I. From Aristotle to Redi.--During the first period, the notion of spontaneous generation was universally accepted, and the whole question of spontaneous origin of life was in a crude and grotesque condition. It was thought that frogs and toads and other animals arose from the mud of ponds and streams through the vivifying action of the sun's rays. Rats were supposed to come from the river Nile, the dew was supposed to give origin to insects, etc. The scientific writers of this period had little openness of mind, and they indulged in scornful and sarcastic comments at the expense of those who doubted the occurrence of spontaneous generation. In the seventeenth century Alexander Ross, commenting on Sir Thomas Brown's doubt as to whether mice may be bred by putrefaction, flays his antagonist in the following words: "So may we doubt whether in cheese and timber worms are generated, or if beetles and wasps in cow-dung, or if butterflies, locusts, shell-fish, snails, eels, and such life be procreated of putrefied matter, which is to receive the form of that creature to which it is by formative power disposed. To question this is to question reason, sense, and experience. If he doubts this, let him go to Egypt, and there he will find the fields swarming with mice begot of the mud of Nylus, to the great calamity of the inhabitants." II. From Redi to Schwann.--The second period embraces the experimental tests of Redi (1668), Spallanzani (1775), and Schwann (1837)--notable achievements that resulted in a verdict for the adherents to the doctrine of biogenesis. Here the question might have rested had it not been opened upon theoretical ground by Pouchet in 1859. The First Experiments.--The belief in spontaneous generation, which was so firmly implanted in the minds of naturalists, was subjected to an experimental test in 1668 by the Italian Redi. It is a curious circumstance, but one that throws great light upon the condition of intellectual development of the period, that no one previous to Redi had attempted to test the truth or falsity of the theory of spontaneous generation. To approach this question from the experimental side was to do a great service to science.