For many years there has been a tendency for workers to search for broad concepts or master factors to explain cyclic changes in numbers of animals. Many writers have proposed that cosmic forces act to effect rather uniform population cycles in many kinds of animals, and that a single grand theory can explain the variations in numbers in different kinds of animals. We have gradually come to believe that such an approach is unnecessary, and may even be undesirable if it deters students from considering the plausibility of other explanations. It may be appropriate to determine, if possible, the reasons why a given population becomes alternately abundant and scarce. If we do this for various kinds of animals, we may then more completely evaluate proposed theories that attempt to cover fluctuations as a whole.