163. Our readers will remark how, by a process of this kind, the primordial potential energy of the visible universe is gradually converted into light and heat, and how this light and heat are ultimately dissipated into space. They will also remark that, as the process goes on, the masses of the universe become larger and larger. In fine, the dissipation of the energy of the visible universe proceeds, _pari passu_, with the aggregation of mass. The very fact, therefore, that the large masses of the visible universe are of finite size, is sufficient to assure us that the process cannot have been going on for ever; or, in other words, that the visible universe must have had its origin in time, and we may conclude that if the visible universe be finite in mass the process will ultimately come to an end. All this is what would take place, provided we allow the indestructibility of ordinary matter; but we may perhaps suppose (Art. 153) that the very material of the visible universe will ultimately vanish into the invisible. 164. There is one peculiarity of the process of development just described, which we beg our readers to note. We have supposed the visible universe, after its production, to have been left to its own laws; that is to say, to certain so-called inorganic agencies, which for want of better knowledge we for the present call forces, in virtue of which its development took place.[51] At the very first there may have been only one kind of primordial atom, or, to use another expression, absolute simplicity of material. As, however, the various atoms approached each other, in virtue of the forces with which they were endowed, other and more complicated structures took the place of the perfectly simple primordial stuff. Various kinds of molecules were produced at various temperatures, and these ultimately came together to produce globes or worlds, some of them comparatively small, others very large. Thus the progress is from the regular to the irregular. And we find a similar progress when we consider the inorganic development of our own world. The action of water rounds pebbles, but it rounds them irregularly; it produces soil, but the soil is irregular in the size of its grains, and variable in constitution. Wherever what may be termed the brute forces of nature are left to themselves, this is always the result: not so, however, when organisms are concerned in the development. Two living things of the same family are more like each other than two grains of sand or than two particles of soil. The eggs of birds of the same family, the corresponding feathers of similar birds, the ants from the same ant-hill, all form groups whose members have a very strong likeness to each other.