Eight Keys to Eden by Mark Clifton — Read

For another week, perhaps ten days or more, since time measurement had lost its meaning, Cal lived among the colonists, watched their complete retrogression into a state of unawareness. Even the speech which they had retained seemed now to thin and falter as the simplifying of their idea-content no longer required its use.

Only Tom and Jed seemed to retain their orientation to the past, the clarity of awareness. These two spent much time together, seemed always available when Cal needed them, yet did not intrude upon his thought. Frank now seemed one with the colonists. Louie lived on the outskirts of the herd, near the colonists but not of them. He had ceased to exhort, warn, command, argue. His face was closed, told nothing of what he was thinking.