CHAPTER XVII

THE EVOLUTION OF PHYSICAL CHARACTERS 368

(1) Preliminary notice of certain difficulties—cause of mutation, strength of selection. (2) During the intermediate period great changes took place, but (3) in general during the first and second periods the tendency was towards the evolution and preservation of certain types, (4) which is comprehensible when the conditions among primitive races are studied. (5) In the third period more variations have been allowed to persist; disease becomes of importance. (6) At all times warfare, migration, and crossing have influenced physical characters. (7) Factors of possible importance other than selection. (8) Conclusion.