Technically, yes, members of different races can interbreed and members of different species cannot. Meanwhile, the Star Trek universe referred to beings from different planets as different species even though, in that universe, they were capable of interbreeding. In that way, basically, all hominids were one species, and green, grey, blue, tan, brown and other colors of skin, bumpy foreheads, pointy ears, horns and whatever mark different races.

Assuming that mut is a scientific term that equals the scientific, not layperson's, meaning of species, then the interbreeding definition is one possible meaning of it, but not the only one. That definition is that a species is that taxonomic classification of creature whose members can mate and regularly produce fertile offspring. A mule, for instance, is the offspring of a male donkey and a female horse, but it is usually infertile, so mules are not considered a separate species; they're hybrids.

But that's not the only scientific definition of species. Depending on the branch of science, a species might be defined by DNA, or morphology, or ecology. It's not so simple as whether or not they can interbreed.

Star Trek's cross-breeding aliens are so outrageously impossible that to even try to make sense of it is an endeavor doomed to failure. Such characters are almost always writers' attempts to introduce a human perspective into an otherwise alien culture, at least in early stories. Once a franchise establishes that hybridization is inexplicably and freely available, hybrid characters start to show up simply because of the sheer impossibility of them not to.

This is an instance where one must simply suspend one's disbelief and move on.

One also has to wonder whether mut is a pun on the English word mutt.

Even the purest Brits are “Anglo-Saxons”, which doesn’t sound very much like a pure race.

The Britons were in Britain long before the Angles, Saxons, and Jutes. So were the Picts, but good luck tracing your ancestry to them.

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SuStel
http://trimboli.name