Having analyzed the sex act, Lucretius then considers conception and what in modern terms would be called genetics. Both man and woman, he says, produce genital fluids that mingle in a successful procreative act. The characteristics of the child are formed by the relative proportions of the mother's "seed" to the father's. A child who most resembles its mother is born when the female seed dominates the male's, and vice versa; when neither the male nor female seed dominates, the child will have traits of both mother and father evenly. Infertility occurs when the two partners fail to make a satisfactory match of their seed after several attempts; the explanation for infertility is physiological and rational, and has nothing to do with the gods. The transfer of genital "seed" "(semina)" is consonant with Epicurean physics and the theme of the work as a whole: the invisible "semina rerum", "seeds of things," continually dissolve and recombine in universal flux. The vocabulary of biological procreation underlies Lucretius's presentation of how matter is formed from atoms.