The first part of this paper2 considered the rate of union in gamete populations, uniformly distributed and in numerical equilibrium. The cases of like gametes, called Isogamy, and that of sexually differentiated gametes, called Anisogamy, were both analyzed. In the latter case, the problem of optimal subdivision of the bulk gametic material between the two kinds has been of interest. Here something similar will be attempted for mating in animal populations. The models will have some meaning only for forms with no tendency to group into colonies and whose males have no parental duty besides fertilization. They apply to populations uniformly scattered over large areas, so, that, although their global number is very high, their density is rather low. Two kinds of animals will be considered: a) bisexuals, by which it is meant that individuals are at the same time, functionally both male and female; fertilization occurs only by mating, in which both partners are fertilized; b) unisexuals, by which it is meant that individuals are specialized to, produce only one kind of gametes. This can occur either through sex reversal in hermaphrodites or through separate sexes. In the former case, individuals may be male when young and revert to females. later, or vice versa3.