32 Finally, Venerable Brothers and Beloved Sons, receive, in the wellknown spirit of zeal for religion and the salvation of souls which consumes you, with docile minds and with the will to obey promptly, this, Our last but most important recommendation of all. The districts confided by the Holy See to your care and labors in order that they too may be added to the Kingdom of Christ the Lord, are for the most part vast in extent. It may thus happen that the number of missionaries belonging to your particular Institute is much smaller than your actual needs require. In this case, just as in well established dioceses members of different religious families, priests, laymen, and nuns of many different Congregations, are accustomed to come to the aid of the bishop, so you also, where there is question of spreading the Faith, of educating the native youth or other similar undertakings, ought not to hesitate to invite and to receive as companions of your labors religious missionaries, even though they be of a different Institute than your own, and also priests or others though they are members of lay Institutes. The Orders and Religious Congregations may well be proud of the missions given them among the heathen and of the conquests made up to the present hour for the Kingdom of Christ. Let them remember, however, that they do not possess the mission fields by a peculiar and perpetual right, but that they hold them solely at the discretion and pleasure of the Holy See which has both the duty and the right to see to it that these missions are well and adequately taken care of. The Roman Pontiff would not be doing his full Apostolic duty if he limited his interest solely to the distribution of missions of greater or lesser extent to one or other Institute. What is of much more importance is that he must always, and with great care, see to it that these different Institutes are sending into the regions confided to them as many qualified missionaries as are needed to carry on in a thorough manner the task of diffusing the light of the truth over the whole extent of these countries.
Source: Rerum Ecclesiae (Vatican.va)