Definitely formulated statistical data of these results are scarcely yet available. But we may study the action of this natural process on one great practical experiment in mental sexual differences which has been going on for some time past. At one time in the various administrations of the International Postal Union there was a sudden resolve to introduce female labour to a very large extent; it was thought that this would be cheaper than male labour and equally efficient. There was consequently a great outcry at the ousting of male labour, the introduction of the thin end of a wedge which would break up society. We can now see that that outcry was foolish. Within recent years nearly all the countries which previously introduced women freely into their postal and telegraph services are now doing so only under certain conditions, and some are ceasing to admit them at all. This great practical experiment, carried out on an immense scale in thirty-five different countries, has, on the whole, shown that while women are not inferior to men, at all events within the ordinary range of work, the substitution of a female for a male staff always means a considerable increase of numbers, that women are less rapid than men, less able to undertake the higher grade work, less able to exert authority over others, more lacking both in initiative and in endurance, while they require more sick leave and lose interest and energy on marriage. The advantages of female labour are thus to some extent neutralised, and in the opinions of the administrations of some countries more than neutralised, by certain disadvantages. The general result is that men are found more fitted for some branches of work and women more fitted for other branches; the result is compensation without any tendency for one sex to oust the other.