THE QUESTION OF SPONTANEOUS GENERATION
Aristotle, though his knowledge of embryology in at least one instance—that of the smooth dog-fish—was very great and very exact, appears at times to have been willing to assume spontaneous generation of such large animals as the eel, for instance, as a common occurrence. But there can be no doubt that even in antiquity common sense sometimes felt itself more or less in opposition to such an idea, and it was natural enough for the men of the seventeenth century, when stirred by the new spirit of scientific research, to seek to solve a problem which has always been of the highest interest, and never far from the minds of thoughtful naturalists.
In this great century the most important investigations of such problems were those of Harvey, Redi, and Swammerdam. Harvey’s embryological observations are far less valuable than his study of the circulation of the blood.〖See Harvey, “On the Motion of the Heart and Blood of Animals,” in H. C., xxxviii 59ff.〗 It may, in truth, be questioned if he surpassed Aristotle in any way as an embryologist. But, at all events, his work served to draw the attention of his successors to this subject, and, however vague his ideas about spontaneous generation in certain lower forms of life, he at least took a firm stand in favor of the theory of generation from the egg in most cases.
The work of Redi is of greater interest and importance. He made elaborate studies of the putrefaction of flesh, saw flies lay their eggs therein, and on gauze when the flesh was protected with it. He saw maggots develop in the unprotected meat, while the use of gauze prevented their development. He found that meat of one kind could support maggots which formed more than one kind of fly, and that the same species of fly could come from different kinds of meat. Hence he concluded that the generation of the fly is from an egg, and that there is no spontaneous generation involved in the putrefaction of meat.
Swammerdam, one of the greatest of naturalists, and many others confirmed the observations and conclusions of Redi, and, by observing again and again normal generation from the egg in many other species of minute organisms, did much to undermine the confidence with which the unaccountable appearance of living things was ascribed to spontaneous generation.
Meanwhile the microscopical studies of Leeuwenhoek had revealed the presence of hosts of minute organisms in putrid fluids and, in the eighteenth century, the problem of spontaneous generation was transferred to the origin of microscopic life. This problem in turn was answered unfavorably to spontaneous generation by Spallanzani. His new method of investigation was to seal up an infusion of meat in a glass flask; next the flask was immersed in boiling water until the contents had been thoroughly heated throughout, and then the behavior of the solution on standing was observed. After thorough heating no signs of putrefaction were revealed to the eye or to the nose; no living things were ever visible in the solution under the microscope. But on admitting the air to the flasks putrefaction soon set in and thus proved that the fault was not with the effect of heat upon what is to-day called the culture medium, but that putrefaction had not previously occurred simply because all germs originally present had been killed by heat; sterilized, in short.