The causes of the relative infertility may be divided into two classes: (1) those which operate by killing persons before they have completed their middle life; (2) those which restrict fertility without killing. Both may be selective in their action. The former kind is alone operative in determining evolution in the animal world and probably also among the less civilised peoples; but, as civilisation advances, the causes of infertility of the other kind increase constantly in effectiveness, while the former operate with less and less intensity. It is through the causes which diminish fertility merely, rather than exterminate individuals, that changes of racial quality of nations are now being, and in the future will be, principally determined. Selection of this kind is usually distinguished from the various modes of natural selection which work by extermination, by the name ‘reproductive selection.’ Briefly, natural selection operates by means of selective death rate, reproductive selection by means of selective birth rate. No doubt, disease, especially in the form of plagues and epidemics, was one of the principal causes of the slowness of increase of population throughout the Middle Ages. And this was probably non-selective as regards mental qualities, although it was strongly selective as regards power of resistance to disease, and has left the European peoples more resistant to most diseases than any other peoples, save perhaps the Chinese[124]. But many other causes of selection were at work. Disease presumably has not affected mental qualities by selection; although by direct action and mental discouragement it may have tended to the decay of civilisations; it has been argued, for example, that malaria played a great part in the decay of classical antiquity, that it was introduced some centuries B.C. and enfeebled the population of Greece and Italy. More interesting, from our point of view of the influences affecting the mental constitution of populations, is the effect of alcohol. Dr Archdall Reid has argued very forcibly that resistance to the attraction of alcohol is a mental peculiarity which a race only acquires through long exposure to the influence of abundant alcohol; that populations are resistant just in proportion to their past exposure to it—as is true in the main of epidemic and endemic diseases—and that in both cases this is due to selection.[125]