" Over and over again in the study of the history of life it appears that what can happen does happen. There is little suggestion that what occurs must occur, that it was fated or that it follows some fixed plan, except simply as the expansion of life follows the opportunities that are presented. In this sense, an outstanding characteristic of evolution is its opportunism.["Meaning of Evolution," 1949, p. 160.] "

― , The Meaning of Evolution: A Study of the History of Life and of Its Significance for Man, Revised Edition