In the search for criteria for specific diagnosis, especially in fossil mammals, too little heed seems to be given by the average systematist to a number of aspects of individual variation which are manifestly such in living forms. As a consequence, our paleontologic literature is burdened with a vexatious synonymy which requires extensive monographic study to unravel. Too often the description is based on excessively fragmentary material, whereas for enduring work not only should the type be as adequately represented in all of its essential parts as possible, but as large a series of individuals as may be had should be used with the idea of detecting such characters as are merely individual and such as are truly diagnostic of the species.
Among recent forms, three directions of variation are easily recognized—those due to age, to sex, and to environment—under the last of which comes the direct influence of . . .