They who have paid attention to the subject have long ago ascertained that the possibility of human existence on the earth depends on conditions altogether of a material kind. Since it is only within a narrow range of temperature that life can be maintained, it is needful that our planet should be at a definite mean distance from the source of light and heat, the sun; and that the form of her orbit should be so little eccentric as to approach closely to a circle. If her mass were larger or less than it is, the weight of all living and lifeless things on her surface would no longer be the same; but absolute weight is one of the primary elements of organic construction. A change in the time of her diurnal rotation, as affecting the length of the day and night, must at once be followed by a corresponding modification of the periodicities of the nervous system of animals; a change in her orbitual translation round the sun, as determining the duration of the year, would, in like manner, give rise to a marked effect. If the year were shorter, we should live faster and die sooner. [Sidenote: Animal and vegetable life interbalanced by material conditions.] In the present economy of our globe, natural agents are relied upon as the means of regulation and of government. Through heat, the distribution and arrangement of the vegetable tribes are accomplished; through their mutual relations with the atmospheric air, plants and animals are interbalanced, and neither permitted to obtain a superiority. Considering the magnitude of this condition, and its necessity to general life, it might seem worthy of incessant Divine intervention, yet it is in fact accomplished automatically. [Sidenote: And also appearances and extinctions determined.] Of past organic history the same remark may be made. The condensation of carbon from the air, and its inclusion in the strata, constitute the chief epoch in the organic life of the earth, giving a possibility for the appearance of the hot-blooded and more intellectual animal tribes. That great event was occasioned by the influence of the rays of the sun. And as such influences have thus been connected with the appearance of organisms, so likewise have they been concerned in the removal. Of the myriads of species which have become extinct, doubtless every one has passed away through the advent of material conditions incompatible with its continuance. Even now, a fall of half-a-dozen degrees in the mean temperature of any latitude would occasion the vanishing of the forms of warmer climates, and the advent of those of the colder. An obscuration of the rays of the sun for a few years would compel a redistribution of plants and animals all over the earth; many would totally disappear, and everywhere new comers would be seen. [Sidenote: Permanence of organisms due to immobility of external conditions.]