A science may affect human life in two different ways. On the one hand, without altering men’s passions or their general outlook, it may increase their power of gratifying their desires. On the other hand, it may operate through an effect upon the imaginative conception of the world, the theology or philosophy which is accepted in practice by energetic men. The latter is a fascinating study, but I shall almost wholly ignore it, in order to bring my subject within a manageable compass. I shall confine myself almost wholly to the effect of science in enabling us to gratify our passions more freely, which has hitherto been far the more important of the two. From our point of view, we may divide the sciences into three groups: physical, biological, and anthropological. In the physical group I include chemistry, and broadly speaking any science concerned with the properties of matter apart from life. In the anthropological group I include all studies specially concerned with man: human physiology and psychology (between which no sharp line can be drawn), anthropology, history, sociology, and economics. All these studies can be illuminated by considerations drawn from biology; for instance, Rivers threw a new light on parts of economics by adducing facts about landed property among birds during the breeding season. But in spite of their connection with biology--a connection which is likely to grow closer as time goes on--they are broadly distinguished from biology by their methods and data, and deserve to be grouped apart, at any rate in a sociological inquiry. The effect of the biological sciences, so far, has been very small. No doubt Darwinism and the idea of evolution affected men’s imaginative outlook; arguments were derived in favour of free competition, and also of nationalism. But these effects were of the sort that I propose not to consider. It is probable that great effects will come from these sciences sooner or later. Mendelism might have revolutionized agriculture, and no doubt some similar theory will do so sooner or later. Bacteriology may enable us to exterminate our enemies by disease. The study of heredity may in time make eugenics an exact science, and perhaps we shall in a later age be able to determine at will the sex of our children. This would probably lead to an excess of males, involving a complete change in family institutions. But these speculations belong to the future. I do not propose to deal with the possible future effects of biology, both because my knowledge of biology is very limited, and because the subject has been admirably treated by Mr. Haldane.[1] [1] See his _Daedalus, or Science and the Future_.