And while, in this superficial solidification, we see under one form howconcentration accompanies loss of contained motion, we see it under anotherform in that diminution of the Earth's bulk implied by superficial corrugation.
Local or secondary integrations have advanced along with this generalintegration. A molten spheroid merely skinned over with solid matter, couldhave presented nothing beyond small patches of land and water. Differencesof elevation great enough to form islands of considerable size, imply a crustof some rigidity; and only as the crust grew thick could the land be unitedinto continents divided by oceans. So, too, with the more striking elevations.
The collapse of a thin layer round its cooling and contracting contents,would throw it into low ridges. The crust must have acquired a relativelygreat depth and strength before extensive mountain systems of Vast elevationbecame possible: continued integration of it made possible great local integrations.
In sedimentary changes a like progress is inferable. Denudation acting onthe small surfaces exposed during early stages, would produce but small localdeposits. The collection of detritus into strata of great extent, and theunion of such strata into extensive "systems," imply wide surfacesof land and water, as well as subsidences great in both area and depth; sothat integrations of this order must have grown more pronounced as the Earth'scrust thickened. §110. Already we have recognized the fact that the evolution of anorganism is primarily the formation of an aggregate, by the continued incorporationof matter previously spread through a wider space. Every plant grows by takinginto itself elements that were before diffused, and every animal grows byre-concentrating these elements previously dispersed in surrounding plantsor other animals. Here it will be proper to complete the conception by pointingout that the early history of a plant or animal, still more clearly thanits later history, shows us this fundamental process. For the microscopicgerm of each organism undergoes, for a long time, no other change than thatimplied by absorption of nutriment. Cells embedded in the stroma of an ovarium,become ova by little else than continued growth at the expense of adjacentmaterials. And when, after fertilization, a more active evolution commences,its most conspicuous trait is the drawing-in, to a germinal centre, of thesubstance which the ovum contains.
Now, however, our attention must be directed mainly to the secondary integrationswhich accompany the primary integration. We have to observe how, along withthe formation of a larger mass of matter, there goes on a gathering togetherand consolidation of this matter into parts, as well as a closer combinationof the parts. In the mammalian embryo the heart, at first a long pulsatingblood-vessel, by-and-by twists upon itself and integrates. The bile-cellsconstituting the rudimentary liver, do not simply become different from thewall of the intestine in which they at first lie, but, while accumulating,they diverge from it and consolidate into an organ. The anterior portionof the cerebrospinal axis, at first continuous with. the rest, and not markedlydistinguished from it, undergoes a union of its rapidly-growing parts; andat the same time the resulting head folds into a mass marked off from thespine. The like process, variously exhibited in other organs, is meanwhileexhibited by the body as a whole; which becomes integrated somewhat in thesame way that an outspread handkerchief and its contents become integratedwhen its edges are drawn in and fastened to make a bundle. Kindred changesgo on after birth, and continue even up to old age. In man, that solidificationof the bony framework which, during childhood, is seen in the coalescenceof portions of the same bone ossified from different centres, is afterwardsseen in the coalescence of bones that were originally distinct. The appendagesof the vertebrae join with the vertebral centres to which they belong: achange not completed until towards thirty. At the same time the epiphyses,formed separately from the main bodies of their respective bones, have theircartilaginous connexions turned into osseous ones -- are fused to the massesbeneath them. The component vertebra of the sacrum, which remain separatetill about the sixteenth year, then begin to unite; and in ten or a dozenyears more their union is complete. Still later occurs the junction of thecoccygeal vertebra; and there are some other bony unions which remain unfinishedunless advanced age is reached. To which add that the increase of density,going on throughout the tissues at large during life, is the formation ofa more fully integrated substance.
The species of change thus illustrated, may be traced in all animals.
