Darwin concluded On the origin of species with the words: 'There is grandeur in this view of life, with its several powers, having been originally breathed into a few forms or into one; and that, whilst this planet has gone cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being, evolved.' This was his first use of the word 'evolution', in any form, in print. He did not invent the term but with his mechanism of 'natural selection', supplemented by ideas about sexual selection, 'divergence' and inheritance, he did describe just how evolution - the continual development of new organisms - could take place.
At last gleams of light have come, & I am almost convinced (quite contrary to opinion I started with) that species are not (it is like confessing a murder) immutable.
Darwin to Joseph Hooker, [11 January 1844]
If, as I believe that my theory is true & if it be accepted even by one competent judge, it will be a considerable step in science.
Darwin to Emma Darwin 5 July 1844
I have lately been especially attending to Geograph. Distrib, & most splendid sport it is,—a grand game of chess with the world for a Board.
Darwin to C. J. F. Bunbury, 21 April 
The facts which kept me longest scientifically orthodox are those of adaptation … This difficulty, I believe I have surmounted.
Darwin to Asa Gray, 5 September 
…if my explanation of these classes of facts be at all right, whole classes of organic beings must be included in one line of descent.
Darwin to Leonard Jenyns, 7 January 
But if (& oh what a big if) we could conceive in some warm little pond … that a protein compound was chemically formed, ready to undergo still more complex changes…
How do new species arise? This was the ancient question that Charles Darwin tackled soon after returning to England from the Beagle voyage in October 1836. Darwin realised a crucial (and cruel) fact: far more individuals of each species were born than could possibly survive.
Origin is 157 years old. (Probably) the most famous book in science was published on 24 November 1859. To celebrate we have uploaded hundreds of new images of letters, bringing the total number you can look at here to over 9000 representing more than 2000 letters, written both before and after the book appeared. You can now see examples of letters to Darwin from nearly 250 different people, and letters he wrote to 150 more.
Although natural selection could explain the differences between species, Darwin realised that (other than in the reproductive organs themselves) it could not explain the often marked differences between the males and females of the same species. So what accounted for these 'secondary sexual characteristics'? The longer manes in male lions and beards in male humans? Antlers or horns being so much smaller, or completely absent, in some female deer or cattle?
It was crucial to Darwin’s theories of species change that naturally occurring variations could be inherited. But at the time when he wrote Origin, he had no explanation for how inheritance worked – it was just obvious that it did. Darwin’s attempt to describe how heredity might work, his 'provisional hypothesis of Pangenesis’, was published in 1868 in his book, Variation of animals and plants under domestication
In a later account of how he had come to the evolutionary ideas published in Origin, Darwin wrote: 'Of all the minor points, the last which I appreciated was the importance & cause of the principle of Divergence' (to Ernst Haeckel, [after 10] August – 8 October ).
The original manuscript about varieties that Wallace composed on the island of Gilolo and sent to Darwin from the neighbouring island of Ternate (Brooks 1984) has not been found. It was sent to Darwin as an enclosure in a letter (itself missing), and was subsequently sent by Darwin to Charles Lyell (letter to Charles Lyell, 18 [June 1858]). The only known version of the text is the one published in Journal of the Proceedings of the Linnean Society (Zoology) 3 (1859): 45–62, and this text is reprinted below.
There are two extant versions of the abstract of Darwin’s theory of natural selection. One was sent to Asa Gray on 5 September 1857, enclosed with a letter of the same date (see Correspondence vol. 6, letter to Asa Gray, 5 September  and enclosure). It is in the hand of Ebenezer Norman, Darwin’s copyist and includes minor alterations and corrections by Darwin. The letter and enclosure are in Gray’s correspondence in the Gray Herbarium of Harvard University.
From a quiet rural existence at Down in Kent, filled with steady work on his ‘big book’ on the transmutation of species, Darwin was jolted into action in 1858 by the arrival of an unexpected letter (no longer extant) from Alfred Russel Wallace outlining a remarkably similar mechanism for species change. This letter led to the first announcement of Darwin’s and Wallace’s respective theories of organic change at the Linnean Society of London in July 1858 and prompted the composition and publication, in November 1859, of Darwin’s major treatise On the origin of species by means of natural selection.
CD gratified that ED wants to translate hisJournal. Will send a copy ofCoral reefs, which contains a fuller treatment of topic. Perhaps ED would insert a note to this effect. Can lend woodcuts fromCoral reefsif ED wants. CD will send a few corrections; he wants to amend way he criticised Agassiz’s glacier theory.
He is also enclosing a questionnaire concerning differences between races or varieties and species, about which he intends to publish sometime.
Queries on ratios of species to genera on southern islands. CD’s observations on distribution of Galapagos organisms, and on S. American fossils, and facts he has gathered since, lead him to conclusion that species are not immutable; "it is like confessing a murder".
Asks whether LJ can throw light on this subject: "What are the checks and what the periods of life by which the increase of any given species is limited?" CD has been driven to conclude that species are mutable; allied species are co-descendants from common stocks.
Further on case ofLycopodium: does JDH know any genera of plants whose species are variable in one continent but not in another? Discussion on variations between floras as regards species richness, and factors affecting geographical distribution. On species, CD expects "that I shall be able to show even to sound naturalists that there are two sides to the question of the immutability of species; – that facts can be viewed and grouped under the notion of allied species having descended from common stocks". Mentions books and papers for and against species mutability. CD believes past absurd ideas arose from no one’s having approached subject on side of variation under domestication.
What does CD think of A. R. Wallace’s paper in theAnnals & Magazine of Natural History["On the law which has regulated the introduction of new species", n.s. 16 (1855): 184–96]? EB considers it good on the whole.
Japanned variety of peacock.
Regional variations in bird species.
EB has little faith in the aboriginal wildness of the Chillingham cattle.
Races of humped cattle of India, China, and Africa.
Indian and Malayan gigantic squirrels, with various races remaining true to their colour, would afford capital data for Wallace, as would the local varieties of certain molluscs. Has Wallace’s lucid collation of facts unsettled CD’s ideas regarding the persistence of species?
Bengal hybrid race of geese is very uniform in colour and as prolific as the European tame goose [seeNatural selection, p. 439].
Will see what he can do for CD with regard to domestic pigeons.
Reports long preparation of work on how species and varieties differ. Agreement with Wallace’s conclusions as reported inAnnals and Magazine of Natural Historyand in his letter to CD of 10 0ct . On distinction between domestic varieties and those in "a state of nature".
On mating of jaguars and leopards, the breeding of poultry, pigeons, etc.
Requests help for his experimenting on means of distribution of organic beings on oceanic islands.
Believes species have arisen, like domestic varieties, with much extinction, and that there are no such things as independently created species. Explains why he believes species of the same genusgenerallyhave a common or continuous area; they are actual lineal descendants.
Discusses fertilisation in the bud and the insect pollination of papilionaceous flowers. His theory explains why, despite the risk of injury, cross-fertilisation is usual in the animal and vegetable kingdoms, even in hermaphrodites.