“It is a law of nature we overlook, that intellectual versatility is the compensation for change, danger, and trouble.”
The Time Traveller proposes that changing and difficult conditions reward mental flexibility, while perfectly stable conditions reduce the need to adapt.
Source details
The Time Machine, Project Gutenberg #35. The Time Traveller states this while speculating about the evolution of the Eloi and Morlocks; it is fictional evolutionary reasoning in a nineteenth-century scientific romance, not a current biological law.