"It is true that in these experiments certain vegetable and animal infusions, after no very long period of time, when examined by the microscope, are found to contain a multitude of minute creatures, of various forms, exhibiting signs of spontaneous motion, and multiplying their species in the usual manner. Some of these are even of a complicated structure, much beyond what might, _à priori_, be expected as the result of the first attempt of inorganic matter to enter into the realms of organic life. The subject has been so frequently discussed, that I need not trouble you with the details of the arguments which have led the most eminent naturalists to believe that these creatures are not really spontaneously engendered, but that they are derived from minute ova which are present in the air, and which, when placed under circumstances favourable to their development, burst into life: in the same way as the egg undergoes those changes which convert its contents into a bird, when placed under the influence of the animal heat of the parent. But even if this view of the matter be not correct, the case is not really altered; for, after all, the _Infusoria_ are never detected except in vegetable and animal infusions, which necessarily presuppose the existence of organic life." But it is one thing to demolish the theory and statements of an antagonist, and another to erect a structure in their place. However completely Spallanzani had demonstrated the faults and untrustworthiness of Needham's results, he had not established the opposite doctrine, and to many it seemed that the very conditions under which his experimentation was conducted, were sufficient to prevent the development of life. But the work begun by Schulze and Schwann and ended by Pasteur apparently has supplied what was wanting in Spallanzani's researches. The evidence is thus admirably summed by Professor Huxley:[141]-- "It is demonstrable, that a fluid eminently fit for the development of the lowest forms of life, but which contains neither germs, nor any protein compound, gives rise to living things in great abundance, if it is exposed to ordinary air; while no such development takes place, if the air with which it is in contact is mechanically freed from the solid particles, which ordinarily float in it and which maybe made visible by appropriate means. "It is demonstrable, that the great majority of these particles are destructible by heat, and that some of them are germs, or living particles, capable of giving rise to the same forms of life as those which appear when the fluid is exposed to unpurified air. "It is demonstrable, that inoculation of the experimental fluid with a drop of liquid known to contain living particles, gives rise to the same phenomena as exposure to unpurified air.