example—the hair on the forearm points outward and upward; on the upper arm down-ward and outward and so on throughout in the human and simian types. Every child comes into the world with a coat of rudimentary hair which is shed at once. Aside from the growth of hair on the head, including the brows and the lashes, the skin is quite free from any noticeable growth of hair for months or even years. Beginning at the age of puberty, however, the growth of hair is very much accelerated over the whole pilous surface of the body, particularly upon the face, in the axilla and over the pubic region. It is a generally recognized law of biology, that, at the period of sexual development, the hairy mammalian character becomes accentuated. The increase in the growth of hair at this time can have only one interpretation, viz., that the ancestors of man represented a very much higher degree of pilosity than is the case with man at the present time. It is interesting to note in this connection the almost universal attempt of men to rid the face of this hairy growth by various devices, either pulling the beard or shaving it. The origin of this custom of depilation probably dates back to the remote past and has been observed as a custom among both savages and civilized peoples.

b. The Voice.—In all animals the voice plays an important part in sexual and social relations. In many animals the voice seems to have almost no other function than as a sex call, or a communication between mates and between parents and young. The human subject illustrates this general biological principle in the profound changes which the voice undergoes at the time of puberty. These changes in the male subject consist in increasing the depth of the larynx, thereby increasing the length of the vocal cords which in turn modifies the pitch of the voice, usually about an octave, making it not only lower but much more pleasing in quality and greatly increased in volume.

c. Bone, Muscle and Gland.—Of incalculably greater importance than the changes described above though perhaps less noticeable to the casual observer, are those physical changes which the body undergoes during the first half of the period of adolescence. I refer to the growth of bone, of muscles and of those internal organs associated with nutrition.

The first step in these profound physical changes is a rapid growth in height that makes itself manifest about the fifteenth year. It is not at all unusual for a boy to grow from four to six inches in a year. This increase in height is very largely due to a lengthening of the thigh and leg bones. In serial homology with the thigh and leg are the bones of the arm and we find that these are undergoing an increase in length commensurate with the increase of the legs. So the boy outgrows his clothes; his coat sleeves are drawn up half way to his elbows and his trousers half way to his knees. The muscles scarcely keep pace with the bones in their growth, and tend to be flabby and to lack usual tonicity. It is difficult for the youth to hold his back straight and his shoulders back; he is awkward and ungainly in his movements and becomes easily fatigued because of the condition of his muscles. But the muscles follow immediately in their development and rapidly gain volume and tonicity, filling out the arms, legs, back and shoulders with large masses of firm muscular tissue. The growth of these muscle masses changes the dimensions of the youth and he fills out in his girths as rapidly as, in the previous period, he increased in length measurements.

All of this increase in bulk can only be accomplished by increased activity of all the nutritive processes. The appetite is practically insatiable; the boy can eat three square meals in the day and lunches between meals. If he wakes up in the night he is hungry. To accomplish the digestion and absorption of this food material, the alimentary tract throughout, and particularly the stomach is greatly increased in size. To accomplish the distribution of the food (blood) the heart also is increased in size and strength. With increased bulk of muscle and increased quantity of food we have increased oxidation in the tissues. This requires increased respiration, which demand is satisfied by rapid development of the respiratory system. The thorax increases in dimensions in all directions; it becomes deeper, broader and longer. Not only does the thorax become more capacious but also more mobile and more responsive to the varying requirements of the system.

If we are interested in the biology of all these changes, we need not go far to discover the natural causes at work to produce them. Nature is preparing in the youth a home builder; it is preparing an individual who can support and protect not only himself, but also a family. This equipment in the case of primitive man must necessarily be one of bone and brawn. While under the conditions of modern society the necessity for bone and brawn is somewhat less marked, the plan of nature is no less evident and no less interesting.

The Genital System.

a. Structural Changes.—The external genitals, besides showing the pudendal pilosity referred to above, are all greatly increased in size. The penis is increased in all of its dimensions, the testes become very much increased in size, the scrotum, probably because of the increased weight of the testes, is also lengthened.

b. Functional Changes.—The testes and associated glandular bodies gradually develop the power of forming perfect semen, capable of fertilizing the human ovum. When these organs thus become capable of procreation, the period of puberty is complete.

In this connection it is important to note that the development of the testes produces a profound effect upon both the physical and mental characteristics of the young man. This effect is produced through a substance formed in the testes and reabsorbed into the body, thus gaining access to the blood where it exerts its mysterious but profound influence. Just how this affects the mind and body will be discussed in detail in a subsequent chapter.

2. PSYCHICAL CHANGES.

Play and Work.

a. Sports.—Most of the higher animals, particularly man, and all races of men, devote a large part of the energies of the adolescent period to sports or games in which individuals contend with each other or teams of individuals contend with opposing teams in games that bring into play the various powers of the neuro-muscular system: such as alertness of all the senses, readiness and correctness of judgment, agility, speed and strength of movement. Sports might be criticised by some because they represent non-productive expenditure of energy. On the other hand, no energy ever expended by man is so highly productive of so precious a material as results from manly athletic sports. The products of these games are the substances consumed by them, paradoxical as that may at first appear. The use of brain, muscle and glands and the consumption of the cell substances of these tissues results in the development of the nerve, muscle and gland cells into a condition larger, better equipped and more responsive than before such use.

Thus, athletic sports, while they make draughts upon the nerves, muscles and glands, develop all of these tissues to a high degree of efficiency. The plan of nature in this instinctive indulgence in sports must be evident. Nature is educating and developing the male animal (man) to the highest possible degree of efficiency, so that sports, instead of being non-productive, lead to the development of structures possessing a high