But with a better and more complete knowledge of organisms and especially of the extremely complicated mode of propagation characteristic of insects, doubts as to _generatio spontanea_ increasingly arose. It was, however, at a comparatively late time, or in the middle of the seventeenth century, that Harvey formulated his famous thesis, “_omne virum ex oro_,” or, as it has been later said, “_omne vivum ex vivo_,” which we may translate thus: “_Life implies life; all living beings descend from previous existing parents_,” or negatively, “_No living being is generated from lifeless matter_.” Thus, for the first time, the idea was pronounced by natural science that life is a specific force; an independent principle, that has not its roots in the material world. As _generatio aequivoca_ leads to materialism, so Harvey’s formula leads to pure idealism. That these consequences should have been seen from the beginning, was so much the less to be expected since even today no such discovery has been made or could have been made, simply because no attention has been given to it. Hitherto the only question has been: Is Harvey’s formula a fact verified by natural science or not? In this form the battle has raged for over two centuries, often with great vehemence, and victory has leaned now to one side, now to the other. Finally, it was agreed that parentless generation was not to be found among the higher forms of animals and plants which could be observed with the naked eye. Büchner himself says it has not hitherto been discovered that any higher or more developed organism may be created by inorganic matter and forces alone. “Today,” he says, “it seems to be a general law of the inorganic world that everything living originates from a parental embryo or else is directly segregated from the mother-body.” But although spontaneous generation of the higher animals and plants seemed doubtful even to Büchner, nothing was at this time settled in regard to the origin of the lower organisms. With the discovery of the microscopical organic world, a new field and one more difficult of access was opened for research. It was now the sudden and unexpected appearance of bacteria, aspergillus and infusoria in places where their previous existence could not be imagined, that maintained the belief in _generatio spontanea_. But by and by we learned to understand the propagation and life also of these low organisms, their ability to withstand very high or very low temperatures, and the facility with which they are spread by the air and, above all, their rapid propagation. It commenced to be more and more evident that even in the micro-organic world no parentless generation exists. The investigations by Spallanzani, and later by Schultze, Schwann, von Dusch and Schröder, were epochal for the establishing of this fact. Their method, however, left some room for criticism which was forcefully pointed out by a great number of scientists, especially by the Englishman Needham.