All animal species have a high potential rate of increase, which must imply a high potential rate of death. Professor Holliday (1) has said: “Something must die, either the sperm, or the egg, or the embyro, or the infant or the adult … Most animals maintain a population at a fairly constant level—there are fluctuations of numbers, but they are fluctuations round a mean. There is a strong element of stability in nature. How is the population fixed at this relatively stable level? Two schools of thought exist. One says that the rules of Malthus apply strictly to the animal world—starvation, disease and predators are the stabilizers and do an immediate and effective job. The other, developed by Professor Wynne-Edwards in Aberdeen, says that animals limit their numbers before starvation does it for them. It is part of the animal's social organization and it is based on instinctive patterns of behaviour. Only a proportion of the population is allowed to breed and those who lose this right in competition become outcasts.”