Classifying Life. Spontaneous Generation

The discoveries made by the microscope in the mid-seventeenth century seemed to blur the distinction between living and nonliving matter. It reopened a question that had seemed on the verge of a settlement. That question involved the origin of life or, at least, of the simpler forms of life.

While it was easy to see that human beings and the larger animals arose only from the bodies of their mothers, or from eggs laid by the mothers, this was not so clear in the case of smaller animals. It was taken for granted until modern times that creatures such as worms and in-sects grew out of decaying meat and other corruption. Such an origin of life from nonlife was referred to as "spontaneous generation."

The classic example presented as evidence for the existence of spontaneous generation was the appearance of maggots on decaying meat. It seemed obvious that these small wormlike organisms had formed out of the dead meat and almost all biologists accepted this fact. One of the few exceptions, however, was Harvey who, in his book on the circulation of the blood, speculated that perhaps such small living things grew out of seeds or eggs that were too small to be seen. (This was an easy point for a biologist to make who was being forced to postulate the existence of blood vessels too small to see.)

An Italian physician, Francesco Redi (1626-97), read Harvey, was impressed, and decided to put the matter to the test. In 1668, he prepared eight flasks with a variety of kinds of meat inside. Four he sealed and four he left open to the air. Flies could land only on the meat in the open vessels and only the meat in those vessels bred maggots. The meat in the sealed vessels decayed and turned putrid but developed no maggots. Redi repeated the experiment, covering some of the vessels with gauze, rather than sealing them completely. In this way, air could get at the meat freely, but flies would still be kept off. Again, no maggots developed.

It seemed then that maggots developed not out of meat but out of the eggs of flies. At this point, biological thinking might well have veered off the concept of spontaneous generation altogether. However, the effect of Redi's experiment was weakened by Van Leeuwenhoek's contemporaneous discovery of protozoa. After all, flies and maggots are still fairly complicated organisms, though simple compared to men. Protozoa were themselves no larger than flies' eggs, if as large, and were extremely simple living things. Surely, they could form by spontaneous generation. The argument seemed upheld by the fact that if nutritive extracts containing no protozoa were allowed to stand, the little creatures soon appeared in large numbers. The matter of spontaneous generation became part of the broader argument that was to reach new intensity in the eighteenth and nineteenth centuries: that of the vitalists versus the mechanists.

The philosophy of vitalism was stated clearly by a German physician, Georg Ernst Stahl (1660-1734). Stahl is most famous for his theories concerning "phlogiston” a substance he supposed existed in substances that, like wood, could burn, or, like iron, could rust. When wood burned or iron rusted, phlogiston (Stahl said) was released into the air. To account for the fact that rusting metals gained weight, some chemists suggested that phlogiston had negative weight. When it was lost the metal therefore grew heavier. This theory proved very attractive to chemists and it was accepted by most of them through-out the eighteenth century.

However, in among Stahl's voluminous writings were also important views on physiology, particularly in a book on medicine which he published in 1707. He stated flatly that living organisms are not governed by physical laws but by laws of a completely different type. Little could be learned about biology, in his view, through the study of the chemistry and physics of the inanimate world. Opposed to him was the Dutch physician, Hermann Boerhaave (1668-1738), the most famous medical man of his times (sometimes called "the Dutch Hippocrates"). In his own book on medicine, he discusses the body in detail and tries to show how all its activity follows the laws of physics and chemistry—the mechanistic view.

For mechanists, who held that the same laws governed both the animate and inanimate worlds, microorganisms had a special importance. They seemed to serve almost as a bridge between life and nonlife. If it could be shown that such microorganisms actually formed from dead matter, the bridge would be complete—and easily crossed.

By the same token, the vitalist view, if valid, would require that, however simple life might be, there must still remain an unbridgeable gulf between it and inanimate matter. Spontaneous generation would not, by the strict vitalist view, be possible.

During the eighteenth century, however, mechanists and vitalists did not line up solidly for and against (respectively) spontaneous generation, for religious views played a role, too. It was felt that the Bible described spontaneous generation in certain places so that many vitalists (who were generally the more conservative in religion) felt it necessary to back belief in the development of life from nonlife.

In 1748, for instance, an English naturalist, John Turberville Needham (1713-81), who was also a Catholic priest, brought mutton broth to a boil and placed it in a corked test tube. After a few days the broth was found to be swarming with microorganisms. Since Needham assumed that the initial heating had sterilized the broth, he concluded that the microorganisms had arisen out of dead material and that spontaneous generation, at least for microorganisms, had been proved.

One skeptic in this respect was the Italian biologist, Lazzaro Spallanzani (1729-99). He felt that the period of heating had been insufficiently prolonged and had not sterilized the broth in the first place. In 1768, therefore, he prepared a nutritive solution which he brought to a boil and then continued to boil for between one half and three quarters of an hour. Only then did he seal it in a flask and now microorganisms did not appear.

This seemed conclusive, but believers in spontaneous generation found a way out. They maintained that there was a "vital principle" in the air, something unperceived and unknown, which made it possible to introduce the capacity for life into inanimate matter. Spallanzani's boiling, they claimed, destroyed that vital principle. For nearly another century, then, the issue was to remain in doubt.