308 ON GENERATION. 



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. 



