Another fundamental genetic consideration is the law of selection. If individuals with worthy traits mate only with individuals who possess worthy traits, a superior stock will be produced. This tendency is very important, since it points the way to a potent method of securing social progress. Eugenic social thought has been developed in part on the basis of the Weismann theory of no or slight transmission of acquired traits. The germ-plasm is transmitted from individual to offspring in a direct line of descent. Injuries to the parent rarely change the nature of the germ-plasm. Only extreme malnutrition or excessive use of alcohol apparently exerts a definite influence on the germ cells. Nature has thus made provision for the protection of germ-plasm, whether strong or defective. Society, then, may encourage the mating of individuals who possess strong physical and mental traits, and discourages the mating of individuals who are defective--thus securing its own positive improvement. Eugenic social thought follows two courses. Restrictive eugenics advocates the segregation of the so-called dysgenic classes, such as the feeble-minded, the insane, and the grossly defective criminal. Public opinion reacts against sterilization; injustice that cannot be remedied may be done through the use of sterilization. Segregation by sexes, while involving expense, is a satisfactory eugenic method of safeguarding society against the reproduction of dysgenic persons. The other trend of eugenic thought supports the raising of the standards of choice in mating. Constructive eugenics, as distinguished from restrictive eugenics, urges a program of education whereby young people will habitually rate one another by physical and mental standards rather than by wealth and class standards. Eugenics disapproves of random mating. It favors assortative mating, because, for example, the “marriage of representatives of two long-lived strains ensures that the offspring will inherit more longevity than does the ordinary man.”[XIX-9] Eugenics thus stresses the importance of teaching young people eugenic ideas, and of training them to be guided by these ideals rather than by caprice and passion.[XIX-10] Eugenic ideals include health, paternity and maternity, and pleasing disposition. Education and character are secondary eugenic ideals of importance. A study of the birth rate shows that the inferior stocks and classes of individuals produce many more children than do the superior groups. Many cultured people do not marry, or if they marry they keep the birth rate very low. As a result, the racial character of a whole people may change within a few generations. The superior strains may be lost and the inferior furnish the entire population.