The hypothesis of spontaneous generation is far older than the theory of transformism. It goes back to the Greek predecessors of Aristotle, at least, and may be of far greater antiquity. It was based, as is well known, upon an erroneous interpretation of natural facts, which was universally accepted up to the close of the 17th century. As we can do no more than recount a few outstanding incidents of its long and interesting history here, the reader is referred to the VII chapter of Wasmann’s “Modern Biology” and the VIII chapter of Windle’s “Vitalism and Scholasticism” for the details which we are obliged to omit. § 2. The Law of Genetic Continuity— From time immemorial the sudden appearance of maggots in putrescent meat had been a matter of common knowledge, and the ancients were misled into regarding the phenomenon as an instance of a _de novo_ origin of life from dead matter. The error in question persisted until the year 1698, when it was decisively disproved by a simple experiment of the Italian physician Francesco Redi. He protected the meat from flies by means of gauze. Under these conditions, no maggots appeared in the meat, while the flies, unable to reach the meat, deposited their eggs on the gauze. Thus it became apparent that the maggots were larval flies, which emerged from fertilized eggs previously deposited in decaying meat by female flies. Antonio Vallisnieri, another Italian, showed that the fruit-fly had a similar life-history. As a result of these discoveries, Redi rejected the theory of spontaneous generation and formulated the first article of the Law of Genetic Vital Continuity: _Omne vivum ex vivo_. Meanwhile, the first researches conducted by means of the newly invented compound microscope disclosed what appeared to be fresh evidence in favor of the discarded hypothesis. The unicellular organisms known as infusoria were found to appear suddenly in hay infusions, and their abrupt appearance was ascribed to spontaneous generation. Towards the end of the 18th century, however, a Catholic priest named Lazzaro Spallanzani refuted this new argument by sterilizing the infusions with heat and by sealing the containers as protection against contamination by floating spores or cysts. After the infusions had been boiled for a sufficient time and then sealed, no organisms could be found in them, no matter how long they were kept. We now know that protozoa and protophytes do not originate _de novo_ in infusions. Their sudden appearance in cultures is due to the deposition of spores or cysts from the air, etc.