The two general theories are catastrophism and gradualism. Catastrophism is the rapid change in conditions found on Earth, such as changes in the atmosphere, that led to the death of most species of dinosaurs. Gradualism says that the dinosaurs died out slowly, over a period of many hundreds of thousands or millions of years. This may be due, for example, to changes of climate caused by continental drift. Some scientists think both theories are correct, with all the changes coming together at the end of the Cretaceous period and leading to the extinction of the dinosaurs.