“Another part was The Rat's training of his memory, and bringing home his proofs of advance at night when he returned from his walk.”
The Rat tracks improvement through repeated memory exercises and concrete evidence of progress, making development something practiced and demonstrated over time.
Source details
The Lost Prince, Chapter XVIII, “Cities and Faces,” Project Gutenberg #384. The narrator describes “Another part” of the boys’ secret-game preparation as The Rat training his memory and bringing home nightly proofs by describing, reciting or roughly sketching what he had observed. It remains fictional training narrative, not an experimentally established memory-training prescription.