French-born American astronomer whose survey of bright, relatively nearby galaxies led to the discovery of the Local Supercluster. In his influential Physics of the Planet Mars (1954) he adopted a cautious position on the question of Martian life, arguing that while low forms of vegetation remained a possibility (see Mars, vegetation on), the "overwhelming majority" of terrestrial organisms would not be able to survive there. However, he pointed out that a second school of thought in the 1950s was more optimistic and emphasized the adaptability of life.