Mike Sutton serves up a fraud story based on letting google search for unique phrases up to Matthew (1831. On Naval Timber and Arboriculture) and then finding them in other sources from book.google.com before 1858, when papers, letters and abstracts of Wallace and Darwin were jointly read before the Linnean Society in London.
He has found, through google, three scholars that belonged to the inner social circle of Charles Darwin and Alfred Wallace that have cited Patrick Matthew (1831).
The three citations are, according to Sutton:
Loudon, J.C. 1832. Matthew Patrick On Naval Timber and Arboriculture with Critical Notes on Authors who have recently treated the Subject of Planting. Gardener’s Magazine. Vol. VIII. p.703.
Chambers, Robert In: Chambers's Edinburgh Journal on Saturday the 24th of March 1832. Volume 1 of that journal containing numbers 1-52 was published in 1833 by Orr and Smith, London.
Selby, P. J. 1842. A history of British forest-trees: indigenous and introduced. London. Van Voorst.
If you actually follow the links to the original sources, however, you will find that only Loudon actually referred to Matthew's ideas that natural selection could transform species, though Loudon called it the puzzling subject of the origin of species and varieties:
"The author introductorily maintains that the best interest of Britain consists in the extension of her dominion on the ocean; and that, as a means to this end, naval architecture is a subject of primary importance and, by consequence, the culture and production of naval timber is also very important. [...] An appendix of 29 pages concludes the book, and receives some parenthetical evolutions of certain extraneous points which the author struck upon in prosecuting the thesis of his book [which it the importance of naval timber]. This may be truly termed, in a double sense, an extraordinary part of the book. One of the subjects discussed in this appendix is the puzzling one, of the origin of species and varieties; and if the author has hereon originated no original views (and of this we are far from certain), he has certainly exhibited his own in an original manner." (Loudon, 1832, pp. 702-703)Robert Chambers
The paragraph in the Chambers's Edinburgh Journal, vol. 1, no. 8, 24 March 1832, p. 63 (see top right corner) is a recipe on how to treat trees so that they yield plank timer. "Divide all branches into leaders and feeders..." and so on about cutting, pruning, thinning etc. As Mike Weale pointed out at the excellent Patrick Matthew Project, PMP (see Excerpt #4), this paragraph is merely an abridged abstract from pp. 8-14 of Matthew's book. It is headed "ON THE TRAINING OF PLANK TIMBER" and ends with ".—Matthew on Naval Timber." That is, the whole recipe is a copy-paste job lacking any original work on the part of the Chambers brothers. I'm not even sure whether this has been written by William or Robert Chambers. It might as well have been written by Matthew himself, possibly on the request of one of the Chambers brothers. Anyway, it is not a citation by Robert Chambers of Matthew (1831) on species transformation through natural selection.
Nobody reading this excerpt could possibly have concluded that the book contained anything of importance concerning the origin of species. Mike Weale considers it an important citation for several good reasons, but not, it must be emphasized, as indicating anything in the direction of plagiarism, which it does not. Nevertheless, Mike Sutton concludes that, because of this excerpt, Robert Chambers must have read and received the passages of the book that are relevant to the idea of natural selection. He further claims that Robert Chambers must also have read and received the addendum to the appendix, in which the idea of natural selection is, for once, in some way combined with that of species transmutation (and catastrophism). And Sutton further claims that Robert Chambers must have transported this idea of combining natural selection with species transmutation to Darwin and Wallace, through his book The Vestiges of the Natural History of Creation. In fact, however, that book does not even mention or imply natural selection (or the survival of the fittest) at all. It is a pamphlet for species trasnformation, okay, but natural selection plays no role in it.
Finally, Selby cited Matthew (1831) about 30 times, but not one of these refers to his ideas about natural selection. It is all about which trees are good for what purpose, how to treat trees, soil, climate, pruning, etc. Admittedly, on page 391, Selby criticizes a claim of Matthew about the "power of occupancy" or competitiveness of tree species. But there is still a huge leap from thinking about competition between species to a theory of evolution by natural selection between variants of a species.
"Matthew, however, in his able treatise on naval timber seems to think that its [the fir's] indigenous location in such districts arises, not so much from preference of soils of the nature above-mentioned, as from its having the more power of occupancy in such soils than any other plant of the country; and this opinion he endeavours to support by stating that the Pinus sylvestris, planted in a good or rich soil, attains larger dimensions and its best timber properties, and that it is only driven from this superior soil by the greater power of occupancy possessed by the oak and other decidous trees, an opinion which we cannot altogether acquiesce, as we see no reason why the fir, it it grows with such additional vigour in a rich soil, as Mr. Matthew asserts, should, at the same time, be unable to maintain a contest with the oak or other tree." (Selby 1942, 391)The significance of this citation is that Selby did not understand Matthew's relative conception of competition. He did not comprehend Matthew's multi-level comparisons. Matthew (1831, 302ff) claimed that pines would do better on rich soils than on poor soils, but since oaks do even better still on rich soils they would exclude the pines from them. Likewise the pines exclude the oaks from poor soils or harsher climates. However, Matthew did not take this consideration as his point of departure for a detailed ecological and evolutionary theory. Instead, he concluded that the tree planter must take care to keep the competitors down, especially in the early seed and seedling stages, if he wants to grow pines on rich soils or oak in Scotland. That is, he used his ecological insights about competition and relative adaptation as a means to criticize other authors and to devise better practical tips on tree growing.
Only in the addendum (pp. 381-88) that Matthew appended to the end of his appendix list of end-notes (A to F), did he replace the tree grower that keeps the competitors down by catastrophes that exterminate the competitors, instead. The remnant species surviving these catastrophes could, only then and only because the competition had been cleared away, radiate into an empty field of existence and diversify. However, given that Selby did not even comprehend Matthew's relative conception of competition, he will most likely not have understood this evolutionary addendum at all.
In conclusion, just finding places, where someone cited Matthew (1831) is not enough to prove that Darwin and Wallace must have know of Matthew's particular combination of natural selection with species transmutation before having their own and different ones.