Page:Popular Science Monthly Volume 68.djvu/493

Rh intraspecific, that of individuals within the species. When members of related species enter into competition in the same region, the struggle becomes in a degree interspecific, one between the species themselves through their actual representatives. But as there is practically no cooperation among members of the same species, except in the family or band relation of the higher animals, the struggle must be in fact always individual. The struggle against the environment in general is in its essence non-competitive, but in the crowd of animals and plants competition becomes part of the environment. Some seeds, we are told, fell on stony ground, but the plants died because they had no depth of earth. Some fell among thorns, and the thorns grew up and choked them. Still others doubtless perished because they were planted too close together. These three causes of the destruction of the great body of animals and plants, the competition with like forms, that with unlike forms, and the pressure of the environment, are present everywhere with all life in varying degrees, and by it all life is forced into lines of adaptation.

With vital characters, selection preserves those which have no utility by the simple action of heredity. All Southdown sheep have tawny faces, although this trait bears no relation to the short firm wool or to the fat and tender flesh, for which traits the Southdown sheep are bred and valued.

In similar fashion, many indifferent characters are traceable in the various breeds and strains of domesticated animals and cultivated plants. We may presume that similar characters in wild animals and plants have been similarly carried along by inheritance from ancestors possessing them. This phenomenon I have elsewhere called 'the survival of the existing.' The actual traits are reproduced by heredity without regard to any question of fitness.

The phenomena of dichromatism belong to the category of individual variation. In the vast majority of animals, we have the dimorphism of sex. In the beginning the embryo is sexless. From the beginning, by forces imperfectly understood, its development must be directed in one way or another. It must assume the structures and functions of one or the other sex. With certain insects, a polymorphic condition exists. With bees, the caste of workers or atrophic females exists, together with the type of drone and queen. With some ants, still other types of individuals occur, but the dimorphism of sex, or the polymorphism of the division of labor, rests on influences perhaps not altogether inherent in the structure of the parent cell.

In dichromatism, the individual from the first shows one or another of two color patterns peculiar to the species. The screech owls are some gray, some rusty red, within the same species, even in the same nest.