CHAPTER XV.

THE LAW OF EVOLUTION CONTINUED.

§ 116. Changes great in their amounts and various in their kinds, which accompany those dealt with in the last chapter, have thus far been wholly ignored-or, if tacitly recognized, have not been avowedly recognized. Integration of each whole has been described as taking place simultaneously with integration of each of the parts into which the whole divides itself. But how comes each whole to divide itself into parts? This is a transformation more remarkable than the passage of the whole from an incoherent to a coherent state; and a formula which says nothing about it omits more than half the phenomena to be formulated.

This larger half of the phenomena we have now to treat. In this chapter we are concerned with those secondary redistributions of matter and motion that go on along with the primary re-distribution. We saw that while in very incoherent aggregates, secondary re-distributions produce but evanescent results, in aggregates that reach and maintain a certain medium state, neither very incoherent nor very coherent, results of a relatively persistent character are produced-structural modifications. And our next inquiry must be-What is the universal expression for these structural modifications?

Already an implied answer has been given by the titleCompound Evolution. Already in distinguishing as simple Evolution, that integration of matter and dissipation of mo

tion which is unaccompanied by secondary re-distributions, it has been tacitly asserted that where secondary re-distributions occur, complexity arises. Obviously if, while there has gone on a transformation of the incoherent into the coherent, there have gone on other transformations, the mass, instead of remaining uniform, must have become multiform. The proposition is an identical one. To say that the primary re-distribution is accompanied by secondary re-distributions, is to say that along with the change from a diffused to a concentrated state, there goes on a change from a homogeneous state to a heterogeneous stato. The components of the mass while they become integrated also become differentiated.*

This, then, is the second aspect under which we lave to study Evolution. As, in the last chapter, we contemplated existences of all orders as displaying progressive integration; so, in this chapter, we have to contemplate them as displaying progressive differentiation.

§ 117. A growing variety of structure throughout our Sidereal System, is implied by the contrasts that indicate an aggregative process throughout it. We have nebula that are diffused and irregular, and others that are spiral, annular, spherical, &c. We have groups of stars the members of which are scattered, and groups concentrated in all degrees down to closely-packed globular clusters. We have these groups differing in the numbers of their members, from those containing several thousand stars to those con

*The terms here used must be understood in relative senses. Since we know of no such thing as absoute diffusion or absolute concentration, the change can never be anything but a change from a more diffused to a less diffused state-from smaller coherence to greater coherence; and, similarly, as no concrete existences present us with absolute simplicity as nothing is perfectly uniform -as we nowhere find complete homogeneity-the transformation is literally always towards greater complexity, or increased multiformity, or further heterogeneity. This qualification the reader must habitually bear in mind.

taining but two. Among individual stars there are great contrasts, real as well as apparent, of size; and from their unlike colours, as well as from their unlike spectra, numerous contrasts among their physical states are inferable. Beyond which heterogeneities in detail there are general heterogeneities. Nebulae are abundant in some regions of the heavens, while in others there are only stars. Here the celestial spaces are almost void of objects; and there we see dense aggregations, nebular and stellar together.

The matter of our Solar System during its concentration has become more multiform. The aggregating gaseous spheroid, dissipating its motion, acquiring more marked unlikenesses of density and temperature between interior and exterior, and leaving behind from time to time annular portions of its mass, underwent differentiations that increased in number and degree, until there was evolved the existing organized group of sun, planets, and satellites. The heterogeneity of this is variously displayed. There are the immense contrasts between the sun and the planets, in bulk and in weight; as well as the subordinate contrasts of like kind between one planet and another, and between the planets and their satellites. There is the further contrast between the sun and the planets in respect of temperature; and there is reason to suppose that the planets and satellites differ from one another in their proper heats, as well as in the heats which they receive from the sun. Bearing in mind that they also differ in the inclinations of their orbits, the inclinations of their axes, in their specific gravities and in their physical constitutions, we see how decided is the complexity wrought in the Solar System by those secondary re-distributions that have accompanied the primary re-distribution.

§ 118. Passing from this hypothetical illustration, which must be taken for what it is worth, without prejudice to the general argument, let us descend to an order of evidence less open to objection.

It is now generally agreed among geologists that the Earth was once a mass of molten matter; and that its inner parts are still fluid and incandescent. Originally, then, it was comparatively homogeneous in consistence; and, because of the circulation that takes place in heated fluids, must have been comparatively homogeneous in temperature. It must, too, have been surrounded by an atmosphere consisting partly of the elements of air and water, and partly of those various other elements which assume gaseous forms at high temperatures. That cooling by radiation which, though originally far more rapid than now, necessarily required an immense time to produce decided change, must at length have resulted in differentiating the portion most able to part with its heat; namely, the surface. A further cooling, leading to deposition of all solidifiable clements contained in the atmosphere, and finally to precipitation of the water and separation of it from the air, must thus have caused a second marked differentiation; and as the condensation must have commenced on the coolest parts of the surface—namely, about the poles-there must so have resulted the first geographical distinctions.

To these illustrations of growing heterogeneity, which, though deduced from the known laws of matter, may be regarded as hypothetical, Geology adds an extensive series that have been inductively established. The Earth's structure has been age after age further involved by the multiplication of the strata which form its crust; and it has been age after age further involved by the increasing composition of these strata, the more recent of which, formed from the detritus of the more ancient, are many of them rendered highly complex by the mixtures of materials they contain. This heterogeneity has been vastly increased by the action of the Earth's still molten nucleus on its envelope; whence have resulted not only a great variety of igneous rocks, but the tilting up of sedimentary strata at all angles, the formation of faults and

metallic veins, the production of endless dislocations and irregularities. Again, geologists teach us that the Earth's surface has been growing more varied in elevationthat the most ancient mountain systems are the smallest, and the Andes and Himalayas the most modern; while, in all probability, there have been corresponding changes in the bed of the ocean. As a consequence of this ceascless multiplication of differences, we now find that no considerable portion of the Earth's exposed surface, is like any other portion, either in contour, in geologic structure, or in chemical composition; and that, in most parts, the surface changes from mile to mile in all these characteristics.

There has been simultaneously going on a gradual differentiation of climates. As fast as the Earth cooled and its crust solidified, inequalities of temperature arose between those parts of its surface most exposed to the sun and those less exposed; and thus in time there came to be the marked contrasts between regions of perpetual ice and snow, regions where winter and summer alternately reign for periods varying according to the latitude, and

regions where summer follows summer with scarcely

an appreciable variation.

Meanwhile, elevations.

and subsidences, recurring here and there over the Earth's crust, tending as they have done to produce irregular distribution of land and sea, have entailed various modifications of climate beyond those dependent on latitude; while a yet further series of such modifications has been produced by increasing differences of height in the lands, which have in sundry places brought arctic, temperate, and tropical climates to within a few miles of one another. The general results of these changes are, that every extensive region has its own meteorologic conditions, and that every locality in each region differs more or less from others in those conditions: as in its structure, its contour, its soil.

Thus, between our existing Earth, the phenomena of