A society for investigating nature and ascertaining truth cannot celebrate its commemoration day more fittingly than by a discussion of its highest general problems. It must be regarded, therefore, with satisfaction that the speaker on such an august occasion as this--the seventy-fifth anniversary of your Society--has selected as the subject of his address a theme of the highest general importance. Unfortunately, it is becoming more and more the custom on such occasions, and even at the general meetings of the great "Association of German Naturalists and Physicians," to take the subject of address from a narrow and specialised territory of restricted interest.