social life without sex can readily be imagined. While the differences between the sexes have been mainly elaborated through the differences of reproductive function, yet these differences have come to be fundamental to the whole nature of the organism. In the higher animals, therefore, the sexes differ profoundly in many ways from each other. Biologists tell us that the chief difference between the male and female organism is a difference in metabolism, that is, in the rapidity of organic change which goes on within the body. In the male metabolism is much more rapid than in the female; hence the male organism is said to be more katabolic. In the female the rapidity of organic change is less; hence the female is said to be more anabolic. Put in more familiar terms, the male tends to expend energy, is more active, hence also stronger; the female tends more to store up energy, is more passive, conservative, and weaker. These fundamental differences between the sexes express themselves in many ways in the social life. The differences between man and woman, therefore, are not to be thought of as due simply to social customs and usages, the different social environment of the two sexes, but are even more due to a radical and fundamental difference in their whole nature. The belief that the two sexes would become like each other in character if given the same environment is, therefore, erroneous. That these differences are original, or inborn, and not acquired, may be readily seen by observing children of different sex. Even from their earliest years boys are more active, restless, energetic, destructive, untidy, and disobedient, while little girls are quieter, less restless, less destructive, neater, more orderly, and more obedient. These different innate qualities fit the sexes naturally for different functions in human society, and there is, therefore, a natural division of labor between them from the first. Indeed, the division of labor between the two sexes may be said to be the fundamental division of labor in human society. The causes which produce sex in the individual are not known to any extent and are probably beyond the control of man. In each species the relative number of the two sexes is fixed by nature, probably through some obscure working of natural selection, and in practically all of the higher species of animals, man included, the number of the two sexes is relatively equal. In human society much depends upon this relative numerical equality of the two sexes. Hence it can be readily seen that it is fortunate that man does not know how to control the sex of offspring, for if he did the numerical equality of the two sexes might be disturbed and serious social results would follow.