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Missing Evidence!
Darwin's Finches
Let us state from the beginning the name of these birds did not come from Charles Darwin. In fact, he admitted early on, he was quite "confused" about these birds, their origin and their comparisons. The term Darwin's Finches was first applied by Percy Lowe in 1936, and popularised in 1961 by David Lack with the second edition of his book Darwin's Finches.
Darwin's Theory is Missing More Than a Few Links
Although Charles Darwin did not mention these finches in his book The Origin of Species, he did record them in his Journal, being mentioned in the first edition (1839), and then having a number of paragraphs and even a picture six years later in the revised edition (1845).
Most of what Darwin said can be found in this:
Seeing this gradation and diversity of structure in one small, intimately related group of birds, one might really fancy that from an original paucity of birds in this archipelago, one species had been taken and modified for different ends... Unfortunately most of the specimens of the finch tribe were mingled together; but I have strong reasons to suspect that some of the species of the sub-group Geospiza are confined to separate islands. If the different islands have their representatives of Geospiza, it may help to explain the singularly large number of the species of this sub-group in this one small archipelago, and as a probable consequence of their numbers, the perfectly graduated series in the size of their beaks. (pp403-420)
Darwin, Journal of Researches into the Natural History and Geology of the countries visited during the voyage round the world of H.M.S. Beagle, revised edition, Henry Colburn 1845.
Darwin did speculate different finches had descended from a common ancestor and had changed to be able to do different things. He wasn't sure which of the different species were from different islands.
He came up with the theory, but not in the details as some people had thought....
Darwin's 'Theory'
Although these birds were to play an important part in the inception of Darwin's theory of evolution by natural selection, during the survey voyage of HMS Beagle Darwin had no idea of their significance. He had learnt how to preserve bird specimens while at the University of Edinburgh and had been keen on shooting, but he had no expertise in ornithology and by this stage of the voyage concentrated mainly on geology and mostly left bird shooting to his servant Syms Covington.
On the Galápagos Islands and afterwards, Darwin thought in terms of "centres of creation" and rejected ideas of transmutation of species.[2] From Henslow's teaching he was interested in geographical distribution of species, particularly links between species on oceanic islands and on nearby continents. On Chatham Island he recorded that a mockingbird was similar to those he had seen in Chile, and after finding a different one on Charles Island he carefully noted where mockingbirds had been caught, but paid little attention to the finches.
When writing up his notes on the way to Tahiti Darwin was astonished to find that all the mockingbirds caught on Charles Island were of one species, those from Albemarle of another, and those from James and Chatham Islands of a third species. As they sailed home about nine months later this, together with other facts including what he'd heard about Galápagos tortoises, made him wonder about the stability of species.
Following his return from the voyage, Darwin presented the finches to the Geological Society of London at their meeting on 4 January 1837, along with other mammal and bird specimens he had collected.
The bird specimens, including the finches, were given to John Gould, the famous English ornithologist, for identification. Gould set aside his paying work and at the next meeting on 10 January reported that birds from the Galápagos Islands which Darwin had thought were blackbirds, "gross-beaks" and finches were in fact "a series of ground Finches which are so peculiar [as to form] an entirely new group, containing 12 species." This story made the newspapers.
Darwin had been in Cambridge at that time. In early March he met Gould again and for the first time got a full report on the findings, including the point that his Galápagos "wren" was another closely allied species of finch.
The mockingbirds Darwin had labelled by island were separate species rather than just varieties. Gould found more species than Darwin had anticipated, and concluded that 25 of the 26 land birds were new and distinct forms, found nowhere else in the world but closely allied to those found on the South American continent. Darwin now saw that if the finch species were confined to individual islands, like the mockingbirds, this would help to account for the number of species on the islands, and he sought information from others on the expedition.
Specimens had also been collected by Captain Robert FitzRoy, FitzRoy’s steward Harry Fuller and Darwin's servant Covington, who had labelled them by island.[7] From these, Darwin tried to reconstruct the locations where he had collected his own specimens. The conclusions led shortly afterwards to his conversion to the idea of transmutation of species.
At that time he was rewriting his diary for publication as Journal and Remarks (later republished as The Voyage of the Beagle) and he described Gould's findings on the number of birds, noting that "Although the species are thus peculiar to the archipelago, yet nearly all in their general structure, habits, colour of feathers, and even tone of voice, are strictly American." In particular, "A group of finches, of which Mr. Gould considers there are thirteen species; and these he has distributed into four new sub-genera. These birds are the most singular of any in the archipelago.... All the species, excepting two, feed in flocks on the ground, and have very similar habits.
It is very remarkable that a nearly perfect gradation of structure in this one group can be traced in the form of the beak, from one exceeding in dimensions that of the largest gros-beak, to another differing but little from that of a warbler."
In 1839 Darwin conceived of his theory of natural selection, and he added more detail to the second edition, published as Journal of Researches in 1845, with an illustration showing the beaks of four finches and the closing remark that "Seeing this gradation and diversity of structure in one small, intimately related group of birds, one might really fancy that from an original paucity of birds in this archipelago, one species had been taken and modified for different ends."
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He discussed this divergence of species of birds in the Galápagos more explicitly in his chapter on geographical distribution in On the Origin of Species:
The most striking and important fact for us in regard to the inhabitants of islands, is their affinity to those of the nearest mainland, without being actually the same species. [In] the Galapagos Archipelago... almost every product of the land and water bears the unmistakeable stamp of the American continent. There are twenty-six land birds, and twenty-five of these are ranked by Mr. Gould as distinct species, supposed to have been created here; yet the close affinity of most of these birds to American species in every character, in their habits, gestures, and tones of voice, was manifest....
The naturalist, looking at the inhabitants of these volcanic islands in the Pacific, distant several hundred miles from the continent, yet feels that he is standing on American land.
Why should this be so? why should the species which are supposed to have been created in the Galapagos Archipelago, and nowhere else, bear so plain a stamp of affinity to those created in America?
There is nothing in the conditions of life, in the geological nature of the islands, in their height or climate, or in the proportions in which the several classes are associated together, which resembles closely the conditions of the South American coast: in fact there is a considerable dissimilarity in all these respects.
On the other hand, there is a considerable degree of resemblance in the volcanic nature of the soil, in climate, height, and size of the islands, between the Galapagos and Cape de Verde Archipelagos: but what an entire and absolute difference in their inhabitants! The inhabitants of the Cape de Verde Islands are related to those of Africa, like those of the Galapagos to America.
I believe this grand fact can receive no sort of explanation on the ordinary view of independent creation; whereas on the view here maintained, it is obvious that the Galapagos Islands would be likely to receive colonists, whether by occasional means of transport or by formerly continuous land, from America; and the Cape de Verde Islands from Africa; and that such colonists would be liable to modification;—the principle of inheritance still betraying their original birthplace."
The conclusion from Darwin's own words is he made a lot of wrong guesses. He was by the way, only an ameture in this field and had no proper credentials for scholarlly investigation nor for offering these supposed "principles".
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