With progressing settlement of the space-relations, the force-relationssimultaneously become more settled; and the exact calculations of physicalastronomy show us how definite these force-relations now are. In short, itneeds but to think of the contrast between the chaos of the primitive nebulaand the ordered relations of the Solar System in the sizes, shapes, motions,and combined inter-actions of its members, to see that increase of definitenesshas been a marked trait of its evolution. §131. From that primitive molten state of the Earth inferable fromgeological data as well as from the nebular hypothesis (probably a liquidshell having a nucleus of gases above the "critical point" of temperature,kept by pressure at a density as great as that of the superjacent liquid)the transition to its existing state has been through stages in which thecharacters became more determinate. A liquid spheroid is less specific thana solid spheroid in having no fixed distribution of parts. Currents of moltenmatter, though kept to certain general circuits by the conditions of equilibrium,cannot, in the absence of solid boundaries, be precise in their limits anddirections: all parts must be in motion with respect to other parts. Buta superficial solidification, even though partial, is a step towards theestablishment of definite relations of position. In a thin crust, however,often ruptured by disturbing forces, and moved by every tidal undulation,fixity of relative position can be but temporary. Only as the crust thickenscan there arise distinct and settled geographical positions. Observe, too,that when, on a surface adequately cooled, there begins to precipitate thewater floating above as vapour, the deposits cannot maintain definitenesseither of state or place. Falling on a solid envelope not thick enough topreserve anything beyond slight variations of level, the water must formsmall and shallow pools over the coolest areas; which areas must pass insensiblyinto others that are too hot to allow condensation. With progressing refrigeration,however -- with a thickening crust, a consequent formation of larger elevationsand depressions, and the precipitation of more atmospheric water, there comesan arrangement of parts which is comparatively fixed; and the definitenessof position increases, until there result continents and oceans -- a distributionthat is not only topographically settled, but presents separations of landfrom water more definite than could have existed when all the uncovered areaswere low islands with shelving beaches, over which the tide ebbed and flowedto great distances.
Respecting the characters classed as geological, we may draw kindred inferences.
While the Earth's crust was thin, mountain-chains were impossibilities: therecould not have been long and well-defined axes of elevation, with distinctwater-sheds and areas of drainage. Moreover, the denudation of small islandsby small rivers, and by tidal streams both feeble and narrow, would produceno clearly-marked sedimentary strata. Confused and varying masses of detritus,such as we now find at the mouths of brooks, must have been the prevailingformations. And these could give place to distinct strata, only as therearose continents and oceans, with their great rivers, long coast-lines, andwide-spreading marine currents.
There must simultaneously have resulted more definite meteorological conditions.
Differences of climates and seasons grew relatively decided as the heat derivedfrom the Sun became distinguishable from the proper heat of the Earth; andthe production of more specific conditions in each locality was aided byincreasing permanence in the distribution of lands and seas. These are conclusionssufficiently obvious. §132. We come now to the evidence yielded by organic bodies. In placeof deductive illustrations, we shall here find illustrations which have beeninductively established, and are therefore less open to criticism. The courseof mammalian development, for example, will supply us with numerous proofsready-described by embryologists.
The first change which the ovum of a mammal undergoes after repeated segmentationhas reduced it to a mulberry-like mass, is the appearance of a distinctionbetween the peripheral or epiblastic cells of this mass and the internalor hypoblastic cells. While growing rapidly the cluster of cells becomeshollow, and the blastodermic vesicle so formed presents a definite contrastbetween the outer layer, or epiblast, and its contents. The mass of hypoblastcells, having at first an indefinite, lens-like figure attached to the insideof the epiblast, spreads out and flattens into a membrane, the boundary ofwhich is irregular -- indefinite alike in form and constitution. And thenthe middle or thicker part presently becomes an opaque circular spot constitutingthe embryonic area: a spot which gradually acquires a pronounced outline.
