The denuding actions of air and water have, from the beginning, been modifyingevery exposed surface: everywhere working many different changes. As alreadysaid (§69) the original source of those gaseous and fluid motions whicheffect denudation, is the solar heat. The transformation of this into variousmodes of energy, according to the nature and conditions of the matter onwhich it falls, is the first stage of complication. The Sun's rays, strikingat all angles a sphere that from moment to moment presented and withdrewdifferent parts of its surface, and each of them for a different time dailythroughout the year, would produce a considerable variety of changes evenwere the sphere uniform. But falling as they do on a sphere surrounded byan atmosphere containing wide areas of cloud, but which here unveils vasttracts of sea, there of level land, there of mountains, there of snow andice, they cause in it countless different movements. Currents of air of allsizes, directions, velocities, and temperatures, are set up; as are alsomarine currents similarly contrasted in their characters. In this regionthe surface is giving off vapour; in that, dew is being precipitated; andin another, rain is descending -- unlikenesses which arise from the changingratio between the absorption and radiation of heat in each place. At onehour a rapid fall in temperature leads to the formation of ice, with an accompanyingexpansion throughout the moist bodies frozen; while at another a thaw unlocksthe dislocated fragments of these bodies. And then, passing to a second stageof complication, we see that the many kinds of motion directly or indirectlycaused by the Sun's rays, severally produce results which vary with the conditions.

Oxidation, drought, wind, frost, rain, glaciers, rivers, waves, and otherdenuding agents effect disintegrations that are determined in their amountsand qualities by local circumstances. Acting on a tract of granite, suchagents here work scarcely an appreciable effect; there cause exfoliationsof the surface and a resulting heap of debris and boulders; and elsewhere,after decomposing the feldspar into a white clay, carry away this with theaccompanying quartz and mica, and deposit them in separate beds, fluviatileor marine. When the exposed land consists of several unlike formations, sedimentaryand igneous, changes proportionately more heterogeneous are wrought. Theformations being disintegrable in different degrees, there follows an increasedirregularity of surface. The areas drained by adjacent rivers being differentlyconstituted, these rivers carry down to the sea unlike combinations of ingredients;and so sundry new strata of distinct compositions arise. And here, indeed,we may see very clearly how the heterogeneity of the effects increases ina geometrical progression with the heterogeneity of the object acted upon.

Let us, for the fuller elucidation of this truth in relation to the inorganicworld, consider what would follow from an extensive cosmical catastrophe-- say a great subsidence throughout Central America. The immediate resultswould themselves be sufficiently complex. Besides the numberless dislocationsof strata, the ejections of igneous matter, the propagation of earthquakevibrations many thousands of miles around, the loud explosions, and the escapeof gases, there would be an inrush of the Atlantic and Pacific Oceans, asubsequent recoil of enormous waves, which would traverse both these oceansand produce myriads of changes along their shores, and corresponding atmosphericwaves complicated by the currents surrounding each volcanic vent, as wellas electrical discharges with which eruptions are accompanied. But thesetemporary effects would be insignificant compared with the permanent ones.

The complex currents of the Atlantic and Pacific would be altered in theirdirections and amounts. The distribution of heat achieved by these currentswould be different from what it is. The arrangement of the isothermal lines,not only on the neighbouring continents but even throughout Europe, wouldbe changed. The tides would flow differently from what they do now. Therewould be more or less modification of the winds in their periods, strengths,directions, qualities; and rain would fall scarcely anywhere at the sametimes and in the same quantities as at present. In these many changes, eachincluding countless minor ones, may be seen the immense heterogeneity ofthe results wrought out by one force, when that force expends itself on apreviously complicated area: the implication being that from the beginningthe complication has advanced at an increasing rate. §159. We have next to trace throughout organic evolution, this sameall-pervading principle. And here, where the transformation of the homogeneousinto the heterogeneous was first observed, the production of many changesby one cause is least easy to demonstrate in a direct way. Heredity complicateseverything. Nevertheless, by indirect evidence we may establish our proposition.