Structure vs fluid. It is taking a step back. They are philosophies, two different world-views. They are philosophies that are intrinsic primarily to our personalities. And they will generally not see eye to eye. They generally dictate our paths in life, particularly in subjects (schooling) we choose, and occupations we pursue, and past-times we enjoy.
This is a different direction, because all you asked me was “How do you know”, and I am not interested in submitting the obvious reply, “How do you not”, the easier path (as is your own). For I cannot relate all the observations I have made in life, and I cannot relate all the things I have read, and I cannot relate all the things I have experienced. They piece together sufficiently for me, and every extra piece of information I gain fits.
I love the scientific method, I fall into structure. When KAr dating makes the claim “it works just not for any age-known samples”, I cringe, for that is science breaking the rules of science. When I heard about a whale found miles inland and at 1000s of feet high, which had been speculated to be 3 million years old, I asked that it be carbon-dated. The reply was “What’s the point? Its 3 million years old so the reading will be non-existent”, to which I cringe for that is breaking the rules of science.
So back to my possible response, which could have been “How do you not”. To which the reply can be “We are here aren’t we”, which is the circular horse before the cart, the chicken before the egg, or is it the egg before the chicken. So I won’t put you into that.
Biologists, and I mean people who actually do biology related work for a living, never touch darwin. It is not about then, it is about now and looking forward. They recognize structure, for they know there are bounds they cannot cross, limits to which they cannot trespass.
If I had to look at the history, I would say darwin had the privilege of fluid, but everything we have learned that is actually accurate and not just a generational peculiarity, has headed into the structured, a very long way now from the fluid.
And finally all dogs, are just wolves. Every single one of them. And the broccoli is just the cauliflower. Nothing more.