Sends a short résumé of his trip on the chance that it will arrive in England earlier than longer letter  which he hopes to send by surer means.
He is "incessantly occupied by new and most interesting animals" and thinks he will be able to do some original work in natural history.
2 Days sail SW of S
My dear Father
I have a long letter, all ready written, but the conveyance by which I send this is so
uncertain.—that I will not hazard it, but rather wait for the chance of
meeting a homeward bound vessel.— Indeed I only take this opportunity as
perhaps you might be anxious, not having sooner heard from me.— All day long
we have been in chace of a packet bound to Rio, & have this evening overtaken
her, tomorrow a boat will go on board of her & this letter will be conveyed to
Rio & from thence to Shrewsbury or to the fire.— We have had a most
prosperous quick & pleasant voyage.— At first.—indeed till
the Canary Islands.—I was unspeakably miserable from sea sickness &
even now, a little motion makes me squeamish.— We did not stop at Madeira,
owing to its blowing fresh, & at the Canary Islands, they wanted to put us in
strict quarantine for 12 days— Sooner than submit to that, we sailed
to Cape de Verds, & arrived at St. Jago on the 16
Natural History goes on excellently & I am incessantly occupied by new & most interesting animals.— There is only one sorrowful drawback, the enormous period of time before I shall be back in England.— I am often quite frightened when I look forward.— As yet everything has answered brilliantly. I like every body about the ship, & many of them very much.— The Captain is as kind as he can be.— Wickham is a glorious fine fellow.— And what may appear quite paradoxical to you, is that I literally find a ship (when I am not sick) nearly as comfortable as a house.— It is an excellent place for working & reading, & already I look forward to going to sea, as a place of rest, in short my home.— I am throughily convinced, that such a good opportunity of seeing the world, might not again for a century.— I think, if I can so soon judge.—I shall be able to do some original work in Natural History.— I find there is so little known about many of the Tropical anima<ls.>
The effect of my sending this letter will <be> to spoil my longer one.—but I was determined not to lose any opportunity (at Cape Verds there was none) & it is doubtful how long it will be before we arrive at Rio.— The Albrolhos banks on coast of Brazil may last us some time.—
As yet I have not felt the heat more than in England.— In about a week it will be widely different.— You will always find my letters home very badly written, as I am exactly in case of having half an hours talk, & then it would be a struggle what should come out first.— This delay in letters will be a lesson not too soon to expect letters.
Give my very best love to everybody & believe me, my dearest Father, Yours | Most affectionate Son | Charles Darwin