Jacob did separate the lambs, such as were ring-straked and brown from the white, as it here follows. He caused the ring-straked and all the brown to go foremost, and the white to follow them, that by the continued beholding of them in the time of their conjunction, they might have their colour more imprinted upon their fancies, and thereby convey it to their young ones. He put them not unto Laban's cattle; which he did upon the same reason, lest the constant beholding of them should make them bring forth the like, i.e. single-coloured ones.
Related Commentaries of Genesis 30:40
Genesis 30:40
40 And Jacob did separate the lambs, and set the faces of the flocks toward the ringstraked, and all the brown in the flock of Laban; and he put his own flocks by themselves, and put them not unto Laban's cattle.