But the boundaries between species are distinct and firm-...
The author neither defines "species" nor gives evidence for such a claim.
The author neither defines "species" nor gives evidence for such a claim.
And a good thing too! Just in the area of North American birds, of which there are less than a thousand species, I can recall at least ten major disputes about speciation. It used to be thought that Baltimore orioles and Bullock's orioles are the same species. Molecular biology now shows they're not even closest relatives. The common towhee was one species, and now is two. The slate colored Junco was several species, and now is one. Ditto the yellow rumped warbler. The crossbill may actually be a complex of at least nine almost indistinguishable but reproductively isolated species.
Nobody with the slightest practical experience of biology would make such an idiotic statement as the one you quoted.