To speak of "the sexual life of the child" seems at first sight to involve a contradiction in terms. It is generally assumed that the sexual life first awakens at the on-coming of puberty (the attainment of sexual maturity of manhood or womanhood); the on-coming of puberty is regarded as the termination of childhood; in fact the term child is usually defined as the human being from the time of birth to the on-coming of puberty. But this contradiction is apparent merely, and depends on the assumption that the on-coming of puberty is indicated by certain outward signs (more especially the first menstruation and the first seminal emission), insufficient attention being paid to the long period of development which usually precedes these occurrences. And yet, during this period of preliminary development, the occurrence of certain manifestations of the sexual life is plainly demonstrable.

The period of childhood is subdivided into several sub-epochs, but the delimitation and nomenclature of these varies so much with different investigators, that to avoid misunderstanding I must first define the subdivisions which I myself propose to employ. If we regard the beginning of the fifteenth year as the termination of childhood, we may divide childhood into two equal periods, the first extending from birth to the completion of the seventh year, the second from the beginning of the eighth to the end of the fourteenth year. I shall in this work designate these two periods as the first and the second period of childhood respectively. In the first period of childhood, the first year of life may be further distinguished as the period of infancy.1 The first and second periods of childhood comprise childhood in the narrower sense of the term. The years that immediately follow the beginning of the fifteenth year I shall denote as the period of youth. Inasmuch as the symptoms of this latter come to differ from those of childhood proper, not abruptly, but gradually, the first years, at least, of youth will often come under our consideration, and I shall speak of this period of life as the third period of childhood. Although childhood in the narrower sense comprises the first and second periods only, childhood in the wider sense includes also the third period. It is hardly possible that any misunderstanding can arise if the reader will bear in mind that whenever I speak of childhood without qualification, I allude only to the period of life before the beginning of the fifteenth year. For all these periods of childhood, first, second, and third, I shall for practical convenience when speaking of males use the word boy, and when speaking of females, the word girl.

The use of this terminology must not be regarded as implying that the distinctions indicated correspond in any way to fixed natural lines of demarcation; on the contrary, individual variations are numerous and manifold. Not only does the rate of development differ in different races (in the Caucasian race, more especially, the age of puberty comes comparatively late, so that among the members of this race childhood is prolonged); but further, within the limits of one and the same race, notable differences occur. More than all have we to take into account the differences between the sexes, childhood terminating earlier in the female sex than in the male—among our own people [the Germans] this difference is commonly estimated at as much as two years. In addition, in this respect, there are marked differences between different classes of the population, a matter to which we shall return in Chapter VI.