The Seedling Stars by James Blish. The Gnome hardcover was in 1956 or 1957 (sources vary); the first paperback was in 1959. This is more a collection of related stories than a fix-up.

Seeding Program - (original title= "A Time to Survive")
The Thing in the Attic
Surface Tension
Watershed

Original appearances from 1942 to 1956 - in Magazine of F&SF, (2) in IF, Super Science Stories, Galaxy. "Surface Tension" as it appears here is combined and revised from "Sunken Universe" (1942) plus "Surface Tension" (1952.)