anch’ egli, e cresce altresi a suo tempo quel verme, che nell’ uovo si racchiude; il qual verme, quando la gallozzola e finita di maturare e che e venuto il termine destinato al suo nascimento, diventa, di verme che era, una mosca....  Io vi confesso ingenuamente, che prima d’aver fatte queste mie esperienze intorno alla generazione degl’ insetti mi dava a credere, o per dir meglio sospettava, che forse la gallozzola nascesse, perche arrivando la mosca nel tempo della primavera, e facendo una piccolissima fessura ne’ rami piu teneri della quercia, in quella fessura nascondesse uno de suoi semi, il quale fosse cagione che sbocciasse fuora la gallozzola; e che mai non si vedessero galle o gallozzole o ricci o cornetti o calici o coccole, se non in que’ rami, ne’ quali le mosche avessero depositate le loro semenze; e mi dava ad intendere, che le gallozzole fossero una malattia cagionata nelle querce dalle punture delle mosche, in quella giusa stessa che dalle punture d’altri animaletti simiglievoli veggiamo crescere de’ tumori ne’ corpi degli animali.”]

It is of great importance to apprehend Redi’s position rightly; for the lines of thought he laid down for us are those upon which naturalists have been working ever since.  Clearly, he held Biogenesis as against Abiogenesis; and I shall immediately proceed, in the first place, to inquire how far subsequent investigation has borne him out in so doing.

But Redi also thought that there were two modes of Biogenesis.  By the one method, which is that of common and ordinary occurrence, the living parent gives rise to offspring which passes through the same cycle of changes as itself—­like gives rise to like; and this has been termed Homogenesis.  By the other mode, the living parent was supposed to give rise to offspring which passed through a totally different series of states from those exhibited by the parent, and did not return into the cycle of the parent; this is what ought to be called Heterogenesis, the offspring being altogether, and permanently, unlike the parent.  The term Heterogenesis, however, has unfortunately been used in a different sense, and M. Milne-Edwards has therefore substituted for it Xenogenesis, which means the generation of something foreign.  After discussing Redi’s hypothesis of universal Biogenesis, then, I shall go on to ask how far the growth of science justifies his other hypothesis of Xenogenesis.

The progress of the hypothesis of Biogenesis was triumphant and unchecked for nearly a century.  The application of the microscope to anatomy in the hands of Grew, Leeuwenhoek, Swammerdam, Lyonnet, Vallisnieri, Reaurnur, and other illustrious investigators of nature of that day, displayed such a complexity of organisation in the lowest and minutest forms, and everywhere revealed such a prodigality of provision for their multiplication by germs of one sort or another, that the hypothesis of Abiogenesis began to appear not only untrue, but absurd; and, in the middle of the eighteenth century, when Needham and Buffon took up the question, it was almost universally discredited.[7]