There are two groups of extinction theories: catastrophic extinction and gradual extinction. Catastrophic extinction would have been caused by a sudden, external event, such as the collision of the earth with an asteroid, or the eruption of a series of gigantic volcanoes. Gradual extinction would have been the result of changes in the earth's land mass and climate shifts. It could also have been because new and better animals won in the struggle for existence.
Until the recent theories about extraterrestrial collisions, some ideas about the disappearance of dinosaurs centered around mammals beating them in the struggle to survive. One theory suggests that mammals killed dinosaurs because they ate dinosaur eggs. Other scientists have suggested that dinosaurs caused their own extinction. According to this theory, too many meat-eating dinosaurs evolved, eating all the plant-eaters, causing all dinosaurs to die. These ideas have the same pitfall described earlier. They explain dinosaur extinction but ignore the extinction of other groups.