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As regards the second problem offered to us by Redi, whether Xenogenesis obtains, side by side with Homogenesis, whether, that is, there exist not only the ordinary living things, giving rise to offspring which run through the same cycle as themselves, but also others, producing offspring which are of a totally different character from themselves, the researches of two centuries have led to a different result.
When the Italian naturalist Redi discovered that an exclusion of flies from meat was all that was necessary to prevent the production of grubs, the doctrine of spontaneous generation was thoroughly upset, for his time at least. But the microscope revealed in "pure" water the presence of thousands of small creatures, the infusoria.
So that M. Pasteur arrived at last at the clear and definite result, that all these appearances are like the case of the worms in the piece of meat, which was refuted by Redi, simply germs carried by the air and deposited in the liquids in which they afterwards appear.
And it was a student trained in the same schools, Francesco Redi a man of the widest knowledge and most versatile abilities, distinguished alike as scholar, poet, physician, and naturalist who, just two hundred and two years ago, published his "Esperienze intorno alla Generazione degl' Insetti," and gave to the world the idea, the growth of which it is my purpose to trace.
However, not three centuries ago, actual experiment gradually convinced the learned that maggots are bred in a dead body only from the eggs laid by parent flies, as shown by the Italian Redi in 1668 who found that no maggots were bred when he simply excluded the flies from access to the dead body by covering it with wire gauze, but that the blow-flies swarmed on the gauze and vainly laid their eggs on it!
A very distinguished Italian naturalist, named Redi, took up the question, at a time when everybody believed in it; among others our own great Harvey, the discoverer of the circulation of the blood.
Once more, an Italian, the Abbé Spallanzani, a worthy successor and representative of Redi in his acuteness, his ingenuity, and his learning, subjected the experiments and the conclusions of Needham to a searching criticism. It might be true that Needham's experiments yielded results such as he had described, but did they bear out his arguments?
From this and from other experiments he was led to formulate the theory that in all cases of apparent production of life from dead matter the real explanation was that living germs from outside had been introduced into it. For a long time this view held the field. Redi was, as his name indicates, an Italian, an inhabitant of Aretino, a poet as well as a physician and scientific worker.
In the middle of the eighteenth century, it was revived, in a new form, by Needham and Buffon; but the experiments of Spallanzani enforced the conclusions of Redi, and compelled the advocates of the occurrence of spontaneous generation to seek evidence for their hypothesis only among the parasites and the lowest and minutest organisms.
In spite of this progress in knowledge the belief in "spontaneous generation" of such excessively minute organisms as the bacteria and yeasts was general until Theodore Schwann in 1836 performed with them just the same experiment as Redi had performed with blow-flies in 1668. The old notions, nevertheless, survive to this day.
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