A bold, up-front assertion that Native Americans were in fact a distinct species from Europeans.
The word "Species" is mixed up in all manner of biological and sociological turmoil. I was even of the impression that becuase of DNA sequencing the term is falling out of favour amongst researchers.
DNA sequencing just replaces it with more accurate things (say the amount of DNA two individuals have in common, estimates of how long ago the last common ancestor lived, etc).
The most commonly used definition of a species today is a group that is capable and commonly practices interbreeding between its members.
There are different species that are capable of interbreeding and even producing fertile offsprings but they do not breed for various reasons this can be as simple as a geographical barrier or as nuanced as a small visual difference like a red patch of feathers vs a black one that restricts them from breeding outside of their group.
A more wider definition of a species is the largest gene pool of individuals technically capable of breeding but this one is not often used.
Overall there would be more loose and more strict definition of species that would be used based on the subject matter at any given moment.
The genetic isolation of Native Americans was clearly a result of a geography.