Your first sentence is true. The second sentence is speculative and inappropriately applied."If it could be proved that any part of the structure of any one species had been formed for the exclusive good of another species, it would annihilate my theory, for such could not have been produced through natural selection."
Darwin's critique of his own work suggested that a Creator might occasionally be expected to co-create organisms with mutual dependencies; a flowers-and-bees and chicken-or-egg conundrum in one pretty package.
He admitted that finding such a situation would indicate that his concept was faulty in that regard. However, no such situation has been observed. Perhaps you can enlighten us with your observation that "one species had been formed for the exclusive good of another species" which would, by Darwin's own admission, show his theory to be incorrect.
Darwin's critique of his own work suggested that a Creator might occasionally be expected to co-create organisms with mutual dependencies; a flowers-and-bees and chicken-or-egg conundrum in one pretty package.
My point is not speculative or inappropriately applied. I simply observe that Darwin's argument was not a scientific argument, but an argument based on natural theology that relies entirely on particular unstated presuppositions and notions regarding the nature of a Creator. Darwin's notions about what a Creator might be expected to do are not scientific; they are metaphysical speculations about the nature of reality, and as such are immune to empirical testing.
He admitted that finding such a situation would indicate that his concept was faulty in that regard. However, no such situation has been observed. Perhaps you can enlighten us with your observation that "one species had been formed for the exclusive good of another species" which would, by Darwin's own admission, show his theory to be incorrect. [emphasis mine]
"Good" is not a scientific, empirically derived concept. It entails philosophical assumptions about the nature of reality. Because certain features of the natural world did not comport with Darwin's view of what an all-powerful, all-benevolent Creator would or should do, he concluded that the Creator is not involved in this world in the slightest. So he reasoned that because one species has not been observed "to have been formed for the exclusive good of another species" therefore they are the result of a process that is unguided and without purpose or design.
While this type of argument has and continues to have great emotional appeal to certain people, Darwin and Darwinist theodicy does not constitute scientific argument.
Cordially,