This compilation of three novellas and a short story was first published in 1957. They are set in the same universe, starting with the colonisation of the Solar System and concluding thousands of years later with humanity widely spread across the galaxy. However, the theme of the stories is very different from the space opera which this might suggest.
The first novella, 'Seeding Program', features the battle over the best method of colonising the Solar System, using three competing methods: building airtight domes around settlements, terraforming the worlds, or modifying human beings so that they could live in very hostile environments, by means of drastic genetic work before conception. Big business strongly favoured terraforming, as it was the most expensive solution and promised big profits over a long period. They were accordingly strongly opposed to the genetic modifications producing 'Adapted' people as these would bring them no such benefits, and closed down the work – but not before a colony of the Adapteds was set up on the surface of Ganymede. The story features one man who was later modified in a similar way and sent to Ganymede as a spy, with the aim of closing down the settlement. Naturally, all does not go to plan.
'The Thing in the Attic' moves far into the future, during the programme of 'seeding the stars' with Adapted humans. In this story, the humans become small, monkey-like, tree-dwelling beings living in a world dominated by a huge forest. It follows the adventures of a small group of rebels who are sentenced to live on the very dangerous forest floor. The theme here is the need to move from their comfortable environment and overcome the dangers of the surface if they are to conquer their world.
The third novella is a well-known classic: 'Surface Tension'. In this case another seeding starship crashes on a wet and almost barren planet, and the only form of humanity they can devise is at the microscopic level, living in ponds. They are left a 'history' in tiny engraved plates, but find it difficult to understand. The epic journey some of them make in a two-inch 'spaceship' crawling from one pond to the next is wonderful.
The final short story, 'Watershed', completes the circle after thousands of years with another seeding ship returning to a ruined and abandoned Earth, ready to seed it with humans modified to live in its changed environment. The key focus here is on the racial discrimination that unmodified humans feel for the Adapted ones – and the fact that the Adapteds are now in a majority, in a huge variety of forms across the Galaxy.
It is easy to dismiss the stories as dated, particularly since we now know that Ganymede doesn't even have the tenuous atmosphere Blish described, and certainly couldn't grow any plants. Even ignoring this, it is hard to imagine that humans could have their biochemistry so drastically modified as to tolerate the conditions in the story. And while Blish's training as a microbiologist shows in 'Surface Tension', the mind boggles at the concept of microscopic humans being as intelligent as full-size ones – and conversing with intelligent microbial life!
Despite these issues, I found the stories intriguing and worth the read. 'Surface Tension', in particular, is one of the standard canon of stories which all SF enthusiasts should read. The meticulous attention to the physical constraints of an alien world recalls Clement's 'Mission of Gravity'.