of their favorable qualities to their progeny. Some individuals will be born with variations from the adapted ancestral type which will prove of decided advantage. These organisms will have a better chance to survive than those that happen to vary from the ancestral type in a disadvantageous direction. And so, in the course of time, as the climate changes in that locality, the plastic group of living plants and animals will be modified and will undergo change from their original characteristics. As intermediate forms perish and new variations appear giving greater advantage in ability to meet the conditions of life, the present inhabitants will differ more and more from the original inhabitants so that if we were to see both side by side we should be led to think that we were observing two quite distinct forms of life instead of related forms. But if we could see the intermediate forms, we could reconstruct the series and understand how one form was descended in almost direct line from another form now quite extinct and with different structure and function. Although the intermediate forms connecting a living group of animals with an older form have long since passed from the surface of the earth, naturalists are able to reconstruct the series of descent with a remarkable degree of accuracy because Nature has preserved for us in the form of fossils the shape and mold in which these creatures were cast millions of years ago. This, in brief, is Darwin's famous doctrine. of the origin of the species by descent under the influence of natural selection. It is the core of the theory of Evolution.

Let us now summarize the points that have been made in this chapter:

(1) The amount of food and space upon the earth for

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. To clearly understand this problem, it is necessary to resort to the theory of inheritance.

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.7b 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, 7-a Metcalf, op. cit., p. 77. 7-b Ibid., p. 73.

7a

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

8

SOMA

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.s

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

8 Metcalf, op. cit., p. 73.