Page:Popular Science Monthly Volume 4.djvu/624

606 was not victorious until, within the last decades, Charles Darwin led upon the hard-fought logic-field an array of facts glittering in their strength. Before a victory had been conquered by the Darwinistic school, the specific forms of vegetal and animal life were held to be immutable. While it was known that among the individuals of any one species certain differences, justifying their being classed as distinct varieties, might arise in the course of successive generations, all such variations were held to be bounded by certain more or less narrow limits of possibility. The facts adduced by the new school of biologists have led to different conclusions, culminating in the assertion that all organic forms are changeable without limit—transmutable without end—capable of a physical, intellectual, and moral elevation, which knows no boundaries.

According to the theory of evolution, the modifications of structure and capability which organic forms are liable to undergo, in the course of generations, may be traced back to three principal causes. The first of these is the influence of physical forces, as, when the strength of the muscles is enhanced by exercise, the mind invigorated by thought, or, inversely, their function impaired by long-continued disuse. The second cause consists in the transmission of these results of individual life to a line of descendants, the effects being compounded, as they pass, with others of the same order. The third cause is to be found in the competition of the forms so produced under circumstances not equally well adapted to their capacities nor sufficient for their coexistence, leading necessarily to the preservation of those races which are best, and the extinction of those which are least, fitted to endure adversity. To this last cause, constantly active in the organic world, the term "natural selection" has been applied, to distinguish it from the artificial selection of the stock-breeder and gardener. The changes wrought in organic forms by the influence of inorganic forces are generally spoken of as variation. But this variation is in reality merely an extension of the principle of competition. Organized beings are brought face to face with the forces of Nature, with the earthquake, the flood, the lightning, and the storm. Often they meet in mortal conflict. The living form sinks to the earth before the power of the thunder-bolt, or the thunder-bolt is conquered by the invention of genius. Death is but a victorious alliance of inorganic forces triumphing over the organic form laid low on the battle-field; life is but the victory of the organic forces over the inorganic hosts. But, do we not also behold a competition taking place between, a struggle for existence. and a natural selection occurring among, inorganic forms? Cast water upon fire; either the water disappears as vapor or the fire is extinguished. A mixture of salt and gravel is brought in contact with water; the salt is dissolved, the gravel remains unaltered. Heat a mixture of salt and sal-ammoniac; the salt persists, while the sal-ammoniac is vaporized.