Genesis 30:41

Overview - Genesis 30
Rachel, in grief for her barrenness, gives Bilhah her maid unto Jacob.
Bilhah bears Dan and Naphtali.
Leah gives Zilpah her maid, who bears Gad and Asher.
14 Reuben finds mandrakes, with which Leah buys her husband's company of Rachel.
17 Leah bears Issachar, Zebulun, and Dinah.
22 Rachel bears Joseph.
25 Jacob desires to depart.
27 Laban detains him on a new agreement.
37 Jacob's policy, whereby he becomes rich.
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Genesis 30:41  (King James Version)
And it came to pass, whensoever the stronger cattle did conceive, that Jacob laid the rods before the eyes of the cattle in the gutters, that they might conceive among the rods.

whensoever the stronger cattle did conceive
As the means which Jacob used would not in general produce similar effects, nay, probably the experiment was never in any other instance tried with effect, it is more in harmony with Divine truth to suppose that he was directed by some Divine intimation; and rendered successful, if not by a direct miracle, yet at least by the Lord's giving a new and uncommon bias to the tendency of natural causes.