On the whole living things evolve from simpler to more complex forms, though not undirectionally. The evolution of more complex species makes the environment more complex and also makes the evolution of even more complex species possible and more selectable. With the evolution of sentients, species can be classified according to their degree of rationality. A more rational species is one whose behaviour (i.e. of its individual members) is controlled (relatively) more by the reward-penalty system than by the automatic, inflexible, programmed responses. In two reasonable simple models, it is shown that a more complex environment favours the evolution of more rational species. This result partly explains the dramatic speed of evolution based mainly on random mutation and natural selection, a speed doubted by creationists.