The Advent of Simon Stark

In this "they come here" SF novel, colonists from another planet have their eye, and their man, on Earth. The Solgrens are ethereal beings who have long since forgotten the burden and the pleasures of being confined in a physical body. More than anything, they are entralled with humans' ability to focus the physical, the mental and the emotional on one project at one time. Will their bodymeld insert accomplish what they put him on Earth to do, or will humanity overtake him? And if it does, what then? Destroy the planet and move on, or opt for Plan B?