In an essay on "Progress: its Law and Cause," elsewhere published,[8] we have endeavoured to show that the transformation of the homogeneous into the heterogeneous, in which all progress, organic or other, essentially consists, is consequent on the production of many effects by one cause--many changes by one force. Having pointed out that this is a law of all things, we proceeded to show deductively that the multiform evolutions of the homogeneous into the heterogeneous--astronomic, geologic, ethnologic, social, &c.,--were explicable as consequences. And though in the case of organic evolution, lack of data disabled us from specifically tracing out the progressive complication as due to the multiplication of effects; yet, we found sundry indirect evidences that it was so. Now in so far as this conclusion, that organic evolution results from the decomposition of each expended force into several forces, was inferred from the general law previously pointed out, it was an example of deductive physiology. The particular was concluded from the universal. We here propose in the first place to show, that there is another general truth closely connected with the above; and in common with it underlying explanations of all progress, and therefore the progress of organisms--a truth which may indeed be considered as taking precedence of it in respect of time, if not in respect of generality. This truth is, that _the condition of homogeneity is a condition of unstable equilibrium_. The phrase _unstable equilibrium_ is one used in mechanics to express a balance of forces of such kind, that the interference of any further force, however minute, will destroy the arrangement previously existing, and bring about a different arrangement. Thus, a stick poised on its lower end is in unstable equilibrium: however exactly it may be placed in a perpendicular position, as soon as it is left to itself it begins, at first imperceptibly and then visibly, to lean on one side, and with increasing rapidity falls into another position. Conversely, a stick suspended from its upper end is in stable equilibrium: however much disturbed, it will return to the same position. Our meaning is, then, that the state of homogeneity, like the state of the stick poised on its lower end, is one that cannot be maintained; and that hence results the first step in its gravitation towards the heterogeneous. Let us take a few illustrations.