guarantees the persistence of the same characteristics. It secures the stability of the race. The steadying influence of heredity has been corroborated by an abundance of statistics. The conservative tendency of heredity always works in the interest of mediocrity. It is constantly operating to bring men and women back to an average type. This may be illustrated from evidence with regard to height. It was found that fathers 72 inches in height had sons whose average height was 70·8 inches; while fathers 66 inches tall had sons with a mean height of 68·3 inches. In each case the average height of the sons showed a tendency to return to the normal. The fact to which this evidence bears witness is true of all human powers and capacities. The dominant tendency in heredity is for men to revert to the normal and average. What then has prevented heredity from reducing all mankind to the same dead level? The answer is found in the fact of variation.
§ 4. The Principle of Change. Variation, as we have seen, is simply one aspect of heredity. It is the tendency to diverge, as contrasted with the tendency to persist. Both tendencies spring from the same germ-plasm. The changes in individuals are really expressions of the vitality of the germ-plasm, just as their persistent similarity is. To the fact of variation is due the diversity of the world of life. No two living creatures are precisely alike. Every fir differs from its neighbour, and no two oak-leaves are exactly similar. Much greater are the differences between human beings, even though they be of the same family. Variation is responsible
