And now let us consider the bearing of this general truth on the evolution of organisms. The germ of a plant or animal is one of these homogeneous aggregates--relatively homogeneous if not absolutely so--whose equilibrium is unstable. But it has not simply the ordinary instability of homogeneous aggregates: it has something more. For it consists of units which are themselves specially characterized by instability. The constituent molecules of organic matter are distinguished by the feebleness of the affinities which hold their component elements together. They are extremely sensitive to heat, light, electricity, and the chemical actions of foreign elements; that is, they are peculiarly liable to be modified by disturbing forces. Hence then it follows, _a priori_, that a homogeneous aggregate of these unstable molecules will have an excessive tendency to lose its equilibrium. It will have a quite special liability to lapse into a non-homogeneous state. It will rapidly gravitate towards heretogeneity. Moreover, the process must repeat itself in each of the subordinate groups of organic units which are differentiated by the modifying forces. Each of these subordinate groups, like the original group, must gradually, in obedience to the influences acting on it, lose its balance of parts--must pass from a uniform into a multiform state. And so on continuously. Thus, starting from the general laws of things, and the known chemical attributes of organic matter, we may conclude deductively that the homogeneous germs of organisms have a peculiar proclivity towards a non-homogeneous state; which may be either the state we call decomposition, or the state we call organization. * * * * * At present we have reached a conclusion only of the most general nature. We merely learn that _some_ kind of heterogeneity is inevitable; but as yet there is nothing to tell us _what_ kind. Besides that _orderly_ heterogeneity which distinguishes organisms, there is the _disorderly_ or _chaotic_ heterogeneity, into which a loose mass of inorganic matter lapses; and at present no reason has been given why the homogeneous germ of a plant or animal should not lapse into the disorderly instead of the orderly heterogeneity. But by pursuing still further the line of argument hitherto followed we shall find a reason.