regards his structure and as regards his life; and evolution has sought to show that the amount of similarity is adequate to prove that the human has, structurally at least, been evolved from the animal. But evolution cannot stop here. The man and the animal alike have something in common with an order of creation which is inferior to both the kingdom of plant-life. The evolution theory, therefore, descends a step further still, and seeks to prove that plant-life was the germ from which the two higher orders were structurally developed. But it cannot stop even here. The man, the animal, and the plant have in their turn something in common with a lower order of creation still; they have elements in their structure which they have derived from the matter of the earth. Evolution, accordingly, seeks to find facts to show that the elements of non-living matter have had something to do with the production of those forms of plant-life which have developed into forms of animal organisation, and which have ultimately flowered into the bodily organisation of man.

Can evolution stop here? that is still impossible. The man, the animal, the vegetable, and the matter of the earth from which they draw their sustenance, have all something in common with an order of existence behind them. That order of existence is force. All forms of earthly matter are subject to one great force-the principle of gravity. The

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human frame, the body of the animal, and the forms of vegetable life, are all attracted towards the earth as their centre. But this attractive power which seems to reside in the matter of the earth does not really reside there. The briefest observation convinces us that, so far from being an active source of gravity, the earth is itself a passive recipient of that force. The matter of the earth is found to be so united to the matter of other worlds as to be incapable of existing if these other worlds were withdrawn. The evolutionist is thus lifted a step further back in his efforts to find the origin of species. One would have thought a priori that the earth would have proved his terminus; it has proved, in truth, only his beginning. It reveals itself as but the fragment of a vast system-a system to whose existence it owes its being, and by whose force its own forces are constituted. Here, then, is a common ground existing between the earth and the whole solar universe -a ground of mutual dependence. The matter of the earth cannot exist in its present form without the matter of the solar heavens; the matter of the solar heavens cannot exist in its present form without the matter of the earth. This mutual dependence of forces indicates that in neither of them does the force find its original seat, and the evolutionist is therefore driven back further still in his search for a first principle. Looking abroad

over the solar universe, he perceives that the mutually dependent forces of the earth and the other planets are themselves dependent on the force of a great central luminary-the sun. To the sun, accordingly, the evolutionist turns his eyes, in the hope of discovering there the original seat of at least that particular system which bears its name. Nor does he fail to find evidence that this central luminary once united within itself all those planetary forms which now circle round it. He sees that the further back he goes in time the sun is of necessity increasing its diameter. A year ago its diameter was 220 feet greater than it is now; a hundred years ago it was four miles greater; a thousand years ago it was forty miles greater; ten thousand years ago it was four hundred miles greater. The evolutionist asks us to conceive what its diameter must have been in periods of indefinitely distant ages. It is not difficult to see that the sun must at that time have been the all of the solar universe, that it must have engulfed within itself all those forms and forces which have now a partially independent being, and that it must have combined in a life of comparative unity the elements of those structures which have since emerged into such striking variety.

Yet there is no reason to believe that the evolutionist has even here reached his terminus. There is no reason to think that the sun, however

much it may be the source of the solar system, is itself anything more than the fragment of a larger and wider system. There is no reason to doubt that the mutual dependence which prevails among planets, prevails likewise among suns, and that our luminary is indebted for its being to the being of a universe beyond it. Once more, therefore, the evolutionist turns his eye backward in search of a principle of unity more distinct and more indisputable than he has hitherto found. He is no longer able to follow the stream of actual facts, for, when he has reached the centre of the solar system, he has almost reached the boundary of direct observation. But where observation fails, imagination begins. The man of science, where the criti-. cal faculty becomes unavailable, appropriates for a time the faculty of the poet, and seeks by a flash of intuition to realise that unity of nature which the limitations of physical sense forbid him to trace or verify.

It is to this leap of the scientific imagination that we are indebted for the theory known as the nebular hypothesis. With the explanation or defence of that hypothesis we have here nothing to do; we are only concerned with it in so far as it is an attempt to reduce the many forms of nature to the one. Viewed in this light, its scope may be thus indicated. The evolutionist conceives that, just as all the planets of the solar system

once formed a part of its central luminary or sun, so all the planets of every other system once formed a part of their central luminary or sun. When the plurality of revolving worlds in the universe has thus been reduced to the plurality of the suns round which they move, the evolutionist goes on to reduce still further this multiplicity of objects. He goes on to conceive a time when the suns themselves were engulfed in a fire more central still. He imagines that at one period the central luminaries of the various planetary systems were all united in the mist of a fire-cloud. He starts from the conception of a vast nebulous mass of heated matter, slowly revolving round its own axis, and gradually cooling down. Within this heated nebulous mass he conceives to have been originally embraced all the forces of the physical universe. He conceives that here were mixed up in an indistinguishable unity the lights of various stars and the stars of various systems-that the planets. were merged in their suns, and their suns were merged in the fire that gave them birth-until all plurality was lost in the indiscriminate commingling of those forces and powers of nature which now exist as seemingly independent elements.

One would think that evolution must here at last have exhausted her possibilities of simplification, must here have finally attained her nearest approach to the discovery of a unifying principle.