security in tortoises

In that key letter to the Harpers of November 24, 1853 might Melville be channeling the optimism of his transplanted Peruvian tortoise hunters?  Retrospectively, even though publication of "Norfolk Isle and the Chola Widow" in Putnam's was a good four months away.  Or prospectively if you run with security prospective as metaphor.  Either way, however much or little he had written so far about Hunilla, the verbal similarities hint of a close identification with her predicament. 

Writing as hunting?  as in "hunt the flying herds of themes" perhaps, "the finest line" says George Monteiro in the first of Melville's manuscript Camoens poems.  Buffaloes there, tortoises here.

Melville to the Harpers in November 1853:

Meanwhile, it would be convenient, to have advanced to me upon it $300. — My acct: with you, at present, can not be very far from square. For the above-named advance — if remitted me now — you will have security in my former works, as well as security prospective, in the one to come, (The Tortoise-Hunters) because if you accede to the aforesaid request, this letter shall be your voucher, that I am willing your house should publish it, on the old basis — half-profits. 

--The Letters of Herman Melville, edited by Merrell R. Davis and William H. Gilman (Yale University Press, 1960) pages 164-165.

Hunilla, Felipe, and Truxill on Norfolk Isle likewise look to tortoises as future security.  Melville wanted cash for literary tortoise hunters; his tortoise hunters want a ride home:

Still, they thought they had, in another way, ample pledge of the good faith of the Frenchman. It was arranged that the expenses of the passage home should not be payable in silver, but in tortoises; one hundred tortoises ready captured to the returning captain's hand. These the Cholos meant to secure after their own work was done, against the probable time of the Frenchman's coming back; and no doubt in prospect already felt, that in those hundred tortoises— now somewhere ranging the isle's interior —they possessed one hundred hostages.
-- Putnam's Monthly Magazine - April 1854