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 th