two suppositions may be entertained regard ing the first origin of Infusoria ; the one, that of their spontaneous generation ; the other, that of their developement or evolution from some pre-existing egg or germ. Those who disbe lieve in the first and adopt the second hypo thesis hold that the ova of the animalcules exist in the substances of the infusions, or are float ing every where in the atmospheric air ; that these ova become developed in that species of infusion only which is suited to serve as their proper nidus or matrix; and that all the varieties of animalculm in different infusions depend upon the infusions being suited, from their composition or the external agencies to which they are subjected, to cause the develop ment of different sorts of ova. The supporters of the hypothesis of Spontaneous Generation hold, on the other hand, that certain changes of composition of the organic molecules in the infusions, in whatever way induced, are the sole cause of the formation of one or other kind of animalcule.
Spallanzani,* one of the most strenuous op ponents of the hypothesis of spontaneous gene ration, shewed by very accurate experiments that no animalcules are formed when the access of air to the infusion is completely prevented, as for example, when it is covered with a little oil, or the vessel containing it is closely sealed ; and he thence concluded that the germs of the ani malcules must exist in the atmosphere; hut the supporters of the hypothesis consider them selves as entitled to hold that no production of animalcules takes place in these circumstances, merely because the exclusion of the air has the effect of preventing that species of decomposi tion which they regard as necessary for the formation of the Infusoria.