The second physiological fact to be noted is the order observed in human development. The faculties grow in a certain determined order. First, those which are needed for simple physical existence; next, those which place the child in fuller relations with Nature; and, lastly, those which link him to his fellows. As digestion is perfected before locomotion, so muscular mobility and activity exist before strength, perception before observation, affection and friendship before love. The latest work of Nature in forming the perfect being is the gift of sexual power. This is a work of development, not simply of growth. There are new organs coming into existence, and the same necessary conditions of gradual consolidation and long preparation for special work exist as in the growth of all the organs of animal life. At the age of puberty, when the special life of sex commences, the other organs of relation--skeleton, muscles, brain--are still carrying on their slow process of consolidation. ‘At eighteen the bones and muscles are very immature. Portions of the vertebræ hardly commence to ossify before the sixteenth year. After twenty, the two thin plates on the body of the vertebræ form, completing themselves near the thirtieth year. Consolidation of the sacrum commences in the eighteenth year, completing after the twenty-fifth. The processes of the ribs and of the scapula are completed by the twenty-fifth year; those of the clavicle begin to form between eighteen and twenty; those of the radius and ulna, of the femur, tibia, and fibula, are all unjoined at eighteen, and not completed until twenty-five. The muscles are equally immature; they grow in size and strength in proportion to the bones, and it is not until twenty-five years of age, or even later, that all epiphyses of the bones have united, and that the muscles have attained their full growth.’[16]