In parshas ויצא, we find our patriarch Jacob shepherding Laban’s flocks. The Torah recounts the story of Jacob’s salary in great detail. Laban and Jacob agreed that Jacob should get “every speckled and dappled lamb, every brownish lamb among the sheep, and the dappled and speckled among the goats”. According to most of the classic commentators, the agreement applied to the newly born only. Those that were already born stayed under Laban’s ownership. In order to prevent disagreements, Laban removed from Jacob’s herd all the speckled and dappled sheep and goats, and gave them to his sons who were to travel three days journey in the opposite direction of Jacob. This in turn diminished the likelihood of speckled and dappled animals to be born in Jacob’s herd. But Jacob came up with a clever trick, to increase the number of speckled and dappled newborns:
וַיִּקַּח לוֹ יַעֲקֹב מַקַּל לִבְנֶה לַח וְלוּז וְעַרְמוֹן וַיְפַצֵּל בָּהֵן פְּצָלוֹת לְבָנוֹת מַחְשֹׂף הַלָּבָן אֲשֶׁר עַל הַמַּקְלוֹת:וַיַּצֵּג אֶת הַמַּקְלוֹת אֲשֶׁר פִּצֵּל בָּרֳהָטִים בְּשִׁקֲתוֹת הַמָּיִם אֲשֶׁר תָּבֹאןָ הַצֹּאן לִשְׁתּוֹת לְנֹכַח הַצֹּאן וַיֵּחַמְנָה בְּבֹאָן לִשְׁתּוֹת:וַיֶּחֱמוּ הַצֹּאן אֶל הַמַּקְלוֹת וַתֵּלַדְןָ הַצֹּאן עֲקֻדִּים נְקֻדִּים וּטְלֻאִים:וְהַכְּשָׂבִים הִפְרִיד יַעֲקֹב וַיִּתֵּן פְּנֵי הַצֹּאן אֶל עָקֹד וְכָל חוּם בְּצֹאן לָבָן וַיָּשֶׁת לוֹ עֲדָרִים לְבַדּוֹ וְלֹא שָׁתָם עַל צֹאן לָבָן:וְהָיָה בְּכָל יַחֵם הַצֹּאן הַמְקֻשָּׁרוֹת וְשָׂם יַעֲקֹב אֶת הַמַּקְלוֹת לְעֵינֵי הַצֹּאן בָּרֳהָטִים לְיַחְמֵנָּה בַּמַּקְלוֹת:וּבְהַעֲטִיף הַצֹּאן לֹא יָשִׂים וְהָיָה הָעֲטֻפִים לְלָבָן וְהַקְּשֻׁרִים לְיַעֲקֹב:
Jacob then took himself a moist rod of poplar and hazel and chestnut. He peeled white strippings on them, laying bare the white of the rodsAnd he stuck the rods which he had peeled, in the runnels in the watering pools, to which the flocks would come to drink, facing the flocks, so they would become heated when they came to drink.Then the flocks became heated by the rods and the flocks gave birth to ringed ones, speckled ones and dappled ones.Jacob segregated the lambs and he put the face of the flocks toward the ringed, and all the brownish ones among Laban’s flocks; he made separate herds of his own and did not mingle them with Laban’s flock’s.Whenever it was mating time for the early-bearing flocks, Jacob would place the rods in the runnels, in full view of the flock to stimulate them among the rods.But when the sheep where late-bearing, he would not emplace; thus, the late-bearing ones went to Laban and the early-bearing ones went to Jacob.
Another way of interpreting the verses is that Jacob’s agreement with Laban was on all the speckled and dappled animals. The question according to this pshat is: who separated the speckled sheep from the rest of the herd and why. According to Sforno this was done by Laban, and this was the first in a series of cheats committed by Laban. But according to ibn Ezra, this was done by Jacob himself and “his sons” the verse refers to, are Reuben and Simon. Ibn Ezra subsequently rejects this pshat, on the basis of Reuben being too young (7 years old) to shepherd.
Either way, what Jacob seems to be doing here, and that is how almost all of the classical commentaries understand it (including Rabbi Akiva in Midrash Tanchuma) is using a method that’s based on the theory of "Maternal Impression" (I’m indebted to DovBear for teaching me the technical term for this). According to this theory, something seen at the time of conception can influence the physical nature of the developing offspring.
The problem with this is that this was scientifically disproven. According to modern science, what the mother observes has no bearing on the fetus’ phenotype.
Now, we can simply say that although Jacob believed that what he is doing is working, all that was really working was Mendel’s Laws of Inheritance. That is actually what R. Shimshon Refael Hirsch say.
However, as DovBear pointed out to me, this doesn’t fit the Torah’s story well. On its face, the Torah is telling us that what Jacob did, actually provided results. Another fellow twiterer (or is it tweeter?) wrote the following:
"Seems if we follow text and what Yaakov said - God made the sheep change colors in Yaakov's favor - the "barber poles" - an aphrodisiac".
In other word's: the rod's served as an aphrodisiac and was not connected with the cange of colors in the animals - that was a miracle.
But is it really true that observable inheritance was disproved? Nowadays I think there is no doubt that it is not true. However in the late 1800s it was still standard science. Here is Shadal’s take on the matter:
While Dr. Blondel’s pshat is very nice, I’m not sure if I agree with his interpretation of וישם דרך שלשת ימים. It seems somewhat forced to me. On the other hand, the dream Jacob saw supports his pshat. For a full analysis of this Shadal by R. Josh Waxman, see here.