retain what? temperature, youth, loftiness, spotlessness, softness

http://claretscience3.wikispaces.com/TOUCH

Like the great dome of St. Peter's, and like the great whale, retain, O man! in all seasons a temperature of thine own. --Moby-Dick, The Blanket (1851) 

One of these little infants, that from certain queer tokens seemed hardly a day old, might have measured some fourteen feet in length, and some six feet in girth. He was a little frisky; though as yet his body seemed scarce yet recovered from that irksome position it had so lately occupied in the maternal reticule; where, tail to head, and all ready for the final spring, the unborn whale lies bent like a Tartar’s bow. The delicate side-fins, and the palms of his flukes, still freshly retained the plaited crumpled appearance of a baby’s ears newly arrived from foreign parts. Moby-Dick, The Grand Armada

In the example above from the Grand Armada chapter of Moby-Dick, comparison with Melville's source shows definitely that retain is Melville's word--one of his own contributions, added during creative revision of the borrowed passage in Frederick Debell Bennett's A Whaling Voyage Round the Globe (Volume 2, pages 167-8), along with vivid figures also not Bennett's: the "reticule"; "Tartar's bow"; and "baby's ears."

When the substance is gone, men cling to the shadow. Places once set apart to lofty purposes, still retain the name of that loftiness, even when converted to the meanest uses. It would seem, as if forced by imperative Fate to renounce the reality of the romantic and lofty, the people of the present would fain make a compromise by retaining some purely imaginative remainder....--Pierre; Or, The Ambiguities (1852)

"... it was a marvel how, under such circumstances, these hands retained their spotlessness."  --The Confidence-Man

For ye who green or gray retain
Childhood's illusion, or but feign;
As bride and suite let pass a bier—
So pass the coming canto here.  --Clarel Part 2 Canto 35, Prelusive

From the August 1853 installment of Scenes Beyond The Western Border; and Scenes and Adventures in the Army:

Scenes Beyond the Western Border
Southern Literary Messenger 19 - August 1853

C.—"Well, well,—I wrote what pleased myself; and,—another object I have, which I did not mention: with scarce a book to read, if one did not write, I fancy the beef and pork and beans would in time form a coating round his brain,—turn it all perhaps to thick and solid skull! How is it with you, Frank? Does yours retain a slight softness?"

From Mardi: And a Voyage Thither, Volume 2 (Babbalanja Falleth Upon Pimminee Tooth And Nail):

For these Tapparians have no brains. In lieu, they carry in one corner of their craniums, a drop or two of attar of roses; charily used, the supply being small. They are the victims of two incurable maladies: stone in the heart, and ossification of the head.