Page:Popular Science Monthly Volume 76.djvu/55

Rh that potato beetles subjected to hot, dry conditions were made lighter in color and that part of their progeny raised under normal environment retained this characteristic.

Mere modifications of muscle or brain, accordingly, as in the trained mind of a savant or the brawny arms of a blacksmith, are not inheritable, but such is not necessarily the case with a quality like smallness of stature due to under-feeding, an influence affecting the mechanism of inheritance itself.

By far the best known of the factors working upon the lower animals is natural selection; that evolution which takes place when, without conscious selective action, one generation has been contributed by a part of the previous one differing from the non-contributing portion, as when wild deer of one generation are descended only from those previously existing deer who have been able to live to maturity because of their superior swiftness. Variations are thus seized upon by natural selection and perpetuated by heredity. The evidence of such a selection in the case of man has been tangibly presented by Sir Francis Galton, Karl Pearson and others working with them.

Natural selection stands opposed to artificial selection, that which is accomplished by conscious effort, such as Burbank's famous work in the production of improved varieties of fruit. It consists of two main processes, each containing several subdivisions, and a common error has been to confine the term natural selection to the first of these main processes, which may be called lethal selection. The case of the deer mentioned above is an instance of lethal selection—that which results from the death of some individuals before reproduction is completed.

The second process of natural selection, of perhaps even more importance at present in man, is reproductive selection, the result, not of premature death, but of a differential number of progeny. This may sometimes counterbalance lethal selection, as in the survival of some of the lower species of animals, whose marvelous reproductive ability preserves them in the face of a very high death rate.

When the cause of the absence of progeny is a failure in the individual to mate at the proper time, it is called sexual selection, but if, for other causes than success in mating, the number of offspring varies among individuals, fecundal selection results.

Sustentative selection, the type of natural selection most commonly recognized, comes from a pressure upon the means of subsistence by proportionately excessive numbers, such as that which sent the successive waves of Aryan migration over Europe. A non-sustentative form of natural selection takes place from the destruction of the individual by some adverse feature of the environment, such as excessive cold, bacteria or some bodily deficiency, and is independent of mere foodsupply. As Plymouth Harbor, for example, kept growing gradually