Early in Suddenly Last Summer Williams lets the play, through Mrs. Venable's narrative, take two of its main characters, Sebastian and his mother, to the Galapagos Islands to witness a grotesque dance of nature: newly hatched sea-turtles hurry from the beach to the protection of the sea, under attack by "flesh-eating birds." From the crow's nest of the ship, Sebastian spends the day viewing the black beach of the volcanic island, literally crawling with life, under attack by the carrion birds, "birds that made the sky almost as black as the beach!" It is a day of attack, killing, and eating, a day that, for Sebastian, defines his view of life and his view of God.

According to Brian Parker, the Encantadas material was one of the last elements that Williams added to the play; it only first appears in drafts associated with pre-production work on the script, when Suddenly Last Summer had been combined with Something Unspoken under the title Garden District.

Herman Melville wrote "The Encantadas, Or the Enchanted Islands" as a series of short travel sketches for Putnam’s Magazine in the March, April, and May 1854 issues. The stories were later collected as part of Piazza Tales, which was published in 1856.

"The Encantadas" relates a tour of the Galapagos Islands as remembered by Melville from his sailor days (he had long since given up the sea). In a series of ten sketches, Melville narrates an island-by-island travelogue of this equatorial landsite, which was later made famous by Charles Darwin, whose visit to the Galapagos prompted his early work in evolutionary theory.

Two critics have discussed the relation of Melville's island sketches to Suddenly Last Summer. James R. Hurt's "Suddenly Last Summer: Williams and Melville," connects the "Sketch Second," as Melville titled his pieces, to the play's moral view.

Melville's "Sketch Second" discusses turtles, but not the turtle's birth and their being killed by predators, the sex and violence, that Sebastian observes in Williams's play. Melville simply describes the turtles of the Encantadas:

   In view of the description given, may one be gay upon
   the Encantadas? Yes: that is, find one the gayety, and he will
   be gay. And indeed, sackcloth and ashes as they are,
   the isles are not perhaps unmitigated gloom.
   For while no spectator can deny their claims to a
   most solemn and superstitious consideration,
   no more than my firmest resolutions can decline to behold
   the spectre-tortoise when emerging from its shadowy recess;
   yet even the tortoise, dark and melancholy as it is upon the
   back, still possesses a bright side; its calapee or breastplate
   being sometimes of a faint yellowish or golden tinge.
   Moreover, every one knows that tortoises as well as turtle
   are of such a make, that if you but put them on their backs
   you thereby expose their bright sides without the possibility
   of their recovering themselves, and turning into view the
   other. But after you have done this, and because you
   have done this, you should not swear that the tortoise
   has no dark side. Enjoy the bright, keep it turned up
   perpetually if you can, but be honest and don't
   deny the black. Neither should he who cannot turn
   the tortoise from its natural position so as to hide
   the darker and expose his livelier aspect, like a great
   October pumpkin in the sun, for that cause declare
   the creature to be one total inky blot. The tortoise
   is both black and bright.
            - Sketch Second

James Hurt discusses the above passage not in terms of nature writing but in terms of a moral vision: Melville's view of the world, Hurt argues, is essentially neutral. There is a black side and a bright side to both turtles and to life, and people who mistakenly view life as only one way or the other are opening themselves up to disappointment or destruction. Melville's Ahab views his great white whale not as an essentially neutral beast, but as an embodiment of evil: Ahab's warped vision ultimately leads to his death. Sebastian, too, argues Hurt, after viewing the black carrion sea birds attacking and eating newly hatched turtles, comes to the conclusion that he has seen the face of God.

"Sebastian's fascination with Melville's account,” Hurt writes, “is consistent, then, with his own fascination with the primeval and with his own vision of the evil face of God. But ironically Sebastian does not see the other theme of The Encantadas: the theme that the universe will be ‘one total inky blot’ for him who sees it thus. And ironically the world which Sebastian sees mirrored in the spectacle of the turtles and the birds will turn and devour him as it devoured the turtles.”

Another critic, Carol F. Reppert, has looked at another sketch in The Encantadas and thinks she has found a similarity between Catharine and a character in Melville’s Sketch Eight, “Norfolk Isle and the Chola Widow.” In the narrative, Melville’s crew discovers a woman shipwrecked on the Norfolk Island in the Encantadas. They rescue her, and Melville tells her story of abandonment, loss, and despair. Reppert does not successfully align the story of the Chola widow with that of Catharine Holly, although her observation that Catharine’s story is too often accepted simply at face value is a good one.