This sentence would imply that Darwin thought it hopeless to rescue Sir J.Franklin's expedition.

If so, the letter must be, at least, as late as 1850.

If the eighteen expeditions mentioned above are "search expeditions," it would also bring the date of the letter to 1850.) LETTER 25.

TO RICHARD OWEN.

Down [March 26th, 1848].
My dear Owen I do not know whether your MS.