Horror

Poul Anderson's "The Green Thumb" ends on a horror fiction note. Joe, challenged about his origin and intentions, has pulled a gun on Pete and Gunnar and then escaped into the forest. Gunnar deduces that the native Nerthusians, who do exist after all, have remained concealed and have sent Joe as a spy. After all, he has the same colour of fur and number of limbs as the native animals with whom he is in tune. 

The horror is in not knowing what the Nerthusians are really like or what they want. This could have been the prelude to a planet-wide war with the natives mobilizing their ecology against the handful of settlers. That is not to be if only because the rest of the series is not about Nerthus although some later events happen there. The next instalment takes us to another human-colonized planet. It was unusual for these two stories to be about one young boy and his relatives on a single planet. The future history of mankind on many planets continues.

It is confirmed that there is a network of intelligent species with interstellar travel so that it is possible to talk about "the Galaxy" in political terms without meaning by that just human civilization in the Galaxy but we are told almost nothing about the other species. As with the same author's later Technic History, we want to know a lot more.