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About the middle of the seventeenth century one Anton Leenwenhoeck lived in the town of Delft, in Holland, as the steward of a judge there. He was accustomed to employ his leisure in making little lenses, wherewith he magnified and examined the structure of such things as butterflies' and gnats' wings. He had great skill of hand, and made several hundred lenses, each one to suit some particular object he wished to examine. With such a simple instrument, in April, 1675, did Leenwenhoeck reveal a new and hitherto unsuspected world of living things. 11e placed under one of his simple microscopes I a glass of rain-water, and perceived a multitude of variously-shaped bodies darting to and fro. Further experiments showed their presence in various organic (that is animal and vegetable) infusions. Hence these animalculae, as their discoverer supposed them to be, have been termed Infusorla. But whence came they? Was it true, as the philosophers of antiquity believed, that organic substances, decaying animal and vegetable matter, with sufficient air and moisture, could, under the influence of heat, beget anew living things? Could "the sun breed maggots in a dead dog, being a god kissing carrion ?" Now it so happens that, thirty-seven years before Leeuwenhoeck's discovery, a similar question had been distrustfully put by a phy sician of Florence, Francois Redi, and had been triumphantly answered in the negative. Redi was not satisfied that the sun could "breed maggots in a dead dog" or in any other kind of dead meat, and he put the question to the test of an experiment as simple as it was con clusive. In hot weather he placed some fresh meat in a jar whose mouth he then covered with gauze, and in another but uncovered jar he placed a similar piece of meat; and he observed that while the exposed meat speedily swarmed with maggots, that under cover, though it became stinking, remained free of maggots. The maggots were not, therefore, the products of putrefaction, but their true Cause was speedily apparent. For he noticed that multitudes of flies buzzed about the gauze, on which maggots were bred from eggs deposited there by the flies.

The maggots were, then, not the products of dead organic matter, but the direct offspring of living things. Could this explanation be applied to flip infusoria of Leeuwenhoeck ? and were they also hatched from eggs, or in some way the offspring of preceding infusoria like unto them? This was a much more difficult question, and not, at first sight, capable of a test so simple as Redi's. Moreover other ob servers flocked to the new field of investigation opened up by the microscope; the simple micro scope was improved on; and, as the instru ment became more and more powerful, smaller and still smaller living things came into view, till some were clearly revealed that would appear as mere points under Leenwenhoeck's simple lens ; while the modern compound microscope now discovers others as invisible to the simple lens as the infusoria are to the naked eye.

The problem to be answered does not alter with the diminution in size ; the same question is raised, Whence come they? In every organic infusion left standing for a day or two, more or less as the weather is cold or warm—in every organic infusion they swarm in myriads. Are they begotten by the mere breaking down, the decomposition, the putrefaction, the death and decay of the organic substance, or are they developed from eggs or in some other way the direct offspring of ancestors like themselves? An other very simple experiment seemed to settle the question, an experiment devised in 1748 by an English Catholic priest named Needham, in association with the French naturalist Buffon, but only perfectly carried out seventeen years later by an Italian philo sopher, the Abbe Spallauzani. Its purpose was the same as Redi's, to keep an organic substance so that nothing could alight on it from without, in order to find whether, under these conditions, living things could develop within it. So minute are the organisms, how ever, that multitudes might be deposited, un seen, from the air during even a moment's exposure. With great ingenuity were the altered circumstances met. It was argued that heat would destroy the organisms, as it destroyed other living things. So an organic

infusion was placed in a flask and boiled, and after it had been boiled for some time the neck of the flask was sealed by melting it in a flame, so that any living thing in the flask must have been killed and no fresh supply could enter. When this had been properly done, no animal cules ever appeared in the flask. The experi ment is open to criticism. When one boils the infusion, the steam with which the flask is filled drives out the air, and by sealing the neck none can enter. It may be that living things can be spontaneously developed from organic infusions, but only in the presence of air, and the experiment has excluded air. In more ways than one was this criticism answered. In 1854 two German observers, Schroeder and Dusch, boiled organic infusions in flasks, but instead of sealing the neck of the flask they simply plugged it, when the steam was issuing from it, with a firm cotton-wool. As the infusion cooled, air entered the flask, but was filtered on its way by passing through the wool. These infusions developed no living things. Than this even a more remarkable demonstration was given by the great French chemist, Pasteur, in 1862. He used a flask having a very long neck with a very fine bore. When the infusion was boiling he bent the neck downwards by the aid of heat. Air could freely enter, but it must pass upwards, not downwards. Any solid particles present in the air could not ascend with it, and so the air when it reached the infusion was free of them. In these flasks also no animalcules were de veloped. But a second ground of criticism remains. It might be said, it is possible that an organic infusion which, before being boiled, might have been capable of spontaneously giving rise to living things, has been rendered incapable of doing so by the boiling, not be cause the heat has destroyed any eggs, seed, or other living thing from which a progeny could have been developed, but because the heat has somehow altered its constitution, just as heat will take the temper out of a spring. This too is easily answered. Break off the sealed end of the flask and leave the flask open but undis turbed, or remove the cotton-wool plug; in a few days the flasks, which for months, or for years maybe, have remained free from putre faction, swarm with life. Or, as Pasteur did, drop a small fragment of the wool plug through which the air has been filtered into the infusion; within hours it is alive. Without doubt, then, the microscopic life that decaying animal sub stances contain in exuberant prodigality is not the outcome of some rearrangement of the par ticles of the dead matter, but is the product of previous life. But these and similar experi ments reveal another fact—organic substances or fluids treated as in these experiments not only remain free from animalcule but exhibit no sign of decay, no evidence of decomposition, no symptom of putrefaction. But let fall into the fluid from the point of a needle, or insert into the organic substance, a tiny speck of matter in which a microscope has exhibited animalculte; at that point putrefaction begins, from that spot it spreads; decomposition where ever the organisms have contrived to push their way, no taint whatever in the spot to which they have as yet been unable to advance, but speedily pervading and permeating the entire fluid or solid till it is one mass of corruption. Thus, just as Redi showed that by excluding the blow-flies dead meat ceased to breed mag gots, even so later experimenters have shown that by excluding the air, or rather by purify ing it from the eggs or seeds, or whatever they be which throng in it, and which it sows on all it collies into contact with—by excluding or purifying the air the dead meat will likewise cease to stink.

This gives a very brief historical summary of the various stages in the discovery of new realms of life, whose inhabitants, though them selves invisible to our eyes, yet obtrude them selves by their ravages before our senses in various offensive ways.

They and their ways must now be described in some more detail, since their significance in men's lives is even more wide and appalling than the early experiments seemed to indicate. •