The problem of animal extinction was reviewed, finding that the literature shows that little evidence exists to conclude that extinction occurs because of Darwinian evolution, i.e., the least fit are more apt to become extinct than the better fit. Researchers have been able to find few consistent differences in biological fitness of animals which become extinct and those that have not. Today, a clear tendency exists for the so-called higher organisms to become extinct, as shown by an evaluation of endangered species lists and a study of animals which have become extinct in recent history. Most types of animals that have become extinct in the past are generally not less fit than surviving types, are very similar to many extant types, and any differences are often irrelevant to survival. The reasons for extinction are either chance or unknown, not a pruning of the inferior species as biological evolution predicts.