...such collateral matter as...the question regarding species... (Matthew 1831)

Some plagiarism theorists try to argue that natural selection was the central theme of the book On Naval Timber and Arboriculture by Patrick Matthew (1831) and that the idea that it was a side issue that could easily be overlooked was a myth created by Charles Darwin, because he had plagiarised Matthew. Any scrap mentioning that the book contains ideas on species and varieties is taken as evidence that natural selection was central to On Naval Timber and could not be overlooked including what Matthew, speaking of himself in the third person, said in his preface: "The very great interest of the question regarding species, variety, habit, has perhaps led him a little too wide."
     However, in the very sentence preceding the one thus quoted, Matthew admits that it's a collateral issue put in only to spice up an otherwise insipid book on planting and timber. Here's the full passage (Matthew 1831, v-vi):

"It may be thought presumptuous in a person who has never had the curiosity to peruse the British classic authors on planting and timber—Evelyn, Hanbury, Marshall, Miller, Pontey—to make experiment of the public sufferance. The author does not, however, think any apology necessary; as, if the public lose time unprofitably over his pages, he considers the blame attachable to them, not to him. A writer does not obtrude as a speaker does, but merely places his thoughts within reach.
       As the subject [planting and timber], notwithstanding its great importance, might, per se, be felt dry and insipid by the general reader, accustomed to the luxuries of modern literature, the author has not scrupled to mix with it such collateral matter as he thought might serve to correct the aridity. The very great interest of the question regarding species, variety, habit, has perhaps led him a little too wide."