Page:Popular Science Monthly Volume 76.djvu/56

52 muddier, the broad crabs were subjected to a heavier death rate, so that the general type of crab in the harbor became comparatively narrow. Selection of this sort may depend on some physical advantage, or sometimes on differences in mentality, as in the survival of races of mankind intelligent enough to overcome a hostile environment—to escape cold by the use of fire or smallpox by the invention of vaccination.

The classification of the modes of natural selection is shown by the following diagram:

An indirect form of sustentative selection may be said to exist when malnutrition has the effect of destroying resistance to an adverse environment, as in the case of tuberculosis. Both modes of lethal selection, moreover, may act upon groups, as well as upon single individuals. Though group selection is best typified by a war between two tribes, such cases as the decline of the Alaskan aborigines illustrate its occurrence without combat.

Throughout the measureless ages before man natural selection worked without let or hindrance.

At last, however, the life of the tree-dwellers evolved that ape-like ancestor of ours, the tailless arboreal mammal with four hands. The hand, originally an adaptation for the purpose of grasping limbs, gradually became fitted for grasping other things than limbs, that is, for "handling" tools. Naturally such an organ made slight variations of brain structure of great advantage, for a better way of handling objects in hunting and fighting might lead to survival, not only of the individual, but also of the tribe. The resulting complexity of brain development brought with it the capacity to think, to hand on thoughts from generation to generation by oral and written tradition, and thus to formulate ideas into science.

Here, however, came a marvelous change in the methods of evolution. Although our primitive ancestors had been, up to this time, like the lower animals, a mere bagatelle under the influences of environment and heredity, thinking man now acquired the power of reflection, and even of discovering and criticizing the laws of evolution themselves. In certain cases we are able to register our dissatisfaction with the course which nature is leading us, and to make definite attempts at turning this course from the merely fittest to our notions of the best. We have learned that the forces at work changing our species are themselves partially under our control, and that, if, like the powers of the