Final answer:
William James is the key figure who focused on how mental activities helped organisms adapt to their environments, establishing the field of functional psychology.
Step-by-step explanation:
The thinker who focused on how mental activities helped an organism adapt to its environment was William James, a pioneer of functional psychology. James, along with peers like John Dewey and Charles Sanders Peirce, supported Darwin's evolutionary theory and its notion that natural selection leads to organism traits, including behaviors, that function for survival and reproduction in a given environment.
Functionalism, the perspective James is known for, emphasizes mental processes in the context of how they help an organism adapt and thrive within its surroundings.
This approach diverges from structuralism by considering the whole mind's functioning, rather than its parts, and combines introspection with objective measures to study behavior and mental activities.