GENERATION, SPONTANEOUS. From the earliest period to the termination, of the middle ages, no one called in question the doctrine that, under certain favorable con ditions, of which putrefaction was one of the most important, animals might be pro duced without parents. Anaximander and Empedocles attributed to this form of generation all the living beings which fist peopled the globe. Aristotle, without com mitting himself to so general a view, maintains that animals are sometimes formed in putrefying soil, sometimes in plants, and sometimes in the fluids of other animals, and lays down the following general principle,-" that every dry subslanv,.which becomes moist, and every moist bony Which is dried, producesliving creatures, 'tialovided it is fit for nourishing them." The views of Lucretius on this subject are shown in the follow ing lines; Nonne vides qumeunque mortt, fluidoque iiquore COlpui a tabuennt, iu narva :Johanna verti? And that " qumdam gignuntur ex non geniis, et sine ffildsimili origine.'' Virgil's directions for the production of bees are .known to every reader of the Geordies, and an expressiOn in the book of Judges (xiv. 14) probably points to a similar opinion.
Passing from classical times to the later period of the middle ages, and the two suc ceeding centuries, we may quote amongst the advocates of this theory Cardan—who, in his treatise De Subtilitate (1342), asserts that water engenders fishes, and that many animals spring from fermentation—Aldrovandus, Licetus, Gassendi, Scaliger, Van Heliuont, who gives special instructions for the artificial production of mice, and Kircher, who in his Mundus Subterraneus (in the chapter De Panspermia Rerum ") describes, and actually figures, certain animals which were produced under his own eyes by the transforming influence of water on fragments of the stems of different plants! Redi, the celebrated Italian naturalist, whose Experiments on the Generation of Insects were published in 1008, seems to have been the first opponent that the doctrine of spontaneous generation encountered. In this work, he proves that the worms and insects which appear in decaying substances are in reality developed from eggs, deposited in those substances by the parents. Leuwenitoek, Swannnerdam, and other eminent naturalists, soon contributed additional facts and arguments in favor of Redi's view; and as from the time of Redi to the present day, the tide of opinion has generally turned strongly against the doctrine in question; it is unnecessary to carry the historical sketch further.
The entozoa, however, continued to be a great stumbling-block. "When," says prof. Owen, " the entozoologist contemplated the koala fixed to the intestine, with its uncinated and suctorious head buried in the mucous membrane, rooted to the spot, and imbibing nourishment like a plant—when be saw the sluggish distoma (or fluke) adher ing by its tucker to the serous membrane of a closed internal cavity, he naturally asked himself how they got there; and finding no obvious solution to the difficulty of the transit on the part of such animals, he was driven to the hypothesis of spontaneous generation to solve the difficulty. It is no wonder that Rudolphi (1808) and Bremser
(1824), who studied the entozoa rather as naturalists than physiologists, should have been led to apply to them the easy explanation which Aristotle had given for the com ing into being of all kinds of vermes—viz., that they were spontaneously generated. No other explanation, in the then state of the knowledge of the development of the entozoa, appeared to be adequate to account for the fact of their getting into the interior cavities and tissues of higher animals." The recent investigations of Von Siebold, Kbehenmeister, Van Beneden, Philippi, etc., regarding the devlopment and meta morphoses of the entozoa, have, however, tended to remove nearly all the difficulties which this subject presented ;. and the advocates of spontaneous generation are fairly driven from this, one of the last of their battle-fields. • The only point at present in dispute is,' whether microscopic organisms (animals or plants) may be spontaneously generated. It is well known that if we examine under the microscope a drop of water in which almost any animal or vegetable substances have been infused, and which contains the particles of such substances in a state of decay or decomposition, it is found to swarm with minute living organisms. The question at issue is this: Are these organisms developed in the water, if the necessary precautions haVe been taken to exclude every animalcule or germ capable of development both from the water and from the air that has access to it? A well-known experiment, devised by prof: Schulze of Berlin (a description of which may be found in Owen's Lectures on the Invertebrate Animals, 2d ed. p. 44), shows that with due precautions in reference to these points, no animal or vegetable organisms are produced. This experiment was continued interruptedly from May 28 until the beginning of August, "and when, at last, the professor separated the different parts of the apparatus, he could not find in the whole liquid the slightest trace of infusoria or confcrvte, or of mold; but all three pre sented themselves in great abundance a few days after he had left the flask standing open." A vessel with a similar infusion, which lie placed near The apparatus, contained vibriones and monads on the second day of the experiment, to which were soon added larger polygastric infusoria.