As the story goes in this part of Genesis 30:
Jacob asks for all the black sheep and striped and spotted goats as payment for services rendered -32. This makes it easy to tell who owns what -33.
Laban removes all such before the division and sends them away-35. Sharp practice.
But Jacob had the flock mate in front of striped branches, producing offspring that were ‘streaked speckled and spotted’ -37-39. I will say ‘marked’ from now on.
He gets the same results by having his flock produced by these methods facing those of Labans flock which were streaked or black. This produces more of the same, which Jacob claims - 40 I thought all those of Laban’s had been sent away in 35, but that’s another question.
It looks very much as if this is meant to produce more of what is wanted. But we know that doesn’t happen that way in real life. Note this is all given as a purely natural description, no suggestion that God is taking an interest.
Are you saying that no more marked goats or black sheep were produced than would normally occur? That would make sense, certainly, but this story seems to say the look-at-the-stripes trick does work. Jacob refines the trick in 41-42 by only working it with strong animals, so the marked goats and black sheep produced are all of good stock.
The heart of the whole thing seems to be 39. Does it mean all the goats were marked when born, and so Jacob’s trick worked? The separation you mention in 40 just looks like Jacob claiming the result, rather than an evidence his trick did not work.
I find this all hard to believe. What am I missing?