The quotation referred to in this question comes from Chapter Eight of this book, which is when March writes to his wife of his experience of working at the cotton plantation belonging to Mr. Canning, where his job is to act as educator to the freed slaves. What March swiftly realises is that how society defines intelligence is something that is so closely bound up with "how lettered" somebody is. However, through his experience of working with these freed slaves, who have been intentionally kept ignorant in order to stop them becoming dangerous or subversive. there are many ways in which these illiterate people show profound intellligence:
Their visual acuity is remarkable, and their memories prodigious. For example, should a steamboat be plying the river, the Negroes can identify the vessel long before it approaches close enough to read the name on her side.
So great is the "intelligence" of this people, therefore, that they have a memory so strong they are able to remember such details as the sound of a steamboat for many years. March therefore writes of how he has discovered that traditional notions of how intelligence can be defined are wanting, as the freed slaves he works amongst are so clearly intelligent, even if it is not in the way that white society would define that term.