Sextus Empiricus, Against the Logicians
1.117-118 = Democritus, 55 B 164 Diels, Die Fragmente der Vorsokratiker
, 4th ed., vol. II, p. 93 (tr. R.G. Bury):
"For animals," he says, "flock together with animals of a like species, as doves with doves and cranes with cranes, and so too all other irrational animals." And it is the same with things inanimate, as one may see in the case of seeds that are being winnowed and in the case of pebbles along the beaches; for in the one case, by the whirling of the sieve lentils are ranged separately with lentils, barley with barley, and wheat with wheat; and in the other case, owing to the motion of the waves, the oblong pebbles are pushed into the same place as the oblong, and the round as the round, as though the similarity in things had a certain force of attraction for them.
καὶ γὰρ ζῶα, φησίν, ὁμογενέσι ζώοις συναγελάζεται, ὡς περιστεραὶ περιστεραῖς καὶ γέρανοι γεράνοις, καὶ ἐπὶ τῶν ἄλλων ἀλόγων· ὡσαύτως δὲ καὶ ἐπὶ τῶν ἀψύχων, καθάπερ ὁρᾶν πάρεστιν ἐπί τε τῶν κοσκινευομένων σπερμάτων καὶ ἐπὶ τῶν παρὰ ταῖς κυματωγαῖς ψηφίδων· ὅπου μὲν γὰρ κατὰ τὸν τοῦ κοσκίνου δῖνον διακριτικῶς φακοὶ μετὰ φακῶν τάσσονται καὶ κριθαὶ μετὰ κριθῶν καὶ πυροὶ μετὰ πυρῶν, ὅπου δὲ κατὰ τὴν τοῦ κύματος κίνησιν αἱ μὲν ἐπιμήκεις ψηφῖδες εἰς τὸν αὐτὸν τόπον ταῖς ἐπιμήκεσιν ὠθοῦνται, αἱ δὲ περιφερεῖς ταῖς περιφερέσιν, ὡς ἂν συναγωγόν τι ἐχούσης τῶν πραγμάτων τῆς ἐν τούτοις ὁμοιότητος.
This might be 68 B 164 Diels-Kranz (unavailable to me).