History of the Intellectual Development of Europe, Volume I (of 2) / Revised Edition

by John William Draper

Effects of the seasons on animals and plants.

In the month of March the sun crosses the equator, dispensing his rays more abundantly over our northern hemisphere. Following in his train, a wave of verdure expands towards the pole. The luxuriance is in proportion to the local brilliancy. The animal world is also affected. Pressed forward, or solicited onward by the warmth, the birds of passage commence their annual migration, keeping pace with the developing vegetation beneath. As summer declines, this orderly advance of light and life is followed by an orderly retreat, and in its turn the southern hemisphere presents the same glorious phenomenon. Once every year the life of the earth pulsates; now there is an abounding vitality, now a desolation. But what is the cause of all this? It is only mechanical. The earth's axis of rotation is inclined to the plane of her orbit of revolution round the sun.

Let that wonderful phenomenon and its explanation be a lesson to us; let it profoundly impress us with the importance of physical agents and physical laws. They intervene in the life and death of man personally and socially. External events become interwoven in our constitution; their periodicities create periodicities in us. Day and night are incorporated in our waking and sleeping; summer and winter compel us to exhibit cycles in our life.

Individual existence depends on physical conditions.

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.

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.

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 And also appearances and extinctions determined. 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.

Permanence of organisms due to immobility of external conditions.

The permanence of organic forms is altogether dependent on the invariability of the material conditions under which they live. Any variation therein, no matter how insignificant it might be, would be forthwith followed by a corresponding variation in the form. The present invariability of the world of organization is the direct consequence of the physical equilibrium, and so it will continue as long as the mean temperature, the annual supply of light, the composition of the air, the distribution of water, oceanic and atmospheric currents, and other such agencies remain unaltered; but if any one of these, or of a hundred other incidents that might be mentioned, should suffer modification, in an instant the fanciful doctrine of the immutability of species would be brought to its true value. The organic world appears to be in repose, because natural influences have reached an equilibrium. A marble may remain for ever motionless upon a level table; but let the surface be a little inclined, and the marble will quickly run off. What should we say of him, who, contemplating it in its state of rest, asserted that it was impossible for it ever to move?