To the average man the division into sexes seems so definitely a fixed scheme of nature that it comes almost as a shock to learn that in thousands of the lowest forms of life there is no such phenomenon. After a time nature seems to be experimenting to see if it offers any advantages. Plant lice have a generation in which both sexes are present and then several in which the males are missing. The hickory phylloxera has a peculiar history. In the spring females only appear and lay their eggs in a leaf ball. These eggs may be either large or small but only one sort comes from a given individual. From the large eggs females are hatched; from the small, males. After fertilization these females lay the large eggs which last over winter to renew the cycle in the spring. Other forms, like earthworms, snails and leeches, are hermaphroditic, and this condition is believed to occur in animals as high as birds and mammals. Curious and bizarre combinations exist such as the blending in one individual of the characters of both sexes, known as gynandromorphism, which is found in insects, occasionally, and birds, rarely. In such a case one side of the body has the color and other characteristics of the male; the other side, those of the female. A bullfinch has been taken with a line on the breast sharply separating the red feathers of the cock on one side from the brown feathers of the hen on the other. Neither side was sexually perfect. It has already been mentioned that the unfertilized eggs of bees develop into drones, the fertilized eggs into workers and queens. Though true hermaphrodites are not found in the highest types of life, there are all sorts of imperfect combinations and malformations which remind us of earlier forms. We are forced to the conclusion that the bisexual forms offer some marked advantages over the uni

sexual forms, and it would seem that these lie in the possibility of combining various strains of ancestry.

If we accept the suggestion that sex is determined by the presence or absence of given determiners in the germ cells we naturally anticipate that the number of boys and girls born will be equal. Curiously enough this does not seem to be quite true if our records are to be trusted. It seems that about 105 boys are born to 100 girls, the exact figures for various European countries being: England, 103.6; France, 104.6; Germany, 105.2; Spain, 108.3. If still-born infants are included the ratio is higher: Germany, 128.3; Italy, 131.1; and France, 142.2. If abortions are counted the difference appears to be even greater.1 We do not know how to explain these returns. They do not necessarily upset our faith in the determination of sex by the chromosomes, for it may be that the cells from which males will result are more active and persistent. It may be that they are less subject to untoward circumstances. Sex ratios are very uneven in many forms of life. Bees and wasps have few males. In some spe cies of nematode worms there is often less than one male to one hundred females and the females are really hermaphrodites. Hybrids of guinea fowl and pheasants produced 74 males to 13 females and crosses between different species of the same genus 72 males to 18 females. This inequality in the number of males and females persists throughout life, though women are often in the majority in the older age groups. Probably conditions of life and labor have much to do with these later differences, and they are not therefore sexual in origin. The following figures from the Census of the United States show this inequality:

1 MORGAN, T. H. Heredity and Sex, pp. 230 ff.

For purposes of analysis we may conveniently divide the differences between the sexes into three groups. We may call these: (1) the primary organs of sex; (2) secondary sexual characters; (3) activities based on sex.

Among the many species of animals it is extremely difficult for us to distinguish the sexes. In countless cases the eggs are fertilized after they leave the body of the female and practically no attention is paid to the young by either parent. With each upward step on the ladder of life the relation between old and young is both longer and more intimate. No one statement of this relationship can be satisfactory for the habits of different species vary. Among birds both sexes usually share in the incubation of eggs and the care of the young, although there are some cases in which both sexes shirk this duty and leave it to other species. When the mammals are reached the connection between mother and child has become peculiarly intimate and her body shows organic differences from that of the male. The mother must shelter and nourish the unborn child and thus certain demands are made upon it which are utterly unknown to the male.

Aside from the reproductive organs there are many physical differences commonly known as secondary sexual characters. Some of these may be merely accidental accompaniments; others seem to be directly sex-linked as if certain chromosome combinations were directly responsible. In many cases it seems that the organs of reproduction actually produce substances, known as hormones, which act as stimulants to growth and directly cause the differences between the sexes. Thus male guinea pigs and rats have been castrated and female ovaries inserted. These ovaries though merely placed under the skin lived and grew. "None of these animals developed male secondary sexual characters; the male external genitalia, for instance, remained immature, and the body assumed the form of the smaller sex, the female. The growth of hair and the deposition of fat were in the direction of the female type. The mammary glands, the nipples, and their surrounding aureolae were typically female. None of these animals showed the characteristic male sexual excitability even in the presence of a female in heat. They very commonly did exhibit the 'tail-reflex' and the 'protective-reflex,' both characteristic of the female, and they were sought by the males, though of course ineffectually. Thus, so far as the secondary sexual characters were concerned, a male animal had been converted, both structurally and functionally, into a female. . . . For this reason it is believed that the hormones which are given out by the reproductive glands, and which serve to excite the development of the secondary sexual characteristics, are not the products of the germ cells proper, the egg cells and sperm cells, but come from the interstitial cells which are in no wise concerned with reproduction." 2 It seems

2 PARKER, G. H. Biology and Social Problems, pp. 52 ff.

likely that the secretion of milk in the breast is caused by some hormone and it may be that other differences between the sexes have a like explanation. Oddly enough we cannot discover that there is any such relation in the case of insects, even though the bodies of the two sexes are often unlike.

The most obvious differences in the human sexes are: (1) the larger size and greater strength of the male; (2) the different proportions of the body, particularly in the shape of the pelvis; (3) the different distribution of hair on the body; (4) the difference in the voice after maturity, resulting from the growth of the larynx.

Studies in England as well as in America indicate that from birth "until the age of nine, boys are above girls in height and weight; at nine and ten are slightly under in height; at ten slightly under in weight; at eleven materially lower in height and weight; at fourteen boys are slightly heavier than girls; at fifteen taller than girls. In other words the pubertal acceleration of growth occurs about three years earlier in girls than boys." It is claimed by many students that the heads of boys are always larger than those of girls despite the superiority of the latter in weight and height during the early teens. The following table shows the relative growth of schoolboys and girls at Battle Creek, Michigan:

3 ELLIS, H. Man and Woman, pp. 32-36.

4 HASTINGS, W. W. In Proceedings Conference on Race Better

ment, p. 618 ff.