Each additional complexity of structure has caused additional complexityof movements; but still, a definite complexity, as is shown by having calculableresults. §141. While the Earth's surface was molten, the currents in the voluminousatmosphere surrounding it, mainly of ascending heated gases and of descendingprecipitated liquids, must have been local, numerous, indefinite, and butlittle distinguished from one another. But when after a vast period the surface,now solidified, had so far cooled that solar radiation began to cause appreciabledifferences of temperature between the equatorial and polar regions, an atmosphericcirculation from poles to equator and from equator to poles, must have slowlyestablished itself: other vast moving masses of air becoming, at last, trade-windsand other such permanent definite currents. These integrated motions, oncecomparatively homogeneous, were rendered heterogeneous as great islands andcontinents arose, to complicate them by periodic winds, caused by the variedheating of wide tracts of land at different seasons. Rhythmical motions ofa constant and simple kind, were, by increasing multiformity of the Earth'ssurface, differentiated into an involved combination of constant and recurrentrhythmical motions, joined with smaller motions that are irregular.
Parallel changes must have taken place in the motions of water. On a thincrust, admitting of but small elevations and depressions, and therefore ofbut small lakes and seas, none beyond small local circulations were possible.
But along with the formation of continents and oceans, came the vast movementsof water from warm latitudes to cold and from cold to warm -- movements increasingin amount, in definiteness, and in variety of distribution, as the featuresof the Earth's surface became larger and more contrasted. The like holdswith drainage waters. The tricklings of insignificant streams over smalltracts of land, were once alone possible; but as fast as wide areas cameinto existence, the motions of many tributaries became massed into the motionsof great rivers; and instead of motions very much alike, there arose motionsconsiderably varied.
Nor can we well doubt that the changes in the Earth's crust itself, havepresented an analogous progress. Small, numerous, local, and like one another,while the crust was thin, the movements of elevation and subsidence must,as the crust thickened, have extended over larger areas, must have continuedfor longer eras in the same directions, and must have been made more unlikein different regions by local differences of structure. §142. In organisms the advance towards a more integrated, heterogeneous,and definite distribution of the retained motion, which accompanies the advancetowards a more integrated, heterogeneous, and definite distribution of thecomponent matter, is mainly what we understand as the development of functions.
All active functions are either sensible movements, as those produced bycontractile organs; or such insensible movements as those propagated throughnerves; or such insensible movements as those by which, in secreting organs,molecular re-arrangements are effected, and new combinations of matter produced.
And during evolution functions, like structures, become more consolidatedindividually, as well as more combined with one another, at the same timethat they become more multiform and more distinct.
The nutritive juices in animals of low types move hither and thither throughthe tissues quite irregularly, as local strains and pressures determine: in the absence of a true blood and a distinct vascular system, there is nodefinite circulation. But along with the structural evolution which establishesa good apparatus for distributing blood, there goes on the functional evolutionwhich establishes large and rapid movements of blood, definite in their coursesand definitely distinguished as efferent and afferent, and that are heterogeneousboth in their directions and in their characters: being here divided intogushes and there continuous.
