Genèse 30:40

And the lambs did Jakob set apart, and place in front of the flocks; all the various coloured and the black among Laban's sheep he set for himself a flock apart, and did not mix them with the sheep of Laban.

AND JACOB SEPARATED THE LAMBS. Rashi comments: “Those sheep that were thus born spotted on the ankles and speckled, he separated and set apart by themselves, thus forming them into a separate flock. The spotted flock he led in front of the ordinary sheep so that the faces of the sheep that followed behind them were gazing at them. This is what Scripture means in saying, And he set the faces of the flocks towards the spotted; the faces of the sheep were directed towards the spotted animals and towards all that were brownish which were found amongst Laban’s sheep. And he put his own droves by themselves, and set them not with Laban’s flocks, as I have already explained.” This is the Rabbi’s [Rashi’s] language. But his words here are not correct. For why did Jacob separate the spotted lambs so that there did not remain in Laban’s flock any speckled or brownish ones, neither in the sheep nor in the goats? And if those that he separated were the ringstraked, speckled and spotted which the sheep had given birth to and which belonged to him, and it was from them that he made this spotted flock, why did he separate only the lambs and did not take also the he-goats and she-goats which were born spotted and make from all of them this spotted flock which he led before the sheep? Moreover, Scripture makes no mention of the fact that brownish ones were born. And again, according to the opinion of the Rabbi [Rashi], there were no ringstraked and spotted among the lambs for these were not his hire. Only the brownish were, and for the brownish he had made no sticks.
But the explanation of the verse is that Jacob separated the lambs from the goats and made from them a separate flock. Now he had a flock of brownish lambs and a flock of spotted and speckled goats. He then had the faces of all the flocks — of the lambs and of the goats — directed towards the ringstraked and towards all the brownish which were in the flock of Laban, since he put the ringstraked before the goats and all the brownish before the lambs, this being in accordance with the opinion of the earlier Rabbis, [Rashi and Ibn Ezra, as explained above], or, according to [Ramban’s] opinion, the ringstraked and all the brownish before the lamb. For the purpose of the separation of the lambs from the goats was on account of the brownish which were his hire from the lambs alone.
The correct interpretation appears to me to be that Scripture is saying, Jacob separated [only] the lambs and set the faces of the flocks, which refer to the above mentioned lambs, to the ringstraked and all the brownish in the flock of Laban, and he put his own droves of the ringstraked and the brownish by themselves, and he set them not with Laban’s flocks, for these, [the ringstraked and the brownish], were his hire. And the meaning of the expression, in the flock of Laban, is that he did so with all of Laban’s flocks but not that they belonged to Laban since the ringstraked among the lambs were Jacob’s.
Now do not ask why Scripture at first says “lambs” and then says “the faces of the flocks” rather than “the faces of the lambs”, for it is normal for Scripture to express itself this way. In this section there is a similar case in connection with the mountain of Gilead.. “The mountain” mentioned at the beginning of the verse is the “mountain of Gilead” mentioned at the end. And the reason Jacob did this with the lambs more than with the goats is that there were no brownish ones among the sticks he put up. It is possible that he knew that because of their heaviness, it is natural for lambs to require many signs to arouse them — more than the light-weight he-goats require.

והכבשים הפריד יעקב, the spotted and striped ones Yaakov had already previously removed from Lavan’s flocks during the first and second year. He now let Lavan’s spotted and striped sheep see the females of the young ones to encourage them to become pregnant with newly born lambs of the skin pattern which they had recently seen while in a state of stimulation. In order to reinforce the imagination of the sheep to be impregnated, he planted the sticks with the peeled parts near the troughs to enhance the chances of the next crop of lambs to display the skin patterns he hoped for.

אל עקוד וכל חום בצאן לבן, this was after Lavan had already reneged and changed the rules of the contract by allocating to Yaakov newly born lambs with different skin patterns than had been agreed to originally.

והכבשים הפריד יעקב, he separated the females from the goats whether male or female, not watering the two species at the same time, in order to prevent goats with undesirable skin patterns being born. None of the goats that were not black belonged to Yaakov [according to Joseph Kimichi’s commentary, there was nothing wrong in what Yaakov did or failed to do, since these goats never had displayed such skin patterns, so why should Yaakov encourage their being born with new skin patterns. Ed.] Yaakov watered the goats elsewhere where they did not observe any sticks or branches that could arouse their imagination. Yaakov did not have any black paint which would have enabled him to paint sticks black. Even if he had possessed such paint, his stratagem would have become public knowledge as naturally black sticks or branches did not exist in that region. He therefore had to content himself with increasing his share of the births of the sheep, foregoing any artificial measures to make the goats throw young ones that were black-skinned. [the interested reader will find that different commentators arrive at different conclusions regarding exactly what was agreed between Yaakov and Lavan, and they each explain his procedures according to their understanding of the agreement. Ed.]

וישת לו עדרים לבדו, from the lambs that were born as his share, year after year, Yaakov formed separate flocks which he kept apart seeing that they amounted to a formidable number of animals. He did not let them graze anywhere near where Lavan’s flocks were grazing. Lavan had displayed envy of Yaakov, and therefore Yaakov did not want to inflame his envy by letting him see his flocks all the time, knowing that Yaakov’s share of the newly born sheep had outnumbered those born as his own share

על צאן, as if the Torah had written עם צאן, “with the sheep.” A similar construction is found in Exodus 35,22 ויבאו האנשים על הנשים, “the men came together with the women.”