Perhaps, however, the simplest way of showing the multiplication of effectsamong phenomena of this order, will be to set down the influences of anymember of the Solar System on the rest. A planet directly produces in neighbouringplanets certain appreciable perturbations, complicating those otherwise producedin them; and in the remoter planets it directly produces certain less visibleperturbations. Here is a first series of effects. But each of the perturbedplanets is itself a source of perturbations -- each directly affects allthe others. Hence, planet A having drawn planet B out of the position itwould have occupied in A's absence, the perturbations which B causes aredifferent from what they would else have been; and similarly with C, D, E,etc. Here then is a secondary series of effects; far more numerous thoughfar smaller in their amounts. As these indirect perturbations must to someextent modify the movements of each planet, there results from them a tertiaryseries; and so on in ever multiplying and diminishing waves throughout theentire system. §158. If the Earth was formed by the concentration of diffused matter,it must at first have been incandescent; and whether the nebular hypothesisbe accepted or not, this original incandescence of the Earth may now be regardedas inductively established -- or, if not established, at least rendered soprobable that it is a generally admitted geological doctrine. Several resultsof the gradual cooling of the Earth -- as the formation of a crust, the solidificationof sublimed elements, the precipitation of water, etc. -- have been alreadynoticed, and I again refer to them merely to point out that they are simultaneouseffects of the one cause, diminishing heat. Let us now, however, observethe multiplied changes afterwards arising from the continuance of this onecause. The Earth, falling in temperature, must contract. Hence the solidcrust at any time existing is presently too large for the shrinking nucleus,and, being unable to support itself, inevitably follows the nucleus. Buta spheroidal envelope cannot sink down into contact with a smaller internalspheroid, without disruption: it will run into wrinkles as the rind of anapple does when the bulk of its interior decreases from evaporation. As thecooling progresses and the envelope thickens, the ridges consequent on thesecontractions must become greater, rising ultimately into hills and mountains;and the later systems of mountains thus produced must not only be higher,as we find them to be, but must be longer, as we also find them to be. Thus,leaving out of view other modifying forces, we see what immense heterogeneityof surface arises from the one cause, loss of heat -- a heterogeneity whichthe telescope shows us to be paralleled on the Moon, where aqueous and atmosphericagencies have been absent. But we have yet to notice another kind of heterogeneityof surface, simultaneously caused. While the Earth's crust was thin, theridges produced by its contractions must not only have been small in heightand length, but the tracts between them must have rested with comparativesmoothness on the subjacent liquid spheroid; and the water in those arcticand antarctic regions where it first condensed, must have been evenly distributed.
But as fast as the crust grew thicker and gained corresponding strength,the lines of fracture from time to time caused in it, occurred at greaterdistances apart; the intermediate surfaces followed the contracting nucleuswith less uniformity; and there consequently resulted larger areas of landand water. If any one, after wrapping an orange in tissue paper and observingboth how small are the wrinkles and, how evenly the intervening spaces lieon the surface of the orange, will then wrap it in thick cartridge-paper,and note both the greater height of the ridges and the larger spaces throughoutwhich the paper does not touch the orange, he will see that as the Earth'ssolid envelope thickened, the areas of elevation and depression became greater.
In place of islands more or less homogeneously scattered throughout an all-embracingsea, there must have gradually arisen heterogeneous arrangements of continentand ocean, such as we now know. These simultaneous changes in the ex tentand in the elevation of the lands, involved yet another species of heterogeneity-- that of coast-line. A tolerably even surface raised out of the ocean willhave a simple, regular sea-margin; but a surface varied by table-lands andintersected by mountain-chains, will, when raised out of the ocean, havean outline extremely irregular, alike in its leading features and in itsdetails. Thus endless is the accumulation of geological and geographicalresults brought about by this one cause -- escape of the Earth's primitiveheat.
When we pass from the agency which geologists term igneous, to aqueousand atmospheric agencies, we see a like ever-growing complication of effects.
