It has been suggested in the literature that fu 弗 is a fusion word of the negator bu 不 and the pronoun zhi 之 and that wu 勿 a fusion word of the negator wu 毋 and the pronoun zhi 之, but there still are many scholars who oppose this hypothesis. In this paper, we give supplementary evidence for this hypothesis based on the phenomena of the complementary distribution of the fused and non-fused forms. In addition, we show that some of the counter evidence is not so convincing as it appears. Specifically, we show the following: First, some of the sentences containing fu 弗 and wu 勿 which have been considered counter evidence to this hypothesis are actually not counter examples! Secondly, though many examples of bu 不 are used as fu 弗 (which have been cited as counter evidence to this hypothesis), most of these examples can be explained in two ways: 1). Fu 弗 was rewritten as bu 不 to avoid violating a taboo in the Han dynasty; 2). Bu 不 might be the better choice between these two negators which modify transitive verbs in some circumstances in Old Chinese. Finally, in this paper we show that the frequencies and functions of fu 弗 and wu 勿 changed with the passage of time, so fu 弗 and wu 勿 were confused with bu 不 and wu 毋, and the original restriction on fu 弗 and wu 勿 gradually disappeared.