The lectures which appear in this volume were delivered at the Academy of Natural Sciences, Philadelphia, in the early months of 1890. They have since been written out, and references added in the foot-notes to a number of works and articles, which will enable the student to pursue his readings on any point in which he may be interested. My endeavor has been to present the results of the latest and most accurate researches on the subjects treated; though no one can be better aware than myself that in compressing such an extensive science into so limited a space, I have often necessarily been superficial. It is some excuse for the publication, if one is needed, that I am not aware of any other recent work upon this science written in the English language.

Philadelphia, August, 1890.