Despite the abundance of the material and the opportunities for observation, we are at present without decisive evidence as to the distinctiveness of sex in any effective way during childhood. Here, as elsewhere, we have to guard ourselves against the influences of nurture in the widest sense of the word; as when, to take an extreme case, we distinguish between the boy and the girl because the hair of the one is cut and of the other is not. The natural, as distinguished from the nurtural, distinctions at this period are probably much fewer than is supposed. It is asserted--to take physical characters first--that the girl of ten gives out in breathing considerably less carbonic acid than her brother of the same age, thus foreshadowing the difference between the sexes which is recognized in later years. If this fact be critically established it is of very great interest, showing that the sex distinction effectively makes its presence felt in the most essential processes of the body. But we should require to be satisfied that the observations were sufficiently numerous, and were made under absolutely equal conditions, and with due allowance for difference in body-weight. They would be the more credible if it were also shown that the number of the red blood corpuscles were smaller in girls than in boys in parallel with the difference between the sexes in later years. Children of both sexes have fewer red blood corpuscles in a given quantity of blood and a smaller proportion of the red colouring matter, or haemoglobin, than adults. Women have very definitely fewer red blood corpuscles than men, and a smaller proportion of haemoglobin, and their blood is more watery. According to one authority this difference in the haemoglobin can be observed from the ages of eleven to fifty, but not before. The specific gravity of the blood is found to be the same in both sexes before the fifteenth year. Thereafter, that of the boy's blood rises, and between seventeen and forty-five is definitely higher than in women of the corresponding age. It thus seems quite clear that, as we should expect, these differences in the blood, which are certainly, as Dr. Havelock Ellis says, fundamental, make their appearance definitely at puberty--a fact which supports the view that fundamental differences of practical importance between the two sexes before that age are not to be found. Careful comparative study of the pulse of children is hitherto somewhat inconclusive, though it is well known that the pulse is more rapid in women than in men.