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134 Religious or ideological motivations might play a role. Stapledon thought that worlds that suffered from the mania of religious imperialism would seek interstellar travel long before economic necessity forced it on them. Science fiction author Stephen Baxter put a more positive spin on the idea: Perhaps we will build starships as we build cathedrals, as repositories of faith sailing into the future.” The most universal motive for interstellar colonization will be the most basic of all: survival. Civilizations that survive for billions of years must migrate, sooner or later; as Sagan put it, their eventual choice, as ours, is spaceflight or extinction. Both Sagan and Chyba believed that this repre- sented a cosmic selection effect, putting pressure on organisms to develop technology.” Even it were not true that survival depends on colonization, Barrow and Tipler speculated that there will be a group in any intelligent species that believes that it does. They will launch the starships.” Some civilizations whose stars were becoming unsuitable may have dealt with this challenge long ago by expanding away from their original homes, colonizing other systems or achieving a sustainable existence in interstellar space. There could be a widespread pattern of technological civilizations expanding outward as their stars age, but at very different times in galactic history. Migration might be a powerful force shaping the presence of intel- ligent beings in the galaxy. Bracewell drew an analogy. The fact that the Earth is populated with intelligent creatures is not because many habitable areas of Earth fostered the evolution of intelligence, but because one area (Africa) was the scene of events. Humans could have walked from Africa to California in less time than it would take for intelligence to evolve in California. Migra- tion is a faster method of civilizing the Galaxy than independent evolution. Interstellar Migration, or Interstellar Cultural Diffusion? For some years now, researchers have debated whether the spread of agricultural techniques into Neolithic Europe resulted from the migra- tion of farmers from the Middle East or from the diffusion of those techniques through cultural transfer from those farmers to an older European population. Recently, some have suggested that both pro- cesses may be involved, as when early farmers intermarried with indig- enous hunter-gatherer females. SETI orthodoxy assumes that the cultural transfer model will prevail at the interstellar level. If interstellar migration occurs, the expansion of a culture could take place by more direct means. Direct Contact