matter of the globe is in the gaseous form. The gases themselves are differentiated into what we call oxygen, nitrogen, carbonic acid, and aqueous vapor, each of which exerts its special influence in the economy of vital existence, and without any one form of which, so far as we can see, life would be impossible. Another portion of the matter composing the earth exists in the liquid state, the chief form of which is water, of which more than half the weight of all organized beings consists. Evidently without water nothing answering to our conception of life could exist. Of the solid matter of the globe there exists the greatest heterogeneity, and it may be classified in a variety of different ways. Many, though probably by no means all, of the so called distinct substances known to us are of direct value in the formation of organic tissues, and certain of them are clearly indispensable, so far as we understand their office; as, for example, lime, phosphorus, iron, etc., etc. Certain of these substances are crystalline, others colloid in their structure, the latter of which possess peculiar adaptations to the formation of organic tissue. Finally, between the solid and the liquid state there exist all grades of transition, thus adding variety to the organic adaptations.
As the universal law of concentration or integration proceeds to reduce all these varied forms to one, and to cement all in a single homogeneous solid, it is met by the powerful but somewhat irregular and erratic force of the solar radiations, reënforcing the inherent cosmical influences already so far overcome in the evolution of the planet as to have brought it to its given state. The result of this conflict of forces is the condition in which we find our globe. Without the aid of the sun's rays, organic evolution might have been impossible. Without the aid of the cosmical force of concentration, in a certain way counteracting without neutralizing them, it would have equally been impossible. With such a predominance of the one as has probably prevailed in the past, or of the other as will probably prevail in the future, the particular form of evolution required to develop what we know as life seems also beyond the range of scientific probability. A few degrees more either of heat or of cold are sufficient to utterly destroy it. Of the latter, we have a near approach to a positive example in the state of things existing in regions round the poles of the earth's axis. Of the former, artificial proofs are easy, and certain desert regions of the globe constitute partial illustrations, easily completed by the imagination.
We thus learn what a precarious thing life is, within what narrow limits it is circumscribed, upon what slender conditions its possibility depends; contemplating which, we may be appalled to reflect how small a portion of the concentrated matter of space must be presumed to fall under these conditions. For, even if every world in space passes through this organic period, its duration must be ephemeral compared with the vast cycle of its existence.