Here are two historical and controversially interpreted statements:
1. In a letter claiming priority for the idea that natural selection could transform species, Patrick Matthew (1860. Nature's Law of Selection. Gardeners’ Chronicle and Agricultural Gazette, April 7, pp. 312-313) also mentioned, where his book has been reviewed, for example:
“reviewed in […] the “Gardeners’ Magazine,” by Loudon”
2. In response, Darwin (1860. Natural Selection. Gardeners’ Chronicle and Agricultural Gazette, April 21, pp. 362-363) readily admitted Matthew’s priority and excused his ignorance of it by:
“neither I, nor apparently any other naturalist, had heard of Mr. Matthew’s views”
Many futile debates can result from this as, for example, the one where the contestants take their different opinions about the question whether Loudon was a naturalist or not as divergent points of departure and then argue about the question whether Darwin did or did not lie, here. Another would be derivative, but still futile, where the contestants argue about the contemporary meaning of the term naturalist in Darwin’s days and whether that would include or exclude Loudon.
These debates are futile, because the first question that needs to be answered, before the above questions can even be fruitfully addressed, is: what valid conclusion can be drawn from the above gives statements? This is different from asking whether Loudon was a naturalist or what that term meant. It is even different from asking whether Darwin did regard Loudon as a naturalist. If the last sentence seems contradictory, remember that we are only asking what logically valid conclusion the above given statements allow us to draw, without external evidence. It's a matter of logic and not of science or history. We are not inquiring into the historical truth about Loudon or Darwin's opinion of him, but only into the logic frame within which any such historic inquiry must fall.
[This is just playing with logic serving to show that those who claim they have proven Darwin to be a liar cannot even apply this simple logic properly. The question, whether or not Darwin did read Matthew's book before 1859, cannot now be ascertained anymore.]
As we can neither ask Darwin nor read his mind, the only way to draw a valid conclusion from the above is to apply logic. The logic that must be applied, here, is the one of syllogisms. A syllogism does not depend on external evidence about the correctness or falsity of the statements that are being assumed (the premises). A syllogism only assure that the conclusion will be valid, if the premises are true. Let’s start with a simple example.
Premise 1: Every L is an M.Of course, conclusion 1 is correct. An L is no N. The semblance of this syllogism with the above given historical statements by Matthew and Darwin can now be increased, step-wise.
Premise 2: No N is an M.
Question: If the above premises were true, which conclusion would be valid?
Conclusion 1: An L is no N.
Conclusion 2: An L is an N.
First, replace that awful word premise by statement. Second, change the numerals and verbs to fit the historic statements by Matthew and Darwin:
Statement 1: L knows M.Of course, conclusion 1 remains the logically valid answer!
Statement 2: No N knows M.
Question: Given the above statements, which conclusion will be valid/consistent/logical?
Conclusion 1: L is no N.
Conclusion 2: L is an N.
Now, replace the letters with words as follows:
“L” by “Loudon”
“knows M” by “had reviewed Mr. Matthew’s views” or by “had heard of Mr. Matthew’s views”
“N” by “is a naturalist”
Statement 1: L knows M. → Loudon had reviewed Mr. Matthew’s views.
Statement 2: No N knows M. → No naturalist had heard of Mr. Matthew’s views.
Question: Which conclusion is correct?
Conclusion 1: L is no N. → Loudon is no naturalist.
Conclusion 2: L is an N. → Loudon is a naturalist.
Still, conclusion 1 remains the logically consistent one.
Rephrase the last syllogism a little more, in order to fit it more closely to the historical statements of Matthew and Darwin. This time, change:
“Statement 1” into “Matthew” and replace “Mr. Matthew’s” by “my” in statement 1
“Statement 2” into “Darwin”
“Conclusion 1” and “Conclusion 2” into “Darwin thought that”
Matthew: Loudon had reviewed my views.Again, given the above historic statements on their own, the valid conclusion remains that Darwin considered Loudon to be no naturalist.
Darwin: (apparently) no naturalist had heard of Mr. Matthew’s views.
Question: Which of the following conclusion is valid?
Darwin thought that Loudon was no naturalist.
Darwin thought that Loudon was a naturalist.
The only alternative
The only possibility in which the valid conclusion would be that Darwin lied, would require further evidence, that is, a letter by Darwin or a notebook entry or anything showing that Darwin did consider Loudon a naturalist:
Matthew: Loudon had reviewed my views.
Darwin 1: calls Loudon a naturalist in a letter or note or anything.
Darwin 2: (apparently) no naturalist had heard of Mr. Matthew’s views.
Question: Which of the following conclusion is valid?This evidence needed for the second premise (Darwin 1) to be true, however, is lacking. The Darwin Correspondence Project lists a letter, where Darwin writes to Jenyns (14 or 21 August 1846):
Darwin thought that Loudon was no naturalist.
Darwin thought that Loudon was a naturalist and lied.
"In such foreign periodicals, as I have seen, there are no such papers, as White, or Waterton; or some few other naturalists in Loudon’s & Charlesworth’s Journal, would have written, & a great loss it has always appeared to me."Unfortunately, this is useless as external evidence on the question whether Darwin included Loudon among the naturalists. Loudon may have been like a commercial editor of some high profile for-profit journal run by professional publishers, or he may have been like one of the many editors of less journalistic and more scientific journals, an established scientist himself who had taken the chore of conducting the journal in question. As that journal was the Magazine of Natural History, and Journal of Zoology, Botany, Mineralogy, Geology and Meteorology, again, Loudon may also have held that post due to his status as a botanist rather than a naturalist.
Some plagiarism theorists take the historic statements by Matthew and Darwin from the beginning of this post as incriminating evidence that 100% prove Darwin to be a liar. Putting it mildly, that is poor logic. Without external evidence about what Darwin considered Loudon to be, the parsimonious conclusion remains that he did not include Loudon among the naturalists. He surely was an improver—a park, garden and landscape designer or architect, a botanist, and an encyclopaedist.
But one cannot simply assume that Darwin did regard Loudon as a naturalist, like Alexander Humboldt, de Candolle or Charles Lyell, then find the above quoted statement of Darwin in contradiction to one's own assumption and, therefore, conclude that Darwin must have lied. That is not just poor logic, but even poorer history.
That said, I again disclaim to draw myself any rash conclusion concerning the question whether Darwin regarded Loudon as a naturalist. This question is open to historic inquiry. But to simply grab the first two statements at hand, charge them with one's own biased assumptions and then claim them to be 100% proof of Darwin's lying and plagiarism is not a historical inquiry.