The principle of sexual selection is applicable primarily to the characteristics of the male; but Darwin supposes them to have been transferred to the other sex, and through them transmitted to the race generally. In his "Descent of Man," he remarks of the actual influence over the race of that principle: "The nervous system not only regulates most of the existing functions of the body, but has indirectly influenced the progressive development of various bodily structures and of certain mental qualities. Courage, pugnacity, perseverance, size and strength of body, weapons of all kinds, musical organs, both vocal and instrumental, bright colours and ornamental appendages have all been indirectly gained by the one sex or the other, through the exertion of choice, the influence of love and jealousy, and the appropriation of the beautiful in sound, colour or form; and these powers of the mind manifestly depend on the development of the brain." That sexual selection has actually resulted in modification of human physical structure, Darwin thinks can be shown by reference to the ancient Persians, whose type was greatly improved by intermarriage with the beautiful Georgian and Circassian women. He refers to several similar cases, and particularly to the Jollofs of West Africa, whose handsome appearance is said to be due to their retaining for wives only their most beautiful slaves, the others being sold. Sexual selection may be operative for the improvement of the race through the action of either man or woman, and the conditions of its activity are different in either case. As to the action of man, Darwin says in relation to primitive peoples: "The strongest and most vigorous men--those who could best defend and hunt for their families, who were provided with the best weapons and possessed the most property, such as a large number of dogs or other animals--would succeed in rearing a greater average number of offspring than the weaker and poorer members of the same tribe. There can, also, be no doubt that such men would generally be able to select the more attractive women. At present, the chiefs of nearly every tribe throughout the world succeed in obtaining more than one wife."