Plato, Laws 708b

οὐ τοίνυν εὔκολος ὁμοίως γίγνοιτʼ ἂν ὁ κατοικισμὸς ταῖς πόλεσιν, ὅταν μὴ τὸν τῶν ἐσμῶν γίγνηται τρόπον, ἓν γένος ἀπὸ μιᾶς ἰὸν χώρας οἰκίζηται, φίλον παρὰ φίλων, στενοχωρίᾳ τινὶ πολιορκηθὲν γῆς ἤ τισιν ἄλλοις τοιούτοις παθήμασιν ἀναγκασθέν. ἔστιν δʼ ὅτε καὶ στάσεσιν βιαζόμενον ἀναγκάζοιτʼ ἂν ἑτέρωσε ἀποξενοῦσθαι πόλεώς τι μόριον· ἤδη δέ ποτε καὶ συνάπασα πόλις τινῶν ἔφυγεν, ἄρδην κρείττονι κρατηθεῖσα πολέμῳ. ταῦτʼ οὖν πάντʼ ἐστὶ

It would not be equally easy for States to conduct settlements in other cases as in those when, like a swarm of bees, a single clan goes out from a single country and settles, as a friend coming from friends, being either squeezed out by lack of room or forced by some other such pressing need. At times, too, the violence of civil strife might compel a whole section of a State to emigrate; and on one occasion an entire State went into exile, when it was totally crushed by an overpowering attack.