The Possessed

An alien race is forced to leave their planet in search of a new home. The alien explorers do not travel in a ship but, interestingly enough, they travel as a swarm through space. When they arrive on our planet, they divide into two swarms, one pursuing a search in outer space for another suitable planet while the swarm remaining here searches for intelligent life. This of course was eons ago. They eventually find what they believe is a suitable species, with an interesting outcome.