Bechorot 7 - Why Is Milk Kosher?
The rule that " whatever comes from kosher is kosher " includes milk. But how do we know that milk is allowed for consumption, given that other parts taken from a live animal are forbidden? Is it because mixture of meat and milk is forbidden, and therefore milk must be permitted? - No, it might be permitted only for benefit. Is it because of Solomon's words "Let the milk of goats be sufficient for your food..." - perhaps that, too, is only to sell it and buy food. Rather, it is because David brought for his brothers cheese made from milk. And if you say there too that he wanted them to sell it - is war the time for trade? Alternatively, it is because Israel is praised as land flowing with milk.
If a non-kosher fish swallowed a kosher fish, the kosher fish is still permitted to be eaten, and if a kosher fish swallowed a non-kosher fish, the non-kosher is prohibited to be eaten - provided that the fish inside was not spawned. Kosher fish lay eggs, and non-kosher spawn their young; all creatures that bear live young suckle them, while all that lay eggs gather food to feed their hatchlings, except for the bat which, though it lays eggs, nevertheless suckles its young; dolphins reproduce as humans, and some say, with humans. Which dolphins are we talking about? - Mermaids whom human males can impregnate.
Art: George Willoughby Maynard - Mermaids