Now, in order to lend some air of probability to the former hypothesis, concerning the necessity for the existence of living ferments, it was incumbent upon its supporters to endeavour to show that the air did contain such a multitude of “germs,” or living things, as were demanded by the requirements of their theory. Spallanzani and Bonnet had, as far as the imagination was concerned, done all that was necessary. They had proclaimed the universal diffusion of “germs” of all kinds of organisms throughout the atmosphere--which were ready to develop, whenever suitable conditions presented themselves. So far, however, this was but another hypothesis. To establish the doctrine that fermentation cannot take place without the agency of living ferments, we cannot receive hypotheses in evidence: facts are needed. These, no one attempted to supply in an adequate manner[13] anterior to the investigations of M. Pasteur. Speaking of his researches, even M. Milne-Edwards says,[14] “Previous to this time, the existence of reproductive particles, or infusorial germs in the atmosphere was nothing more than a _plausible hypothesis_, put forward in order to explain the origin of such creatures in a manner conformable with the general laws of reproduction; but it was only a mere supposition, and no one had been able actually to see or to handle these reproductive corpuscles.” [13] M. Pouchet and others had examined the dust which _settles_ on objects, and amongst much débris of different kinds had found comparatively few ova or spores. He had not, however, up to this time, filtered the air, so as to see what germs might be detected floating about in the atmosphere. [14] ‘Anat. et Physiol. compar.’ t. viii. p. 264. We have to look, therefore, to M. Pasteur’s investigations, and to others which may have been since conducted, for all the scientific evidence in support of what has been called the “Panspermic hypothesis.”