Chapter XV: The subject-matter of Chapter XV will be confined to the

influence of the environment upon human bodily and mental characters. The influence of the environment as a stimulus affecting the direction in which mental characters are used will be left to a future chapter.

We next turn to inquire what influence changes in the germ-cells have had. Before we can make any progress it is necessary to ask what characters—physical and mental—have their basis in the germ-cells, and an inquiry into this subject will occupy Chapter XVI. In Chapters XVII and XVIII we ask what evidence there is of selection and other factors throughout human history which we have reason to suppose have a bearing on change in the average germinal constitution of the human species. This will among other things necessitate an examination of the facts in Chapters VII, VIII, and X in order to ascertain what bearing they have upon selection. Having thus some knowledge of what is inherited, and having discussed the occurrence of such factors as in human history may have brought about changes from one generation to another in the character inherited, we shall be in a position in Chapter XVII to attempt an estimate of the results which the factors—selection and others—have had upon physical characters. We shall find that as regards these characters the position is more or less clear; we shall find, that is to say, that the influence of selection and other factors, considered in conjunction with the influence of the environment discussed in