force. Observe, however, that, if here we meet with a truly fruitful agency, capable of accomplishing difficult feats of new combination and delicate equilibrium, we meet with it _here first_; and the moment we fall back from the line of sentient life, and quit the scene of this eager, aggressive, and competing power, we part company with all principle of progress; and consequently lose the tendency to that increasing complexity of structure and subtlety of combination which distinguish the organic from the inorganic compounds. Below the level of life, there is no room for the operation of "natural selection." Its place is there occupied by another principle, for which no such wonders of constructive adaptation can be claimed;--I mean, the dynamic rule of _Action on the line of least resistance_,--a rule, the working of which is quite in the opposite direction. For evidently it goes against the establishment of unstable conditions of equilibrium, and must therefore be the enemy rather than the patron of the complex ingredients, the precarious tissues, and the multiplied relations, of sentient bodies; and on its own theatre must prevent the permanent formation of any but the simpler unions among the material elements. Accordingly, all the great enduring masses that form and fill the architecture of inorganic nature,--its limestone and clay, its oxides and salts, its water and air,--are compounds, or a mixture, of few and direct constituents. And the moment that life retreats and surrenders the organism it has built and held, the same antagonist principle enters on possession, and sets to work to destroy the intricate structure of "proximate principles" with their "compound radicals." With life and mind therefore there begins, whether by modified affinities or by removal of waste, a _tension_ against these lower powers, carrying the being up to a greater or less height upon the wing; but with life it ends, leaving him then to the perpetual gravitation that completes the loftiest flight upon the ground. Within the limits of her Physics and Chemistry alone, Nature discloses no principle of progression, but only provisions for periodicity; and out of this realm, without further resources, she could never rise.