Charles Darwin was born on 12 February 1809. Here are a few excerpts from letters Darwin wrote or was sent on his birthday:
Two letters written by his sister Susan while he was on the Beagle voyage:
I must begin this folio by wishing you joy my dear Charley of being this day23 years old; and I heartily hope it may find you happy, and that you may continue so for many and many a year to come.
Darwin, S. E. to Darwin, C. R., 12 Feb 1832. See the letter
It is always my fate to write to wish you joy upon your Birthday; but thank goodness this is the last I can do so, into foreign parts.— You are today 27—and I hope all the rest of your life you may spend very happily amongst us.—
Darwin, S. E. to Darwin, C. R., 12 Feb 1836. See the letter
One sent to his cousin, William Darwin Fox, as Darwin was writing Origin of Species:
My abstract is the cause, I believe of the main part of the ills to which my flesh is heir to; but I have only two more chapters & to correct all, & then I shall be a comparatively free man.—
Darwin, C. R. to Fox, W. D., [12 Feb 1859]. See the letter
And here are a few birthday letters we haven’t published yet:
Letter 10841: To Adrian Anthoni von Bemmelen and H. T. Veth, 12 February 1877
I received yesterday the magnificent present of the Album, together with your letter. I hope that you will endeavour to find some means to express to the two hundred en seventeen distinguished observers and lovers of natural science, who have sent me their photographs, my gratitude for their extreme kindness. I feel deeply gratified by this gift, and I do not think that any testimonial more honourable to me could have been imagined. I am well aware that my books could never have been written and would not have made any impression on the public mind, had not an immense account of material been collected by a long series of admirable observers; and it is to them that honour is chiefly due.
I suppose that every worker at science occasionally feels depressed, and doubts whether what he has published has been <wor>th the labour which it has cost him; <but for the remaining years of> my life, wh<enever> <I want cheering,> I will look at the <portraits of my> distinguished co-w<orkers> <in the> field of science, and remem<ber> <their> generous sympathy. When I die the album will be a most precious bequest to my children. I must further express my obligation for the very interesting history contained in your letter of the progress of opinion in the Netherlands with respect to Evolution, the whole of which is quite new to me. I must again thank all my kind friends, from my heart for their ever memorable testimonial, and I remain, | Sir | Your obliged and grateful | Servant | Charles R. Darwin
Letter 11355: From W. D. Fox, 12 February 
Broadlands, | Sandown, I.W.
My dear old Friend
For surely I may call you so, as you and one other, are all left of our friendships at a time when life was glorious. It is not bad now tho’ I am nearly 73—and few have been happier than I have. I rejoiced to hear from your Son at Southampton that you had all a most happy family party at Christmas— (How I should have liked a peep at you all from behind a curtain). May you have many happy Christmas’s—and therefore many happy returns of your Birthday.
I saw one day, that you were born in 1809— I am glad to hear you are four years younger than myself— I always thought you were only two.
We were so glad to hear of W Es marriage. He is just the man to make a splendid Husband—and seems as happy as possible. You and I have been so happy in our marriages that we can only wonder all do <no>t marry. But it is not always that a man can meet with his double.
We have an anxious house this winter in nursing one of my daughters who I believe slept in a damp bed last summer, and has never been well since. We have feared (& still do) consumption—but all the Drs say there is no really consumptive symtoms, but that there is inflammatory action of the air cells, complicated with Asthma.
A fortnight since, my only comfort in watching her, was that Asthma was the then agent of evil, and so it proved—and ever since she has been improving, but I have more fear than hope of the issu<e> I have, like all the world, (except I hope you and yours) been laid up with Bronchitis, and have not been out of doors this year.
However I am much better, and we are longing for sun and warm air for our dear Invalid.
But why bother you with these troubles— When I sat down I only meant to congratulate you and Mrs Darwin on your Birthday, and wish you all blessings thro the remainder of your life.
Mrs Fox joins most heartily with me in these wishes
Ever Dear old Darwin | Yours affectly | W. D. Fox
Letter 11358: To Ernst Haeckel, 12 February 
Down Beckenham Kent
My dear Friend
I thank you sincerely for all your most kind words & good wishes on my birth-day. My health has been better of late, & I am able to do every day what I consider a fair amount of work, but what you would consider a mere trifle. Considering my age it will be the more prudent course for me not to attempt to write on large & difficult subjects of a general nature, but to use my remaining strength in studying small special points, & thus to aid, as far as I can, the noble cause of science. To you & others must be left the extending & fortifying the principles of Evolution.— I am now working with the aid of my son Francis chiefly on the growth, movements & development of seedling plants; & we have observed some new & curious facts.—
I rejoice to hear that you are well, & that you are hard at work, about which I did not in the least doubt. As you say time time is what is wanted most.—
With all good wishes to you & yours & with many thanks
Yours very sincerely | Charles Darwin
Provenance: Ernst-Haeckel-Haus, Friedrich-Schiller-Universität, Jena
Letter 11874: From Zoological Station of Naples, 12 February 1879
The Zoological Stations of Naples and the naturalists diferent nations there assembled presents there warmest congratulations to the veteran of Modern Zoology on the occaison of this seventieth Birthday
Letter 13678: From Max Steffen & six other German students | 12 February 1882
Febr. 12th. 1882.
On the 75^th. return of your birth-day seven German students drink on your health in the old Teutonic manner.
Albrecht von Bockelmann, stud. rer. nat.
Theodor Bombe, stud. phil. et geogr.
Hermann Engelmann, stud. phil.
Arthur Petry, stud. rer. nat. et geogr.
H. Scheer, stud. rer. nat.
B. < > stud. phil.
Max Steffen, stud. phil.