"Indeed, in the long run, extinctions of species are as inevitable as the deaths of individual animals, and it may be that the causes of extinctions are as varied as the causes of individual deaths. A wave of extinctions—a sudden diminution in the number of species—is analogous to a sudden big drop in the size of a human population, an event that deserves to be explained even though the individual people would inevitably have died sooner or later anyway. Catastrophes in human populations have many causes: war, famine, and pestilence are the possibilities that first spring to mind. There may be equally many causes for evolutionary catastrophes, as waves of extinctions could well be called. Another possibility, however, is that extinctions come in waves that are part of a recurring cycle. It would then be the cycle itself, rather than each individual wave in the cycle, that would need to be explained. If there is such a cycle, it presumably follows a cycle in the inorganic world, such as cyclic climactic changes."