Now this is not convincing, because scientific literature on the status of the Golden Pippin as a variety of the wild crab apple, its incipient decay, and what could be done against it, was as important an issue in the 18th century as naval timber used to be. The sailors were fine with Rum, but the rest of the population needed its cider. The decay and degeneration of cider apple varieties was at least as severe a crisis as the lack of oak forest trees for naval timber.
This is a tale of the crab apple, the Golden Pippin, and what Darwin may actually have meant by his cryptic note: "They die; without they change; like Golden Pippens. it is a generation of species. Like generation of individuals."
Let's go straight for Thomas Knight (1801. A Treatise on the Culture of the Apple & Pear, and on manufacturing of Cider and Perry. 2nd Edition. London: Longman, Rees; and White). This book opens with a salvo: stating up front that animals and plants vary more strongly under domestication than in nature and that humans profit from this by artificial selection:
"The effects of cultivation on the animal and vegetable system are extremely similar. A change in form, in colour, and in size or stature, takes place in each; and in each those changes appear to arise from similar causes—from a more abundant and regular supply of nourishment than is afforded in a state of nature, with a favourable climate, or protection from the bad effects of an indifferent one. The offspring of every plant and animal, when unchanged by cultivation, bears a very close resemblance to it's parents; but amongst the cultivated kinds of each, it is extremely various; still, however, generally shewing some similarity to them. By taking advantage of incidental variations, and by propagating from those individuals which approach nearest to our ideas of perfection, improved varieties of fruit, as well as of animals, are obtained." (p. 3)From there, he directly plunges into the topic of Apple cultivation.
"The Apple (on the culture of which I propose to offer some observations in the following pages) is not the natural produce of any soil, or climate; but owes its existence to human art and industry; and differs from the crab, which is a native of every part of England, only in the changes which cultivation has produced in it." (p. 5)Alas, the best varieties of apples trees are in decay and degeneration.
"The Moil, and its successful rival the Redstreak, with the Musts and Golden Pippin, are in the last stages of the decay and the Stire and Foxwhelp are hastening rapidly after them." (p. 6f)After pointing out the difference to animals, which cannot be propagated by vegetative (asexual) means, he advances an explanation that could, with anachronism, be called one of cloning the trees through grafting.
"The art of the planter readily divides a single tree into almost any number that he wishes; but the character of the new trees, thus raise, is very essentially different from that of a young seedling plant; they possess a preter-natural maturity, and retain the habits and diseases of the tree of which they naturally formed a part." (p. 8)The cure, of course, is breeding afresh from seeds and crossing breeds (p. 37). Astonishingly, Knight also provides a qualitative observation that is strikingly reminiscent of Mendel's later quantitative proof of segregation:
"If the male and female be taken from two permanent varieties of different characters, the immediate offspring will present a mixture of both characters, in nearly an equal proportion; but the progeny of this offspring will be extremely various. Some will take nearly the form of their male, and others of their female ancestry, and it will be long before a new permanent character is acquired." (p. 88f)Let's leave Thomas Knight, here, and see how Sir Humphry Davy (1815. Elements of Agricultural Chemistry. New York: Eastburn, Kirk & Co.) picked up the ball and ran with it. After recapitulating the experiments and findings of Knight (1801), he continues with the selection that is necessary after crossing breeds:
"The power of the horticulturist extends only to the multiplying excellent varieties by grafting. They cannot be rendered permanent; and the good fruits at present in our gardens, are the produce of a few seedlings, selected probably from hundred of thousands; the result of great labour and industry, and multiplied experiments.Given all this, what could Charles Darwin have been thinking about, when he wrote in his notebook about Golden Pippens that die without change and the generation of species being like generation of individuals?
The answer should be clear now. Thomas Andrew Knight (1801) claimed that apple varieties that were propagated by grafting were nothing more than parts of the trees they stemmed from. Nothing rejuvenated them and they died from old age ("they die, without they change"), no matter on what youthful stock they were grafted. What Knight (1801) had argued quite forcefully was that they were not to be counted as two generations but merely as one individual having been divided by horticulturists ("It is a generation of species. Like generation of individuals"). What does that mean? It means that Charles Darwin has read Thomas Knight's Treatise shortly before he made his cryptic note.