Man's place in the universe. A study of the results of scientific research in relation to the unity or plurality of worlds.

The first serious attempt by a biologist to evaluate the likelihood of life on other planets. Wallace concluded that the Earth was the only planet in the solar system that could possibly support life, mainly because it was the only one in which water could exist in the liquid phase. More controversially, Wallace maintained that it was unlikely that other stars in the galaxy could have planets with the necessary properties, as the existence of other galaxies had not yet been proved.