The author includes logical evidence that Indian workers and formerly enslaved people didn't get along because wages went down.
In the passage, it states:
"The Indian coolies and the ex-slaves, who resented the newcomers flooding the colonies and driving down wages, were instant rivals."
In this case, the Indian coolies are the newcomers and the ex-slaves are the ones resenting them. Therefore, the ex-slaves resent the Indian coolies because they were driving down wages.