Sentence:  Such characters , with their low existence and often low morality , produce humorous effects in his novels and tales , as they did in the writing of Longstreet and Hooper and Harris , but it need not be added that he gives them far subtler and more intricate functions than they had in the earlier writers ; nor is there <WSD>need</WSD> to add that among them are some of the most highly individualized and most successful of his characters . 
Meaning of the word marked with <WSD> tags:  anything that is necessary but lacking