The dolphins are the second most intelligent species on the planet Earth, only surpassed by mice, although many outside observers don't know about the mice. They long ago knew of Earth's planned destruction and tried to communicate this to humans who misinterpreted it as "amusing attempts to punch football or whistle for tidbits." The last ever dolphin message was misinterpreted as a surprisingly sophisticated attempt to do a double-backward somersault through a hoop while whistling the "Star Spangled Banner," but was, in fact, a message. The message was "so long, and thanks for all the fish.
The dolphin has developed a rather peculiar ability, which exploits the Plural nature of their galactic sector, not dissimilar to that evolved by the Babel fish, which is this: In the picosecond before inevitable calamity, dolphins instantly wink into existence in all other possible probabilities in the Whole Sort of General Mish Mash.
The disappearance of the dolphins was reported in The Times newspaper with the title "Dolphins vanish".
This bridging between probabilities allowed the dolphins, after the initial destruction of the Earth by the Vogons and before the final, to replace an Earth that was destroyed with another one from somewhere else in the Mish Mash
The dolphins, before leaving, left a message to the humans they felt close to in the form of a glass fish bowl, on which were inscribed the words "So long, and thanks for all the fish." In the film, this was a song which they sang at the start of the film.
"For instance, on the planet Earth, man had always assumed that he was more intelligent than dolphins because he had achieved so much—the wheel, New York, wars and so on—whilst all the dolphins had ever done was muck about in the water having a good time. But conversely, the dolphins had always believed that they were far more intelligent than man—for precisely the same reasons.”
Dolphins are mentioned in:
- The Douglas Adams book The Hitchhiker's Guide to the Galaxy
- The 2005 film The Hitchhiker's Guide to the Galaxy
- The Douglas Adams book So Long, and Thanks for All the Fish