of the plastic virtue by which this species of animal is per- petuated.
But as in some species there appears to be no occasion for males, females sufficing of themselves to continue the kind ; so do we discover no males among these, but females only, con- taining the fertile rudiments of eggs in their interior ; in other species, again, none but males are discovered which procreate and preserve their kinds by emitting something into the mud, or earth, or water. In such instances nature appears to have been content with a single sex, which she has used as an in- strument adequate to procreation.
Another class of animals has a generative fluid fortuitously, as it were, and without any distinction of sex ; the origin of such animals is spontaneous. But " as some things are made by art, and some depend on accident, health for example," 1 so also some semen of animals is not produced by the act of an individual agent, as in the case of a man engendered by a man ; but in some sort univocally, as in those instances where the rudiments and matter, produced by accident, are susceptible of taking on the same motions as seminal matter, as in " animals which do not proceed from coitus, but arise spontaneously, and have such an origin as insects which engender worms." 2 For as mechanics perform some operations with their unaided hands, and others not without the assistance of particular tools ; and as the more excellent and varied and curious works of art re- quire a greater variety in the form and size of the tools to bring them to perfection, inasmuch as a greater number of motions and a larger amount of subordinate means are required to bring more worthy labours to a successful issue art imitating nature here as everywhere else, so also does nature make use of a larger number and variety of forces and instruments as necessary to the procreation of the more perfect animals. For the sun, or Heaven, or whatever name is used to designate that which is understood as the common generator or parent of all animated things, engenders some of themselves, by accident, without an instrument, as it were, and equivocally; and others through the concurrence of a single individual, as in those instances where an animal is produced from another animal of the same genus
1 Arist, Phys. lib. i, cap. 1 . Mb. lib. ii, cap. 3.
