Sentence:  Nevertheless, because of the frequent transposition between the two sexes of the traits supposedly specific of each -- a far more frequent transposition than the conventional division between the sexes affords opportunity to observe, the woman being often the more aggressive, the man the more retired of the two—to assign <WSD>forehandedly</WSD> one or the other complement to one or the other sex is arbitrary and without warrent. 
Meaning of the word marked with <WSD> tags:  Ahead of time, in advance.