Imágenes de páginas





THERE can be no doubt that the question of the origin of organic life must have exercised as much fascination in the earliest ages as it does to-day. The attention of poets and philosophers was naturally arrested by this question, and they believed that the living always arose from the non-living.

The doctrine of spontaneous generation was cepted article of faith with the writers of the Middle Ages, until an Italian chemist improved upon this by giving a recipe for creating mice! However, it was not till 1668 that Francesco Redi, by the simple device of covering a jar with wire-gauze, actually demonstrated that the maggots present in putrid meat did not arise de novo. In 1675 Leeuwenhoeck, "the father of microscopy," by the aid of a simple lens, discovered the presence of extremely small organisms in putrescent fluids. Later, the development of the compound microscope showed microbes everywhere, and a veritable germ-mania prevailed.

The question of the origin of these minute germs was thus once more brought to the front; but no real advance was made till the middle of the eighteenth century, when an English divine, Needham, instead of relying on mere assertions, conceived the happy idea of putting the matter

to the test of an experiment. He boiled infusions for several minutes in flasks, which were then hermetically sealed, and found that organisms rapidly developed in the contents. He concluded, therefore, that the germs originated spontaneously in the lifeless constituents of the liquid.

Needham's observations were combated by an Italian divine, Spallanzani, who claimed that in infusions enclosed in air-tight vessels, and boiled for a whole hour, no organisms appeared.

But Needham objected that Spallanzani, by the excessive heat which he employed, had so altered the air in the vessels, that it was no longer suitable for the development of life. To meet this objection, Schultz, in 1836, only half filled the flasks with the infusions, into which the external air (passed through bulbs containing sulphuric acid) was sucked daily. On examining the contents after several months, no organisms were detected ; but they soon appeared when the flask was opened, and exposed to the air.

But objectors still urged that the treatment of the air, although not violent, had nevertheless altered its composition.

Schröder modified Schultz's experiment by making the incoming air pass through cotton wool, instead of sulphuric acid. This treatment could not be called “violent," and yet the air completely lost the power of decomposition. These experiments certainly suggested that there was something in the air, capable of giving rise to organised beings.

Now, what is the nature of this something ?

Before answering this question, it will be as well to pause at this stage, and to interpret some of the results thus far achieved. Perhaps the most instructive feature of this controversy is the progressive limitation of cases

of spontaneous generation. Beginning with the higher animals, it became more and more limited till mice and flies were gradually excluded. It is obvious that the cause of decomposition is not a gas, for that would not be excluded by filtration through cotton wool, but is something discontinuous, something particulate. Further, as these particles are destroyed by heat and sulphuric acid, they are probably of organic origin.

The doctrine of spontaneous generation was thus apparently lost, but the results obtained by the experimenters were by no means uniform, and, in many instances, previously boiled liquid underwent decomposition. In 1858 Pouchet appeared as an advocate of this theory, contending that organic molecules can be derived from previously living molecules. His wellconducted experiments and bis zeal brought him many adherents, and spontaneous generation was once more in high favour.

Things were at this pass, when Pasteur undertook the study of this subject and, by a series of classical researches, demonstrated that it is possible to preserve any organic substance provided the heating be sufficiently prolonged and the external air be carefully excluded. He showed further that the atmospheric dust is the exclusive cause of life in organic infusions, and demonstrated this by an ingenious experiment. He boiled the liquid in a flask, the neck of which was drawn out in the form of an “S,” and then left it for some time. The fluid did not decompose, although it was in contact with the external air, because the dust was arrested in the bend of the tube. But if the vessel were violently shaken, the dust was dislodged and, entering the fluid, rapidly brought about its decomposition. “And therefore," exclaimed Pasteur in the course of his memorable address, “I

could point to that liquid and say to you, I have taken my drop from the immensity of creation, and I have taken it full of the elements appropriated to the development of inferior beings. And I wait, I watch, I question it, begging it to recommence for me the beautiful spectacle of the first creation. But it is dumb, dumb since these experiments were begun several years ago; it is dumb because I have kept it from the only thing man cannot produce, from the germs which float in the air, from life, for life is a germ and a germ is life. Never will the doctrine of spontaneous generation recover from the mortal blow of this simple experiment.”

Pasteur, however, was somewhat premature in his conclusion, as decomposition still occurred even with careful precautions. This was found to be due to the presence of "

spores or seeds, and, with the discovery of these resistant forms, the doctrine of spontaneous generation received its final death-blow. Omne vivum e vivo was thus shown to be true not only with regard to the higher beings, but of the lowest unicellular organisms as well.

This leads us naturally to the question: Whence came the ultimate and primary creature? How did life originate on our globe? The hypothesis of Lord Kelvin, that life was brought to the earth by shooting meteors from other planets, evades the question and only throws it one step further back. It would then be asked: How did life originate on these heavenly bodies? If not the result of a miracle, it must have been due to spontaneous generation. As a matter of fact, we have in the “nitrifying" (see p. 38) and other allied bacteria, some of the representatives of the earliest forms of life, which are ceaselessly at work in producing the living from the non-living materials. Evidently these organisms must

have played an all-important role in the evolution of life on our globe.

Strictly speaking, the experiments cited above merely show that spontaneous generation has not been proved experimentally. But it does not follow therefrom that spontaneous generation is impossible. Although the actual production of the living from the non-living substances cannot be imitated in the laboratory, yet there are certain considerations which show thai such changes can take place. Scholl has demonstrated that ferments, which may be said to occupy an intermediate position between the lifeless proteids and the living cells, may be rendered less active by heat and rejuvenated by suitable treatment. Prof. Bose of Calcutta has recently shown that metals respond to electric and other stimuli much in the same way as the nerves of animals do. These results indirectly support the theory of spontaneous generation, for they tend to obliterate the line of demarcation between the organic and the inorganic, the living and the non-living.

This recognition of the hypothesis of spontaneous generation can do no harm. It is in complete harmony with the law of origin from ancestors, which, as we have already seen, is capable of universal application. And, although the experimental proof be wanting, the possibility of spontaneous generation must be frankly admitted, if bacteriology is to take its rank among the exact sciences.

« AnteriorContinuar »