“Slowly he turned the pages, scanning the pictures and the text for a repetition of the combination B-O-Y. Presently he found it beneath a picture of another little ape and a strange animal which went upon four legs like the jackal and resembled him not a little. Beneath this picture the bugs appeared as: A BOY AND A DOG There they were, the three little bugs which always accompanied the little ape.
And so he progressed very, very slowly, for it was a hard and laborious task which he had set himself without knowing it — a task which might seem to you or me impossible — learning to read without having the slightest knowledge of letters or written language, or the faintest idea that such things existed.
He did not accomplish it in a day, or in a week, or in a month, or in a year; but slowly, very slowly, he learned after he had grasped the possibilities which lay in those little bugs, so that by the time he was fifteen he knew the various combinations of letters which stood for every pictured figure in the little primer and in one or two of the picture books.”

Source: Tarzan of the Apes (1912), Ch. 7 : The Light of Knowledge