plant and animal use is limited; many more individuals are born than can survive; the result is a perpetual struggle for survival.

(2) The fittest individuals tend to be the ones that survive; the battle is to the strong, the race is to the swift.

(3) The individuals so selected transmit many of their favorable qualities to their offspring by heredity.

(4) But although heredity produces a wonderfully exact copy of the parent in the child, there is never precise reduplication. There is latitude for individual variation. If among the innumerable multitudes of individual variations that may occur, one chances to appear which, no matter in how slight a degree, gives the individual possessing it advantage in the struggle, that individual is bound to be favored with longer life and larger number of progeny-with survival, in short.

But the theory of natural selection proposes to explain only those characters which give advantage in the struggle for existence. It does not explain the existence of certain characters which do not give definite advantage to their possessors and yet tend to persist from generation to generation. Some of these characters, like the brilliant plumage of certain birds (peacock and peahen), would seem to be of positive disadvantage by making them conspicuous to their enemies. To account for these markedly contrasted sex-characters, Darwin advanced the theory of Sexual Selection. He believed that the individuals possessing the brilliant coloring were more attractive to those of the opposite sex and so had a better chance to mate than their fellows of a more sober hue. By the laws of heredity the brilliant plumage was transmitted, and the less attractive individuals, not

securing mates, or at any rate less robust mates, would have fewer progeny and eventually their line would die out. There were also combats between rival males for the possession of females as well as the preferential mating where the female chooses or seems to choose. There is little reason to doubt the effect of selection where there is combat among males. For when the younger or weaker candidates are killed, or expelled from the herd, or left unmated, there is discriminate elimination, the progeny inherit the strong constitutions of their parents. But as to preferential mating, the theory has broken down rather badly under criticism since Darwin's time.

There is one other point of considerable importance which must be discussed before we can understand the real significance of natural selection. It is the alleged inheritance of acquired characters. The athlete has larger and more developed muscles than the average man. Do his children inherit larger and more developed muscles? Many years ago the naturalist Lamarck advanced a theory that modifications induced in the structure of the parent by adaptation to its surroundings were inherited by the offspring. His classic illustration of this theory was the giraffe. The entire frame of the giraffe has been adapted to support an enormously long neck which is of use to the animal in reaching the foliage of trees. Lamarck thought that the ancestors of the giraffe had ordinary necks but had increased the length of them through many successive generations by constantly stretching to reach high foliage. Moreover, when the neck became so long as to require for its support special changes in the general form of the animal, these

7 Thomson & Geddes, op. cit., p. 172.

changes brought about the dwindling of other parts from which so much activity was no longer required. The result was that the whole organization of the animal became more and more adapted to browsing on high foliage. This same principle was applied to explain many other structural peculiarities. Before we can have a satisfactory understanding of this problem it is necessary to resort to the theory of inheritance explained in chapter I.

In speaking of inheritance we said that the parent was rather the trustee of the germ-plasm than the producer of the child. In higher plants and animals the function of reproduction is not performed by the body as a whole, but is given over to special groups of cells, the germ cells, constituting the ovaries and testes. It is from these cells that new individuals arise. In view of this the problem we have just been considering is not so simple. For example, how can the enlargement of a muscle due to exercise, so affect the germ cells, which lie at some distance from the muscle in question, as to cause the new individual, which shall arise from these germ cells, to have the corresponding muscle in its own body enlarged? Under ordinary conditions it is only the germ cells in the body which have any descendants in the following generation. In the body there are muscle cells, bone cells, nerve cells, etc. Weismann used the term soma to include all the cells of the body which are not germ cells. Now the whole body of the offspring comes from the union of two germ cells; an egg from one parent and a spermatozoon from the other. No somatic cell gives rise to any part of the offspring. While the fertilized egg is developing into an adult organism it divides into a number of portions called blastomeres,

some of these form the germ cells of the new individual, the remainder become its soma. The germ cells of one generation are thus derived almost directly from the germ cells of the preceding generation. One can now understand more clearly the significance of the theory of the continuity of germinal plasm. Professor Metcalf

FIGURE 7. Diagram of Inheritance of Body Cells and Germ Cells. has illustrated this principle by the simple diagram shown in figure 7.8

The diagram shows that both the germ cells and the soma of any generation are derived from the germ cells alone of the preceding generation. No modification in a somatic cell of the parent could, therefore, cause a corresponding modification in the soma of the child; because the soma of the child is descended from the parental germ cells. In the case of the athlete the enlarged muscles would mean modification in the soma, but this modification would not appear in his child because only the germ cell is inherited, not the soma.

Modifications of the soma are of two kinds: first, those produced by the effect of the environment upon the organism; and second, those resulting from the reac

Metcalf, op. cit., p. 73.

tion upon itself of the activity of the animal or plant." The occupation of a blacksmith is one which develops the muscles of the arm by the continuous and vigorous form of exercise of hammering iron. If the blacksmith has a son who becomes a bookkeeper, does the son have any stronger right arm than he would have had if his father had been an office clerk? Certainly the size of a muscle is increased by use, and decreased size results from disuse. Are these effects inherited by the offspring? One point must be noted carefully: the fact that the blacksmith does develop strong muscles as a result of the exercise shows that he must have an inborn capacity for developing strong muscles by exercise. But if the blacksmith inherited from his parents the ability to develop strong muscles by exercise, his son in turn would inherit from him the same ability. Exercise or the lack of it would therefore only bring out the latent tendency or simply leave the natural tendency to work itself out. The innate capacity would be inherited, not the accentuated development induced by exercise. The child is not the child of the biceps muscle of the parent, but the child of the germ cells of the parent. The biceps muscle of the parent has little to do with these germ cells. How therefore, could the use of biceps muscle in the arm of the parent so affect the offspring that he would have stronger biceps than if his parent had not developed his own through exercise? There is little evidence to support the doctrine of transmission of acquired characteristics.

But if there is no direct transmission of the individual modifications produced by environment, wherein does the importance of function and environment consist?