THAT portion of Prof. Karl Pearson's letter in NATURE of Nov. 10 which deals with chemical problems is largely based on misconceptions of the meaning of terms used by chemists. Thus, after quoting a statement of mine regarding optically active compounds, he says: “An optically active compound means merely a preponderance of one kind of enantiomorph.” That is precisely what it does not mean; that would be an optically active mixture. No chemist ever uses the word “compound” when he means “mixture”; I meant one kind of enantiomorph and one only. Moreover, I explained this point in detail in my first reply. Prof. Percy Frankland, as a chemist, has of course found no difficulty in following my meaning; he says that the question which I raised was: “the possibility of producing, without the interference of a living agency, an optically active substance unaccompanied by its enantiomorph.” A great part of Prof. Pearson's letter is therefore devoted to combating an opinion which I never expressed, and I am consequently relieved from the necessity of further discussing this part, or of calling attention to similar misconceptions which it contains. One point, however, I must notice. Prof. Pearson complains that I have supposed that he meant “twenty” molecules and no more, when in reality he referred to twenty tosses of a coin; and he adds that he was willing to assume the formation of a million molecules. I was led to take his words in the former sense by my impression—as it now appears, a mistaken impression—that he really understood that I was arguing about single asymmetric compounds; and I imagined that he purposely assumed the formation of only a small number of molecules in order that they might conceivably be all of one kind of asymmetry.