This, though Darwin, as a pigeon fancier, might’ve come up with it without his hypothesis on his own. Breeders of dogs, pigeons, and peas knew the hard way how the rules work, without the details.
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Actually, in a sense this is true–every facet of biology does seem to scream out “design”. There’s absolutely no way all the amazingly complex and intricate function of form and behavior we see in living things from amoebas to zebras could possibly have “just happened” or have evolved by “random chance”.
But Darwin’s insight was that evolution could produce such apparent “design” without a designer:
Richard Dawkins has also expounded very eloquently on both the absolutely stunning amount of design and complexity in living organisms, and also on how the theory of evolution by natural selection allows for design without a designer; see for example The Blind Watchmaker.
Darwin’s genius was that his idea (that breeding caused creation of species) was so obviously correct to members of his own class and country.
Darwin tied biological creation to the Creation Myth of the English Upper Class (breeding).
The idea of Breeding was /not/ the ‘theory of evolution’. Darwins ‘theory of evolution’ was (1) That breeding could cause the origination of species. (2) That breeding provided a mechanism for /evolutionary/ (gradualist) change. (3) That breeding provided a mechanism for non-observable creation. (4) That breeding provided a non-supernatural mechanism for origin of species. (5) That breeding as origin of species provided an explanation for and was compatible with geological evidence (the cutting-edge science of the day)
The educated English scientific class of the day understood selective breeding, which they took to apply not just to horses, but to themselves. They believed, mostly, in a gradual, evolutionary, natural (non-supernatural) origin of species. They just hadn’t made the link between the two: natural (non-supernatural) selection and the origin of species. When the idea was proposed, it burst on them like a flash of light: why didn’t I think of that?
tbf, it can be easier to think of evolutionary theory as an aspect, perhas even a consequence, of The Enlightenment/Age of Reason. It sure as hell didn’t appear out of thin air. Nor did it to Charles given his grandfather Erasmus was a great proponent:
Darwin’s great contribution to the field was the introduction of a mechanism. It was pretty obvious that species seemed to flow a bit from one to the next. What Darwin came up with was HOW this happened; through the mechanism of natural selection.