Keep Pace Meaning

Example : 1912: Edgar Rice Burroughs, Tarzan of the Apes, Chapter 12  As he had grown older, he found that he had grown away from his people. Their interests and his were far removed. They had not kept pace with him, nor could they understand aught of the many strange and wonderful dreams that passed through the active brain of their human king. So limited was their vocabulary that Tarzan could not even talk with them of the many new truths, and the great fields of thought that his reading had opened up before his longing eyes, or make known ambitions which stirred his soul.