in all animals which can move about, the sexes are separated, one individual being male and one female, though both are the same in species, as with man and horse. but in plants these powers are mingled, female not being separated from male. wherefore they generate out of themselves, and do not emit semen but produce an embryo, what is called the seed. empedocles puts this well in the line: ‘and thus the tall trees oviposit; first olives . . . ’ for as the egg is an embryo, a certain part of it giving rise to the animal and the rest being nutriment, so also from a part of the seed springs the growing plant, and the rest is nutriment for the shoot and the first root.
in a certain sense the same thing happens also in those animals which have the sexes separate. for when there is need for them to generate the sexes are no longer separated any more than in plants, their nature desiring that they shall become one; and this is plain to view when they copulate and are united, that one animal is made out of both.
it is the nature of those creatures which do not emit semen to remain united a long time until the male element has formed the embryo, as with those insects which copulate. the others so remain only until the male has discharged from the parts of himself introduced something which will form the embryo in a longer time, as among the sanguinea. for the former remain paired some part of a day, while the semen forms the embryo in several days. and after emitting this they cease their union.
and animals seem literally to be like divided plants, as though one should separate and divide them, when they bear seed, into the male and female existing in them.
in all this nature acts like an intelligent workman. for to the essence of plants belongs no other function or business than the production of seed; since, then, this is brought about by the union of male and female, nature has mixed these and set them together in plants, so that the sexes are not divided in them. plants, however, have been investigated elsewhere. but the function of the animal is not only to generate (which is common to all living things), but they all of them participate also in a kind of knowledge, some more and some less, and some very little indeed. for they have sense-perception, and this is a kind of knowledge. (if we consider the value of this we find that it is of great importance compared with the class of lifeless objects, but of little compared with the use of the intellect. for against the latter the mere participation in touch and taste seems to be practically nothing, but beside absolute insensibility it seems most excellent; for it would seem a treasure to gain even this kind of knowledge rather than to lie in a state of death and non-existence.) now it is by sense-perception that an animal differs from those organisms which have only life. but since, if it is a living animal, it must also live; therefore, when it is necessary for it to accomplish the function of that which has life, it unites and copulates, becoming like a plant, as we said before.
testaceous animals, being intermediate between animals and plants, perform the function of neither class as belonging to both. as plants they have no sexes, and one does not generate in another; as animals they do not bear fruit from themselves like plants; but they are formed and generated from a liquid and earthy concretion. however, we must speak later of the generation of these animals.
