We will take the Ruff as an example. According to Mr. Edmund Selous, pairing, in this species, is promiscuous--the Ruffs are polygamous, the Reeves polyandrous. Suppose, then, that upon this island of some few miles in circumference, whereon his investigations were made, the movements of neither Ruff nor Reeve were subject to control, that the birds wandered in all directions, and that the union of the sexes were fortuitous, would the result have been satisfactory? We must remember that the Reeve requires more than one Ruff to satisfy her sexual instinct; we must also bear in mind the possibility that the functioning of her instinct may be subject to some periodicity, and we ask whether, under these circumstances, accidental gatherings would meet all the requirements of the situation. Now, manifestly, she must be in a position to find males when her appropriate organic condition arises. But in the absence of any system in the distribution of the sexes, how could delay be avoided, or how could a uniform discharge of the sexual function be assured? There is, however, a system. In the first place, there are the assembly grounds to which the birds repair season after season; and then, on the assembly grounds, there are the territories, represented, as Mr. Selous tells us, by depressions where the grass by long use has been worn away, and each depression is owned by one particular Ruff. The assembly grounds have the effect of splitting up and scattering the birds, and the number of Ruffs at any one particular meeting place is limited by the territories; with the result that Ruffs fit to breed are evenly distributed and always to be found in certain definite places, and the Reeves know by experience where to find them. The advantage of this territorial system is therefore apparent. Instead of this district being overcrowded and that one deserted; instead of there being too many of one sex here and too few of the other sex there; instead of a high percentage of individuals failing to procreate their kind, just because circumstances over which they have no control prevent their discovering one another at the appropriate time--each sex has its allotted part to play, each district has its allotted number of inhabitants, and the waste of energy and the loss of time incurred in the process of mating is reduced to a minimum.