I would submit that his theory was inevitably and catastrophically flawed from the very beginning - His premise is that 'life changes because it must', whereas the truth lies closer to 'life changes because it CAN'. He envisions a need to exploit resources which stems from the idea that existing resources are insufficient; ergo, modifications over time allow the species to better exploit what it could not exploit before.
What he failed to account for is a myriad of other species better adapted to exploit that very resource (no matter what it is). The specialist species (probably of several varieties) already far exceed the ability of an interloper, by their own established specializations. Hence the interloper, presumed by Darwin to be in desperate straits, cannot compete, and would predictably die out (as we all know species under duress are wont to do)...
This is the thing he missed, and the thing that explains his errata. The adaptation of species happens because it can. An environment has to exist wherein a resource is not being exploited by any specialist species. The profit of adapting drives the adaption, the resource produces a flourishing of the species, and that flourishing drives the adaptation even further. This points to a much faster adaptation than current evolution theory allows for, and is more in line with proofs available to us today in the near past where such adaptation can be observed. This is what the Galapagos Finches demonstrate.
But his basic theory has been confirmed innumerable times: 1) descent with modifications (mutations)[...]
In this I have no argument, with the exception that I will fervently declare that such is restricted to species and does not extend to genre (kind).
and 2) natural selection ("survival of the fittest") accumulating small changes over many generations can result in separated populations which no longer interbreed and are therefore classified as different species.
This I would largely deny. As I said before, I think adaptation is rather quick, and and is driven by an ability to profit from a resource that is not currently being exploited, by a species with enough depth to withstand the change (and provide it by genetic diversity). This is what we can observe in real time, without (too much) extrapolation...
Darwin produced no theories regarding origin of life on Earth, but merely speculated on something possibly growing within a "primordial soup". Today there are several working hypotheses on origin of life, none strongly confirmed, but Darwin's basic evolution idea is considered as much fact as theory.
And in the assumption of 'fact', the error remains. Note that I do not mean Darwin himself any offense (except in that he missed), at least as far as 'Origin of Species' goes - Albeit that many of his comments thereafter bring offense. My backhanded compliment was that at least he took scientific method seriously, and strove to provide errata, something that is lost upon today's generation.
Despite your neutral, even scholarly, tone your points are all "catastrophically flawed", and amount to nothing more than denial of the obvious -- all of which I'll explain in depth as time permits, which sadly, is not now...
I'd call your criticism of Darwin totally trivial, to the point of being meaningless.
"Can" versus "must" is a ridiculous distinction, worthy of those ancient theologians who debated angels on pin heads.
You might just as well condemn Newton's laws of motion because Newton knew nothing of Einstein's relativity theory.
Roamer_1 : "I will fervently declare that such is restricted to species and does not extend to genre (kind)."
The word "species" is a scientific term of biological classification, as are words like "genus", "order", "phylum", etc.
All such terms are defined by science, in order to classify every living thing on Earth.
But there are no biological terms "genre" or "kind".
Indeed "kind" is a strictly religious term, which defies all attempts to specify scientifically.
Therefore, regardless of how "fervently" you declare it, your words are meaningless.
As for the process of speciation itself, there are abundant though highly incomplete records of it in fossils, and corresponding records in comparisons of various species DNA.
What these all illustrate is that the more ancient two species common ancestors, the more different are both their DNA and physical morphology.
In short, they confirm Darwin's theory.
Roamer_1 : "This I would largely deny. As I said before, I think adaptation is rather quick, and and is driven by an ability to profit..."
Only anti-evolutionists distinguish between "adaption" and "evolution", or "micro" versus "macro".
In real science, they are precisely the same thing, one shorter-term, the other longer-term.
But the process is the same throughout, as first described by Darwin: 1) Descent with modification and 2) Natural Selection.
Roamer_1 : "...at least he took scientific method seriously, and strove to provide errata, something that is lost upon today's generation."
No, most of what you call "errata" has long since been fully investigated and resolved.
That's why basic evolution is not an issue to science, it's not even just a theory, but is strongly considered more - or - less fact.