The poor performance
A naive and surprised reaction will be based on the fact that the only work on forest trees one usually ever heard of is Patrick Matthew's book On Naval Timber and Arboriculture, because it has some relation with natural selection. Thinking that Matthew was the only source for anything on forest trees will lead to an interpretation as follows:
First Interpretation: The fact that Darwin shows interest in forest-trees and their variability in nursery gardens proves that he had read Matthew (1831).
However, this is not the only possible interpretation, especially if you know something about the pertinent literature from around 1831.
Second Interpretation: There were others before Matthew (1831) writing about forest trees and their cultivation. An interest in the issue does neither prove nor disprove a knowledge of Matthew (1831).
Finally, having actually read and understood Matthew (1831) can even lead to the opposite of the first interpretation.
Third Interpretation: The question whether forest-trees sport more in nurseries than in nature as a consequence of the special conditions in nurseries, like manuring, indicates that Darwin had not read Matthew (1831) or else he would not have asked this. Matthew (1831, 308) had made it clear that the variability only appears to be increased under domestication, because of the lack of natural selection there. For Darwin to get there, it would still take years of studying barnacles and other organisms.
"Man's interference, by preventing this natural process of selection among plants, independent of the wider range of circumstances to which he introduces them, has increased the difference in varieties, particularly in the more domesticated kinds; and even in man himself, the greater uniformity, and more general vigour among savage tribes, is referrible to nearly similar selecting law—the weaker individual sinking under the ill treatment of the stronger, or under the common hardship." (Matthew 1831, p. 308)One could, then, continue with a useless controversy about this entry and what it signifies about Darwin's knowledge or ignorance of Matthew (1831) or quip something like: "Had Darwin asked whether Forrest Gump sports much in nursery gardens, I'd surely know the answer." However, the entry also provides the chance to do something more interesting.
The historical performance
Let's first look at the context of Darwin's notebook entry. The very first question at page 3 about crossing two halfbred (we'd say heterozygous) animals and their offspring being uniform already shows that Darwin was on the track of hybridizers* in particular rather than on Matthew's general observations about the variability of trees in forests and nurseries.
* [By the way, Gregor Mendel was one of these hybridizers, and he would later answer Darwin's question with a resounding "No." He started from two strains of peas that differed in one trait (e.g., colour of the flowers), but were truebred (that is, homozygous) for that trait. Crossing this parental P-generation yielded a halfbred (that is, heterozygous) F1-generation, which was was uniformly showing the dominant trait. But crossing these halfbred F1 plants with each other yielded an F2-offspring showing the parental traits in the ratio 3:1. Hence, the answer is "No!" to Darwin's first question at page 3: "If two halfbred animals exactly alike be inbred, will offspring be uniform?"]
Furthermore, Darwin's entries at page 21 of the same notebook show that he had a particular problem with trees and inbreeding. He believed that some degree of out-breeding was necessary for the health of organisms but could not understand how that should be possible in in mixed forests. Here, the scattering of trees of the same species should lead to close inbreeding between flowers of the same tree despite insects or wind.
Darwin even wondered whether fruit trees can flower and their fruit ripen in Scotland—a question he'd never have jotted down had he been in the know of Matthew's writings.
"(2) History of fruit trees far north in Scotland — do they flower — do they live healthily, or does fruit merely not ripen. — The point to attend to is whether good & plenty of pollen is produced. & 2d if so, whether concepcion takes place, — the mere fact of seeds ripening has scarcely any no relation to hybrids." (Darwin's Q&E notebook, p. 21)At the same time, experiments by hybridizers, like William Herbert, had shown that hybridization can boost the variability of the offspring generally referred to as "sporting." If trees were inbred in mixed forests, and their out-breeding had similar effects as the hybridizing in experiments by hybridizers, the close proximity of trees of the same species in nurseries should lead to an increase of sporting in their offspring.
Darwin's further question about the manuring of trees in nurseries ("
"In fairness, it may be proper to explain, that the greater part of the trees we have thus cultivated have been of Pyrus, although we commenced the practice with common forest trees—yet the pear and apple vary nothing from the oak and ash in the primary stage of life" (emphasis added, Matthew 1831, p. 215, footnote †)That means, had Darwin read Matthew (1831), he might not have asked this question about the effects of domestic conditions on variability in the first place.
The analytical performance
The scan and the transcript provided by Darwin Online are not very good. Let's take a closer look at a better scan provided by the American Museum of Natural History.
The first thing to notice, here, is that the transcription (by David Kohn) has added a footnote "a" after "forest-trees." What does it say? It says: "aDo forest-trees] ‘No’ added over ‘Do’"
Apparently, we must look closer still!
What we see, now, is that Darwin has overwritten the first word, either "Do" with "No" or vice verse. Here are samples of Darwin's "D" and "N" from other pages in the same notebook as well as the Do/No overwrite at page 3 in large.
Taking the question mark at the end of the sentence as an indication that the original sentence must have been a question, the original first letter must have been a "D" in "Do forest-trees sport much in nursery gardens?" That means, Darwin has overwritten the "D" with an "N" and thus turned a question into an answer. He apparently concluded something about the effect of either out-breeding or manuring in nurseries. So here's an interesting project for an accomplished sleuth in trying to find out what this overwrite signifies.