Spawning Ground

"Spawning Ground" by Lester Del Rey is a science fiction story taking place when the Solar System is about to be destroyed, compelling humankind to seek refuge on other planets. It's about how humans change when they live in new places and face strange conditions. Captain Gwayne and the crew of the Pandora land on a planet where they find blob-like things and human-like people. When two young crew members go missing, Gwayne hurries to save them and learns that the planet's inhabitants can change humans. He also finds Captain Hennessy, who has changed a lot, and learns that the blob creatures have secrets. Gwayne figures out they can't go back to Earth and that they must stay on the planet in order for humanity to continue.