At last gleams of light have come, & I am almost convinced (quite contrary to opinion I started with) that species are not (it is like confessing a murder) immutable.
It wasn't the first time Darwin revealed how significant the doctrine of immutability was in mid nineteenth century thought, and it demonstrates once again the importance of historical context.
To understand the evolution genre one must understand the history of thought behind it. In this case, one of the several metaphysical motivations for evolution was (and is) the claim that if God created the species they would be fixed. Indeed, divine creation would produce a static, unchanging world.
This thinking is often associated with the great eighteenth century Swedish botanist Carl Von Linne, or Linnaeus. At one time he advocated the fixity of species concept and later was troubled when he discovered hybrids—species that are produced by the crossing of two related species.
Linnaeus softened his doctrine of fixity of species, but this was inconsequential. His system with its conception of species became deeply rooted, and the nineteenth century began with the notion of species as immutable still strongly in place. This notion was increasingly being challenged but it was nonetheless a major obstacle for Darwin to overcome.
It was therefore highly significant when Darwin became persuaded that related populations of birds he saw at the Galapagos were actually different species. If there was the slightest foundation for this idea, Darwin had anticipated in a famous notebook entry, it "would undermine the stability of species."
Today's readers often fail to understand the significance. After all, what can be so important about some different birds on some islands? Certainly the birds did not suddenly reveal to Darwin how fishes could change to amphibians, or how amphibians could change to reptiles, or how reptiles could change to mammals. Rather, the revelation was that the idea of divine creation was suddenly becoming untenable. The crucible for Darwin was not an abundance of positive evidence for evolution but rather negative evidence against creation.
Evolutionist Ernst Mayr has pointed out that Darwin's conversion from creationist to evolutionist was due to three key scientific findings and later reinforced by several additional findings. These scientific findings were all findings against creation. In other words, the key evidence that swayed Darwin was not direct evidence for evolution but rather evidence against creation that indirectly argued for evolution.
And as Mayr further points out, the doctrine of fixity of species was a key barrier to overcome in order if the concept of evolution was to flourish:
Darwin called his great work On the Origin of Species, for he was fully conscious of the fact that the change from one species into another was the most fundamental problem of evolution. The fixed, essentialistic species was the fortress to be stormed and destroyed; once this had been accomplished, evolutionary thinking rushed through the breach like a flood through a break in a dike.
The pre-Darwinian metaphysic was that species were fixed and essentialistic. Evidence for small-scale change argued against the old view and in so doing became an important proof text for evolution.
This is the story behind Darwin's concerns. And it explains why today evolutionists casually claim their theory is a fact--after all, we have discovered adaptation. If there is change, then divine creation is false, and if creation is false then evolution, in one form or another, is true.
Metaphysical claims such as these mandate evolution. They underwrite the fact of evolution. The rest is just research problems on how evolution occurred—the theory of evolution.