On the other hand, it seems clear as regards respiration that as early as the age of twelve there are definite differences between the sexes. Several thousands of American school children were examined, and between the ages of six and nineteen the boys were throughout superior in lung capacity. The girls had almost reached their maximum capacity at the age of twelve, and thereafter the difference, till then slight, rapidly increased.[6] It appears that from eight to fifteen years of age a boy burns more carbon than a girl, the difference, however, being not great. But at puberty the boy proceeds to consume very nearly twice as much carbon per hour as his sister. Perhaps the matter need not be pursued further. It is sufficient for us to recognize that puberty is really the critical time, and that in the consideration of womanhood we may, on the whole, be justified in looking upon the problem of the girl before that age as almost identical with her brother's. Yet we must be reasonably cautious, since our knowledge is small, and there is some by no means negligible evidence of fundamental physiological differences between the sexes before puberty, relatively slight though these may be. Therefore, though on the whole we need make few distinctions between the girl and her brother, and though we are doubtless wrong in the magnitude of the practical distinctions which we have often made hitherto, yet we must remember that these are going to be different beings, and that the main principles which determine our nurture of womanhood may be recalled when we are doubtful as to practice in the care of the girl child. Physiological distinctions, we have seen, probably exist during these early years, but are of less importance than we sometimes have attached to them, and of no importance at all compared with what is to come. Psychological distinctions, we may believe, are still more dubious. For instance, it is generally believed that the parental instinct shows itself much more markedly in girls than in boys, and the commonly observed history of the liking for dolls is quoted in this connection. As this instinct bears so profoundly upon the later life of the individual, and as we may reasonably suppose the child to be the mother of the woman as well as the father of the man, the matter is worth looking at a little further. But, in the first place, it has been asserted that the doll instinct has really nothing whatever to do with the parental instinct in either sex. Psychologists, whom one suspects of being bachelors, tell us that what we really observe here is the instinct of acquisition: it really does not matter what we give the child, though it so happens that we very commonly present it with dolls; it is the lust of possession that we satisfy, and in point of fact one thing will satisfy it as well as another.