Page:Popular Science Monthly Volume 63.djvu/19

Rh from their unimproved relatives. Variation is not a consequence of adaptation; adaptation is a result of variation.

Heredity and variation are not two opposing forces, the one tending to preserve and the other to destroy the specific type; they are two closely adjacent aspects of the single process of organic succession. The permanence of types is not secured by stable or unchanging characters, but by individual diversity or inconstancy, and the consequent power to move in advantageous directions. Organisms are so constituted that the persistent repetition of the same form or character complex is not possible; the supposition of a non-progressive heredity comes from the pre-evolutionary period. Heredity does not oppose variation; evolution is the inheritance of variations, facilitated by cross-fertilization. The causes of variations are also the causes of the accumulation of variations, and of the resulting diversity of species. Variation and cross-fertilization are the means, while selection and isolation are the incidents, of a continuous organic motion. Species are not normally at rest, nor are their motions predetermined by external forces or by internal mechanisms; they are not compelled in one direction, but must move in some direction, as variation and environment permit.

Static theories are further inadequate because they neglect the fact that change or biological motion is necessary to maintain the vigor and efficiency of the organism. A kinetic theory, on the other hand, recognizes such motion as normal, and as facilitated by cross-breeding, instead of being hindered. In whatever environment and however propagated, organisms of all types and all categories of complexity are changing or evolving, though with unequal rapidity. Organisms multiplied asexually and thus connected only in simple or linear series make slow progress in comparison with groups in which variations can be distributed through cross-fertilization. The more complex the organic structure the greater the necessity that it be supported, as it were, by many diverse, intergrafting lines of descent. The reasons for this have not been explained, but for purposes of expression it may be ascribed to a special property or requirement called symbasis, served at