Biologists have tended to assume that closely related species will have similar cognitive abilities. Johan J. Bolhuis and Clive D. L. Wynne put this evolutionarily inspired idea through its paces. Darwin's theory of evolution by natural selection is broadly accepted among biologists, but its implications for the study of cognition are far from clear. Few within the scientific pale would argue against the proposition that life on Earth has evolved and that this general principle can be extended to the process of thought.