The New Student's Reference Work/Spontaneous Generation of Life
Spontaneous Genera′tion of Life, the doctrine that life in some form is developed from non-living matter. The ancients believed that frogs and eels sprang from mud, that insects were generated in dew, that decaying meat bred maggots, that the appearance of other forms of life was to be accounted for by spontaneous generation from lifeless matter. This belief was quite general until the 17th century, when the Italian, Francesco Redi, made experiments to test the truth of it. He placed bits of meat in open jars, and others in jars that were covered by fine gauze, and watched for the development of maggots. The blow-flies visited the uncovered meat, but were prevented from reaching the other. Of course, maggots hatched from the eggs, but did not appear in the protected meat, and, by this simple experiment, he proved that maggots did not arise spontaneously from the meat. This was about 1660. He made further experiments and reached the conclusion, not as a matter of opinion, but as a result of experiment, that life arises only from antecedent life and does not develop spontaneously.
This position was quite generally accepted for larger animals, but with the introduction of the microscope, a new world of extremely minute living beings was made known, and doubts began to be entertained that those minute beings must always have parents like themselves. It is a well-known fact that fluids, like clear mutton-broth or water, in which any vegetable or animal substance has been soaked, will, if allowed to stand, soon become teeming with microscopic life. The possibility of this life arising by spontaneous generation formed a new phase of the question. Needham and Buffon attempted to put the matter to test by boiling infusions, to kill all germs that might exist in them, and corking them tightly. In course of time these fluids became cloudy and were overrun with microscopic life. But about 1775 a priest of the period named Spallanzani, showed that their experiments had not been conducted with sufficient care. He boiled infusions for three quarters of an hour in flasks, and, while the fluid was still boiling, heated the necks of the flasks and, drawing them into a fine point, he closed them by heat. No microscopic life appeared in the flask treated in this way, and the conclusion was drawn that spontaneous generation of life was disproved.
About this time oxygen was discovered and shown to be necessary to all forms of life. This brought a new point of view, and objection was made to Spallanzani's form of experiments on the score that oxygen was excluded by sealing the necks of the flasks. In order to test this objection, Schulze and Schwann in 1836 took up the question again and devised a means of admitting oxygen into the flasks containing fluids that had been boiled. The necks of the flasks were drawn out long, and convoluted, and left open at the ends. In some cases the inlet was heated and air drawn in. In other cases the air was passed through bulbs containing chemicals that did not alter the oxygen. It was soon discovered that a plug of cotton-wool in the necks of the flasks would act as a filter. It permits the air to pass through and arrests all floating particles and, therefore, does not allow dust or germs to pass through with the air. A flask, or test-tube, is partly filled with an organic infusion; the latter is boiled and, while boiling, a plug of cotton-wool is pushed into the mouth of the vessel. On cooling, the air passes through this plug, but the smallest solid particles are caught in the meshes of the cotton-wool. Organic fluids treated in this way have been known for years to show no sign of microscopic life.
The question was regarded as settled, but in 1859 it was unexpectedly opened in Paris by Pouchet, who claimed that life was developed in organic fluids even after boiling and preventing the entrance of floating particles from the air. Pasteur, however, demonstrated that Pouchet had been careless, and had admitted solid particles to the flask in the mercury, through which the oxygen had been introduced into the flasks. This closed the question as far as it could be answered by experiment. Professor John Tyndall added something to the certainty of the matter by an ingenious experiment. He had an air-tight box made with glass-windows, through which he passed a strong beam of light in a darkened room. The floating particles in the air, contained within the box, fell of their own weight, and came to rest on the sides and bottom of the box, which had been smeared with a sticky substance to hold them. He could tell when the air was completely cleared of these particles by his beam of strong light, because any minute floating particle would become illuminated and reflect the light. When they had entirely come to rest, he called the air “optically pure.” Through the bottom of this box he had inserted a number of test-tubes with their mouths opening into the chamber, and, by means of a long funnel and a hole pricked in a rubber membrane at the top, he was able to introduce a variety of organic fluids into these test-tubes. The ends of the test-tubes outside of the box were now inserted into boiling oil and all germs of life that might be contained in the fluids were completely killed. He was able to expose the fluids in these open test-tubes to the optically pure air for months, without any appearance of microscopic life in any of them. To demonstrate that the fluids were capable of supporting such life, he had simply to open for a moment an air-tight door in the back of the chamber and admit some of the outside air. When this was done, the fluids became turbid in a short time, and were teeming with microscopic life. Thus it was demonstrated that microscopic life came from floating germs in the air, and is not spontaneously generated in organic fluids. See Tyndall in Pop. Sci. Mo. (vol. 12, 1878) and in Floating Matter of the Air.