The same principles are followed by horticulturalists; but the variations are here often more abrupt. No one supposes that our choicest productions have been produced by a single variation from the aboriginal stock. We have proofs that this is not so in some cases, in which exact records have been kept; thus, to give a very trifling instance, the steadily-increasing size of the gooseberry may be quoted.
What proofs was Darwin pointing at? Did he leave records of the items he had taken as proofs?
Further, what information do we have nowadays? Presumably record-keeping has become a much easier task down the ages, and we have a deluge of information to pick from. What examples are there of, say, plants that can no longer pollinate or be pollinated from the aboriginal stock (or an intermediate, for that matter). Surely there are many.
Of course, not being able to reproduce is part of the definition of being a separate species - or at least one of the definitions, depending on who you listen to! (An upcoming post on this is in the writing...)