Since he has the name denoting breadth (namely Japhet), being expanded in his growth and increase, that part of the things of the world which have been assigned by nature for the use of mankind, that is to say, the earth, can no longer hold him, therefore he passes over into the other part, that is to say, the sea and the islands belonging to it. This is the literal meaning of the statement. But if we look to its inner sense, all the external blessings which are bestowed by nature, such as riches, and honour, and principalities, are lavished and poured forth in every direction on those men into whose hands they come, and are also extended widely to others who are not so much within reach; so that in a greater, or at all events, in no less a degree do they surround and hem the man in, in accordance with the greediness of the lovers of riches and glory, since they are eager for principalities, and are never satisfied because of their insatiable desires.
Japhet in Jewish Tradition
The Midrash of Philo 2:2