explained only by accepting certain landmarks of order and law. These, however, are understandable in a sense which the scepticism of Hume and his modern imitators does not admit. For instance, the law of natural selection is understood, if we understand (a) that the supply of chemical nourishment for plants and organic nourishment for animals is strictly limited, (b) that local groups of different species are constantly competing for larger shares of the limited supply, while animals feed on plants, some animals on other animals, and parasitic micro-organisms on higher organisms of all sorts, and (c) that both individuals and groups which are ill-adapted to obtain nourishment or to escape or resist hostile attacks tend to disappear, while those which become better adapted in either respect tend to survive and multiply. Similarly, the law of action in the line of least resistance is in itself understandable; for instance, in the case of water rising in a common pump. When the atmosphere in the barrel and suction-pipe is withdrawn, while the atmospheric pressure on the surface of the water external to the pipe continues as before, it stands to reason that a column of water will be freed to ascend the pipe for some distance. Other laws, like the law of gravitation, may be at present unexplained uniformities of action, but science always strives to make them relatively understandable. Many suppose that gravitation itself may be ultimately explained by ethereal strain or pressure, or in some other way, which would enable us to see mentally why bodies gravitate as we see why water rises in a pump.
Let us now turn from ideas of causation to ideals, or ideas of objects which may and should be humanly pursued. The fundamental difference of a consciously sought end from a natural effect is, of course, that there is no guarantee that the end will be attained. If it is to be realized, it must be, in the first place, willed with sufficient sincerity, and, in the second place, pursued with sufficient knowledge of the means which may bring it about and the obstacles which would otherwise prevent its attainment. There may be a theoretical necessity of attainment or non-attainment according to the strength of our motives, the amount of our relevant knowledge, and the
