“The Declaration of Independence calls Native Americans “merciless Indian savages.”’
With one major, notable difference, slime. Most of the founders (if not all) understood it as a cultural thing not a state of being. And some of them were savage in nature. It may also be noted that the early immigrants to America did not consider all of them savages. By contrast, Darwin pointed to their savagery as a state of being and something that would eventually be conquered by the more civilized.
But who’s counting facts?
As I alluded to in my previous post, your criticism of using the term “savage” is based on a linguistic anachronism. The term did not have the connotations or meaning that it does today. It just meant technologically undeveloped or uncivilized (in writings from that time you can even find phrases like “savage flowers” for wild flowers).
Some at the time even believed in the idea of the “noble savage” - with the modern definition of the word, that would be an oxymoron.
However, so many saw the “savages” as being violent, cruel, and bloodthirsty that the word not only began to pick up such connotations, but even began to mean that by definition.
So when Darwin calls a group of people savages, he was referring to them culturally. In fact Darwin spends a great deal of time in Descent of Man arguing that the advancement of civilization in europe was due to cultural, not biological, evolution (although there are a few times he seems to waffle on that idea and suggest that some groups are mentally inferior; he may have been unsure himself if it was wholly cultural or partly innate):
“The western nations of Europe, who now so immeasurably surpass their former savage progenitors, and stand at the summit of civilization, owe little or none of their superiority to direct inheritance from the old Greeks, though they owe much to the written works of that wonderful people.”
As for the American Founders believing that the savages were only different culturally, it’s hard to argue that with Washington making statements like the Indians having “nothing human except the shape”. And in Darwin’s time there was a popular idea among Creationists, particularly in America, called “polygenism”. Many of the scientists who opposed Darwin were polygenists. This idea said that various human races were completely unrelated. Thus many of them believed that only whites were descended from Adam and Eve, and so other races were perhaps not even really human.
There were many cultures Darwin encountered during his trip that he liked and many he didn’t like. Darwin was certainly no cultural relativist - if he didn’t like a culture - most notably the Fuegians, he said so.