55 In former times the vast missionary field was not limited within the set confines of various ecclesiastical territories, nor was it entrusted to different religious institutes to be worked along with a growing. native clergy. This, as all know, generally obtains today. It even sometimes happens that some mission territories are entrusted to the members of a particular province of a religious institute. We see the utility of this, of course, since by this method the organization of Catholic missions is conveniently facilitated. This arrangement, however, may give rise to serious inconveniences, which must be remedied as far as possible. Our predecessors have touched this point in the letters, which We have already referred to. In this matter they have laid down wise norms. We repeat them here and ratify them, paternally exhorting you "to accept and comply with them religiously in keeping with your well known zeal for religion and the salvation of souls. In those territories which the Apostolic See has entrusted to your zeal to be won to Christ our Lord, it sometimes happens, since they are often very extensive, that the number of missionaries each of you has from his own religious institute is far less than what is needed. In similar circumstances, even in fully established dioceses, additional priests, brothers, and sisters from different religious families come in and help the Bishop. So, too, in the missions, do not hesitate to summon to your aid as your co-workers missionaries who are not of your own religious family, whether they be priests or belong to lay institutes. They can be called to help in spreading the Faith, to educate the native youth, and to engage in other missionary activities. Let religious orders and congregations take legitimate pride in the foreign missions entrusted to them, as well as in the harvest of souls so far won for Christ's Kingdom. But let them remember that they have not received their portion of the Lord's vineyard by a kind of private title in perpetuity. Rather they hold it at the will of the Holy See, whose right and responsibility it is to see that it is fully developed. The Roman Pontiff does not fulfill his apostolic duty merely by portioning out larger or smaller mission territories among different religious institutes. What is more important, he must make it his continual and anxious care that these institutes send into the territories entrusted to them missionaries sufficient in numbers and especially in apostolic quality to preach the Gospel successfully throughout the whole territory."
Source: Evangelii Praecones (Vatican.va)