Талмуд к Берешит 1:21

Rebbi Jeremiah said, Cahana asked R. Simeon ben Laqish¹¹¹The abbreviation ריש לקיש is Babylonian. The entire paragraph has a parallel in Gen.rabba 7(6); it is mentioned in Babli Baba Qama 55a.: What is the rule for someone who mates sea animals¹¹²In this version, one speaks about aquatic mammals who do copulate.? He said to him, it is written about them (Gen. 1:21): “By their kinds¹¹³As explained in the next version, “by their kinds” is written to forbid cross-mating..” Rebbi Aḥa did not say so, but Rebbi Aḥa used to say in the name of Rebbi Simeon ben Laqish, wherever “by their kinds” is written, kilaim applies to it. Rebbi Cahana objected, it is written about the animals of the sea¹¹⁴In Gen.rabba, one reads “fish.” The verse speaks of all sea creatures, including fish, sea reptiles, and sea mammals. It is difficult to see how copulation could apply to fish. “by their kinds;” kilaim should apply to them! Rebbi Yose ben Rebbi Abun said, here did Cahana spread his net over Rebbi Simeon ben Laqish and caught him¹¹⁵Since R. Simeon had no answer to his objection.. Rebbi Jonah said, I can explain it, because of one who leads¹¹⁶Mishnah Kilaim 8:2 states that any two kinds of animals that are kilaim with one another may not be driven or led when harnessed together and may not be used for plowing together.. He brings a string and binds it to the gills of a white fish¹¹⁷Greek λεῦκος, name of a fish; cf. also λευκός “white”. and a green fish; they rub one another and spawn¹¹⁸Hence, while copulation is a notion not applicable to fish, any actions to induce cross-breeding are still forbidden. (The Babli, Baba Qama55a, does not consider the crossing of fish species, only that of marine mammals.).