Adam Clarke on Genesis 30:40

Jacob did separate the lambs, etc. - When Jacob undertook the care of Laban's flock, according to the agreement already mentioned, there were no party-colored sheep or goats among them, therefore the ring-streaked, etc., mentioned in this verse, must have been born since the agreement was made; and Jacob makes use of them precisely as he used the pilled rods, that, having these before their eyes during conception, the impression might be made upon their imagination which would lead to the results already mentioned.