Thus, then, is that increase of heterogeneity which is not a trait ofEvolution, distinguished from that increase of heterogeneity which is. Thoughin disease and after death, individual or social, the earliest modificationsare additions to the pre-existing heterogeneity, they. are not additionsto the pre-existing definiteness. From the outset they begin to destroy thisdefiniteness, and gradually produce a heterogeneity that is indeterminateinstead of determinate. As a city, already multiform in its variously-arrangedstructures of various architecture, may be made more multiform by an earthquake,which leaves part of it standing and overthrows other parts in differentways and degrees, but is at the same time reduced from orderly arrangementto disorderly arrangement; so may organized bodies be made for a time moremultiform by changes which are nevertheless disorganizing changes. And inthe one case as in the other, it is the absence of definiteness which distinguishesthe multiformity of regression from the multiformity of progression.
If advance from the indefinite to the definite is an essential characteristicof Evolution, we shall of course find it everywhere displayed; as in thelast chapter we found displayed the advance from the homogeneous to the heterogeneous.
To see whether it is so, let us now consider the same several classes offacts. §130. Beginning, as before, with a hypothetical illustration, wehave to note that each step in the evolution of the Solar System, supposingit to have originated from diffused matter, was an advance towards more definitestructure. As usually conceived, the initial nebula was irregular in shapeand with indistinct margins, like those of nebulae now existing. Having partially-differentproper motions, the parts of its attenuated substance, while being drawntogether, generated, by the averaging of their motions, as well as by changesin the directions of these motions, a certain angular momentum; and the entiremass as it concentrated and acquired rotation must have assumed the formof an oblate spheroid which with every increase of density, became more specificin outline, and had its surface more distinctly marked off from the surroundingvoid. Simultaneously, the constituent portions of nebulous matter, insteadof moving round their common centre of gravity in various planes, as theywould at first do, must have had these planes more and more merged into asingle plane, that became less vague as the concentration progressed -- becamegradually defined.
According to the hypothesis, change from indistinct characters to distinctones, was repeated in the evolution of planets and satellites. A gaseousspheroid is less definitely limited than a liquid spheroid, since it is subjectto larger undulations of surface, and to greater distortions of general form;and, similarly, a liquid spheroid, covered as it must be with waves of variousmagnitudes, tidal and other, is less definite than a solid spheroid. Thedecrease of oblateness which goes along with increase of integration, bringsrelative definiteness of other elements. A concentrating planet having anaxis inclined to the plane of its orbit, must, while very oblate, have itsplane of rotation much disturbed by external attractions; whereas its approachto a spherical form, involves a smaller precessional motion, and less markedvariations in the direction of its axis.
