What's interesting, though, are two things (both of which I was unaware of when I read this book way back in the early '70s): the first is the statement that prior to Star Trek science fiction on television did not have recurring characters. I don't believe that's actually true. Both Lost In Space and Voyage To The Bottom Of The Sea debuted a year or two before Star Trek and both had a cast of recirring characters. I also believe a few short lived series produced in the 1950s had recurring characters.
What page and passage are you referring to? As I've understood it, the idea was that TOS was the first
adult science fiction drama with continuing characters -- that the only previous SF shows that had been done as adult dramas had been anthologies such as
Science Fiction Theater, The Twilight Zone, and
The Outer Limits. There had certainly been kids' SF/fantasy shows with continuing characters, like
Adventures of Superman and
Tom Corbett, Space Cadet, but none aimed at adults, as far as I can recall.