"There is overwhelming evidence that Linneaus did not
believe in fixity of species."
It's a bit complicated.
I'll start with the observation that L. rejected the early evolutionary
theory by Buffon. (Not that it was worth much, AFAIK...)
L. believed that the genera (literal meaning: something along the line of
"clans") had been created, while species (literally: "views") could arise by
hybridization of other species of the same genus. (In plants, at least, he
suggested several concrete examples.) This went so far that to him
extinction was impossible -- if a species would die out, it could always
arise anew, in identical form, by hybridization of two others.
Systema Naturae has a religious hymn every few pages.
That said, the good man was pretty far ahead of his time in other respects.
Remember that he called the chimpanzee (erm... including the orang-utan)
*Homo troglodytes*. In a private letter he even wrote he should have fused
this genus with *Simia* (all monkeys) because there were no "genus-level
differences", but this, he wrote, would have led to an even bigger uproar.
Now keep in mind that T. H. Huxley put us into Kingdom Psychozoa over 100
years later.