Darwin also set himself the task of showing _how_ evolution might have taken place. He pointed to the influence of the environment, to the effects of use and disuse, and to natural selection. It is to the last theory that his name is especially attached. He appealed to a fact familiar to everyone, that no two individuals are identical and that some of the differences that they show are inherited. He argued that those individuals that are best suited to their environment are the most probable ones to survive and to leave most offspring. In consequence their descendants should in time replace through competition the less well-adapted individuals of the species. This is the process Darwin called natural selection, and Spencer the survival of the fittest. Stated in these general terms there is nothing in the theory to which anyone is likely to take exception. But let us examine the argument more critically. [Illustration: FIG. 71. Series of leaves of a tree arranged according to size. (After de Vries.)] If we measure, or weigh, or classify any character shown by the individuals of a population, we find differences. We recognize that some of the differences are due to the varied experiences that the individuals have encountered in the course of their lives, i.e. to their environment, but we also recognize that some of the differences may be due to individuals having different inheritances--different germ plasms. Some familiar examples will help to bring home this relation. If the leaves of a tree are arranged according to size (fig. 71), we find a continuous series, but there are more leaves of medium size than extremes. If a lot of beans be sorted out according to their weights, and those between certain weights put into cylinders, the cylinders, when arranged according to the size of the beans, will appear as shown in figure 72. An imaginary line running over the tops of the piles will give a curve (fig. 73) that corresponds to the curve of probability (fig. 74). [Illustration: FIG. 72. Beans put into cylindrical jars according to the sizes of the beans. The jars arranged according to size of contained beans. (After de Vries.)] [Illustration: FIG. 73. A curve resulting from arrangement of beans according to size. (After de Vries.)] If we stand men in lines according to their height (fig. 75) we get a similar arrangement. [Illustration: FIG. 74. Curve of probability.] [Illustration: FIG. 75. Students arranged according to size. (After Blakeslee.)]