CHAPTER XIV.

THE LAW OF EVOLUTION.

107. Deduction has now to be verified by induction. Thus far the argument has been that all sensible existences must, in some way or other and at some time or other, reach their concrete shapes through processes of concentration; and such facts as have been named have been named merely to clarify the perception of this necessity. But we cannot be said to have arrived at that unified knowledge constituting Philosophy, until we have seen how existences of all orders do exhibit a progressive integration of Matter and concomitant loss of Motion. Tracing, so far as we may by observation and inference, the objects dealt with by the Astronomer and the Geologist, as well as those which Biology, Psychology and Sociology treat of, we have to consider what direct proof there is that the Cosmos, in general and in detail, conforms to this law.

In doing this, manifestations of the law more involved than those hitherto indicated, will chiefly occupy us. Throughout the classes of facts successively contemplated, our attention will be directed not so much to the truth that every aggregate has undergone, or is undergoing, integration, as to the further truth that in every more or less separate part of every aggregate, integration has been, or is, in progress. Instead of simple wholes and wholes of which the complexity has been ignored, we have

here to deal with wholes as they actually exist-mostly made up of many members combined in many ways. And in them we shall have to trace the transformation as displayed under several forms—a passage of the total mass from a more diffused to a more consolidated state; a conrurrent similar passage in every portion of it that comes to have a distinguishable individuality; and a simultaneous increase of combination among such individuated portions.

§ 108. Our Sidereal System by its general form, by its clusters of stars of all degrees of closeness, and by its nebulæ in all stages of condensation, gives us grounds to suspect that, generally and locally, concentration is going on. Assume that its matter has been, and still is being, drawn together by gravitation, and we have an explanation of all its leading traits of structure-from its solidified masses up to its collections of attenuated flocculi barely discernible by the most powerful telescopes, from its double stars up to such complex aggregates as the nubeculæ. Without dwelling on this evidence, however, let us pass to the case of the Solar System.

The belief, for which there are so many reasons, that this has had a nebular genesis, is the belief that it has arisen by the integration of matter and concomitant loss of motion. Evolution, under its primary aspect, is illustrated most simply and clearly by this passage of the Solar System from a widely diffused incoherent state to a consolidated coherent state. While, according to the nebular hypothesis, there has been going on this gradual concentration of the Solar System as an aggregate, there has been a simultaneous concentration of each partially-independent member. The substance of every planet in passing through its stages of nebulous ring, gascous spheroid, liquid spheroid, and spheroid externally solidified, has in essentials paralleled the changes gone through by the general mass; and every satellite has done the like. Morcover, at the same

time that the matter of the whole, as well as the matter of each partially-independent part, has been thus integrating, there has been the further integration implied by increasing combination among the parts. The satellites of each planet are linked with their primary into a balanced cluster; while the planets and their satellites form with the Sun, a compound group of which the members are more strongly bound up with one another than were the far-spread portions of the nebulous medium out of which they arose.

Even apart from the nebular hypothesis, the Solar System furnishes evidence having a like general meaning. Not to make much of the meteoric matter perpetually being added to the mass of the Earth, and probably to the masses of other planets, as well as, in larger quantities, to the mass of the Sun, it will suffice to name two generally-admitted instances. The one is the appreciable retardation of comets by the ethereal medium, and the inferred retardation of planets a process which, in time, must bring comets, and eventually planets, into the Sun. The other is the Sun's still-continued loss of motion in the shape of radiated heat; accompanying the still-continued integration of his mass.

§ 109. To geologic evolution we pass without break from the evolution which, for convenience, we separate as astronomic. The history of the Earth, as traced out from the structure of its crust, carries us back to that molten state which the nebular hypothesis implies; and, as before pointed out (§ 69), the changes classed as igneous are the accompaniments of the progressing consolidation of the Earth's substance and accompanying loss of its contained motion. Both the general and the local effects may be briefly exemplified.

Leaving behind the period when the more volatile elements now existing as solids were kept by the high temperature in a gascous form, we may begin with the fact that until the Earth's surface had cooled down below 212°,

the vast mass of water at present covering three-fifths of it, must have existed as vapour. This enormous volume of disintegrated liquid became integrated as fast as the dissipation of the Earth's contained motion allowed; leaving, at length, a comparatively small portion unintegrated, which would be far smaller but for the unceasing absorption of molecular motion from the Sun. In the formation

of the Earth's crust we have a similar change similarly caused. The passage from a thin solid film, everywhere fissured and moveable on the subjacent molten matter, to a crust so thick and strong as to be but now and then very slightly dislocated by disturbing forces, illustrates the process. And while, in this superficial solidification, we see under one form how concentration accompanies loss of cotained motion, we see it under another form in that diminution of the Earth's bulk implied by superficial corrugation.

Local or secondary integrations have advanced along with this general integration. A molten spheroid merely skinned over with solid matter, could have presented nothing beyond small patches of land and water. Differences of elevation great enough to form islands of considerable size, imply a crust of some rigidity; and only as the crust grew thick could the land be united into continents divided by oceans. So, too, with the more striking elevations. The collapse of a thin crust round its cooling and contracting contents, would throw it into low ridges: it must have acquired a relatively great depth and strength before extensive mountain systems of vast elevation became possible. In sedimentary changes, also, a like progress is inferable. Denudation acting on the small surfaces exposed during early stages, would produce but small local deposits. The collection of detritus into strata of great extent, and the union of such strata into extensive "systems," imply wide surfaces of land and water, as well as subsidences great, in both area and depth; whence it

follows that integrations of this order must have grown more pronounced as the Earth's crust thickened.

§ 110. Already we have recognized the fact that organic evolution is primarily the formation of an aggregate, by the continued incorporation of matter previously spread through a wider space. Merely reminding the reader that every plant grows by concentrating in itself elements that were before diffused as gases, and that every animal grows by re-concentrating these elements previously dispersed in surrounding plants and animals; it will be here proper to complete the conception by pointing out that the early history of a plant or animal, still more clearly than its later history, shows us this fundamental process. For the microscopic germ of each organism undergoes, for a long time, no other change than that implied by absorption of nutriment. Cells imbedded in the stroma of an ovarium, become ova by little else than continued growth at the expense of adjacent materials. And when, after fertilization, a more active evolution commences, its most conspicuous trait is the drawing-in, to a germinal centre, of the substance which the ovum contains.

Here, however, our attention must be directed mainly to the secondary integrations which habitually accompany the primary integration. We have to observe how, along with the formation of a larger mass of matter, there goes on a drawing together and consolidation of the matter into parts, as well as an increasingly-intimate combination of parts. In the mammalian embryo, the heart, at first a long pulsating blood-vessel, by and by twists upon itself and integrates. The bile-cells constituting the rudimentary liver, do not simply become different from the wall of the intestine in which they at first lie; but, as they accumulate, they simultaneously diverge from it and consolidate into an organ. The anterior segments of the cerebro-spinal axis, which are at first continuous with the rest, and distinguished