It is inconsistent with the nature of life - as revealed by the record of the past - for a species to remain in an environmental niche when the opportunity exists for escape. Most individuals of the species remain within the security and comfort of the environment to which they have become adapted... [But] certain individuals will always probe the limits of their environment. These adventurous few are the vanguard of a new development in the evolution of life... As most fish remained in the water, and most apes remained in the forest, just so, in tomorrow's world most of us will remain on the earth... But a small percentage of the human species... will leave us, and their descendants will spread out into the galaxy.
Robert Jastrow
Introduction to The Next Ten Thousand Years by Adrian Berry, 1974