The greater precocity of girls is another aspect of the biological factor in sexual mental differences. It is a psychic as well as a physical fact. This has been shown conclusively by careful investigation in many parts of the civilised world and notably in America, where the school system renders such sexual comparison easy and reliable at all ages. There can now be no doubt that a girl at, let us say, the age of fourteen is on the average taller and heavier than a boy at the same age, though the degrees of this difference and the precise age at which it occurs vary with the individual and the race. Corresponding to this is a mental difference; in many branches of study, though not all, the girl of fourteen is superior to the boy, quicker, more intelligent, gifted with a better memory. Precocity, however, is a quality of dubious virtue. It is frequently found, indeed, in men of the highest genius; but, on the other hand, it is found among animals and among savages, and is here of no good augury. Many observers of the lower races have noted how the child is highly intelligent and well disposed, but seems to degenerate as he grows older; In the comparison of girls and boys, both as regards physical and mental qualities, it is constantly found that while the girls hold their own, and in many respects more than hold their own, with boys up to the age of fifteen or sixteen, after that the girls remain almost or quite stationary, while in the boys the curve of progress is continued without interruption. Some people have argued, hypothetically, that the greater precocity of girls is an artificial product of civilisation, due to the confined life of girls, produced, as it were, by the artificial overheating of the system in the hothouse of the home. This is a mistake. The same precocity of girls appears to exist even among the uncivilised, and independently of the special circumstances of life. It is even found among animals also, and is said to be notably obvious in giraffes. It will hardly be argued that the female giraffe leads a more confined and domestic life than her brother.