ON GENERATION. 457 



seed having preceded their birth ; and whilst some of them are 

 generated from the earth, or putrefying vegetable matter, like 

 so many insects, others are produced in animals themselves and 

 from the excrementitious matters of their parts." Now the whole 

 of these, whether they arise spontaneously, or from others, or in 

 others, or from the parts or excrements of these, have this in 

 common, that they are engendered from some principle adequate 

 to this effect, and from an efficient cause inherent in the same 

 principle. In this way, therefore, the primordium from which 

 and by which they arise is inherent in every animal. Let us 

 entitle this the primordium vegetale or vegetative incipience, 

 understanding by this a certain corporeal something having life 

 in potentia ; or a certain something existing per se, which is 

 capable of changing into a vegetative form under the agency of 

 an internal principle. Such primordia are the eggs of animals 

 and the seeds of plants ; such also are the conceptions of vivi- 

 parous animals, and the worm, as Aristotle calls it, whence in- 

 sects proceed : the primordia of different living things conse- 

 quently differ from one another ; and according to their diver- 

 sities are the modes of generation of animals, which nevertheless 

 all agree in this one respect, that they proceed from the vegetal 

 primordium as from matter endowed with the virtue of an 

 efficient cause, though they differ in respect of the primordium 

 which either bursts forth, as it were, spontaneously and by chance, 

 or shows itself as fruit or seed from something else preceding it. 

 Whence some animals are spoken of as spontaneously produced, 

 others as engendered by parents. And these last are again distin- 

 guished by their mode of birth, for some are oviparous, others 

 viviparous, to which Aristotle 1 adds a vermiparous class. But 

 if we take the thing as simple sense proclaims it, there are 

 only two kinds of birth, inasmuch as all animals engen- 

 der others either in actu virtually, or in potentia po- 

 tentially. Animals which bring forth in fact and virtu- 

 ally are called viviparous, those that bring forth potentially 

 are oviparous. For every primordium that lives potentially, 

 we, with Fabricius, think ought to be called an egg, and we 

 make no distinction between the worm of Aristotle and 

 an egg, both because to the eye there is no difference, 



1 Hist. Anim. lib. i, cap. 5. 



