that, here and now, a direction is to be observed in the evolution of inorganic matter, by which natural operations are tending to become less active, and the amount of available energy is diminishing. If it continues indefinitely, first life, and later on all activity and change whatsoever will cease. There is a tendency towards death and towards unchanging inactivity. The next great category is that of the organic, of living matter. We have to consider its origin and later history. So far as constitution goes, living matter is merely a special and highly complicated form of ordinary matter; and there can be no reasonable doubt that it has originated naturally from non-living matter. While the _main_ direction of the inorganic has been towards degradation of energy, it has shown another subsidiary direction towards the production of more and more complex forms of matter. If our general ideas are correct, there must have been a time when matter in our ordinary sense of the word did not exist--there can have been no atoms, only free electrons. From this state, there evolved one in which the various electron-systems that we call atoms first appeared; later still, atoms could join with atoms to produce molecules. Leaping over vast periods, we would come to the time when radiation had brought the temperature of the earth surface below 100 degrees centigrade; water then could form from steam and solution occur. Through solution, all soluble elements, which would otherwise remain locked in the inactivity of the solid state, are enabled to enter upon a new phase of mobility, of chemical life, as we may say. Only in water could colloid carbon compounds first be built up, and only from such substances could life originate. Living substance, or at least much of it, must be formed of molecules containing thousands of atoms, each atom in its turn a system of circling electrons. Here already is a vast increase of complexity: it remains to be seen whether the same tendency is perpetuated later. The evolutionary concept is to biology what the doctrine of the conservation of energy has been in the physico-chemical sciences--an indispensable preliminary to proper methods of attack. But while great stress has been laid on the various _methods_ by which evolution may be supposed to have taken place--natural selection, Lamarckism, orthogenesis and the rest--biology has concerned herself comparatively little with the _form_ of the process in itself. But it is here that evolution becomes of value to us in our present search; for once more we become aware of a direction. Partly from the direct evidence of palaeontology, partly from indirect evidence, but along many converging lines, we can form an idea of this direction which in broad outlines is unassailable.