I'm trying to recall a story where humans have died off. Dolphins are repopulating the land and a series of surgical procedures is involved. The final procedure is to close the blowhole.
There may have been a "love story" where one dolphin was going on land and the other was staying behind.
I believe that this was in a collection of short stories (possibly including "Examination Day") from the mid seventies. The story would have been targeted at seventh and eight graders.
Destiny and the Dolphins (1969) by Roy Meyers.
A nuclear holocaust destroys everything. There's only three humans left I believe and they repopulate the Earth.
This is the only review I could find of it that references what the story is about. It's the third book in the Dolphin Trilogy.
In Destiny and the Dolphins, a nuclear holocaust destroys all of civilization, leaving the three of them to repopulate the world. It is up to them to work out how they will engineer a new society.
