Such an interpretation would imply some reasonable assumption respectingthe pre-existing distribution of the stellar matter and of the matter formingnebulae, and we have no warrant for any assumption. If we allow imaginationto range back through antecedent Possibilities and probabilities, we seeit to be unlikely that homogeneous matter filled the space which our SiderealSystem now fills, at a time immediately preceding its initiation. Ratherthe evidence which the heavens present implies that the distribution outof which the present distribution arose, was irregular in all respects. Thoughcertain traits of our galaxy suggest that it has a vague individuality, andthat, along with their special motions, its stars have some general motion;yet the evidence forces on us the conclusion that many varieties of changeshave been simultaneously going on in its different parts. We find nebulaein all stages of concentration, star-clusters variously condensed, groupsof larger stars approximating in different degrees, as well as regions likethose which the nubeculae occupy, presenting complex structures and apparentlyactive changes. The most which can be said respecting this total distributionis that, subject as all parts of our Sidereal System are to the law of gravitation,the heterogeneities it exhibits, everywhere implying a progressing concentration,that is, integration, point backward to a less heterogeneous state and pointforward to a more heterogeneous state. But, leaving aside this too transcendentquestion, we may without undue rashness consider from the evolution pointof view the changes to be anticipated in one of those collections of matterdescribed as a diffused nebulosity, or one of those more distinct ones ofwhich the outlying parts are compared to wisps of cloud blown about by thewind. The only evolutional process which can at first be displayed is theprimary one of integration -- the gathering together through mutual attractionof the parts; for in this early stage in which indefiniteness and incoherenceare so fully exemplified, there does not yet exist such an aggregate as iscapable of exhibiting secondary re-distributions: we have only the dispersedcomponents of such an aggregate. Contemplating, then, only the process ofintegration, we may, without asking anything about the previous history ofan irregular nebula, safely assume that its parts have their respective propermotions; for the chances are infinity to one against a state of rest relativelyto one another. Further, the chances are infinity to one against their propermotions being such that during concentration they will cancel one another: the motion of some part, or the resultant of the motions of several parts,will constitute a proper motion distinct from that which mutual gravitationgenerates -- a motion which, unless just counterbalanced by an opposite one(again an infinite improbability) will generate rotation. It may, indeed,be argued that, apart from any pre-existing proper motions of its parts,a nebulous mass, if irregular, will acquire rotation while integrating; sinceeach outlying fragment, arriving after the rest have been gathered together,is infinitely unlikely to fall into the mass in such a manner that its motionwill be entirely cancelled by resistance; but, falling into it so as to bedeflected laterally, will have its motion of approach so changed in directionas to become in part a motion of revolution: a resultant of all such motions,largely conflicting, being an eventual rotation of the mass. It must not,however, be assumed that this will necessarily be the rotation of a solitaryaggregate. The great nebula in Andromeda does not appear on the way to forma single body; and is an advanced spiral of which the that in Canes Venaticiouter parts have a tangential motion too great to permit of their being drawninto the centre. Rather the apparent implication of the structure is thatthere will be formed a cluster of masses revolving round a common centreof gravity. Such cases, joined with those of the annular nebula, suggestthat often the processes of integration result in compound structures, variousin their kinds, while in other cases, and perhaps most frequently, singlemasses of rotating nebulous matter are formed.
Ignoring all such possibilities and probabilities, however, and limitingour attention to that form of the nebular hypothesis which regards the solarsystem as having resulted from a rotating spheroid of diffused substance;let us consider what consequence the instability of the homogeneous necessitates.
