 Understanding Spencer's theoretical orientation requires some background in 19th-century biology.  Darwin's theory of evolution was not the first modern attempt to account for diversity of life on the planet without invoking an act of divine creation (Levins and Lewontin, 1985, 27–28).  Jean-Baptiste Lamarck (1744–1829) explained the apparent match between each animal species and its environment by positing that individual animals could acquire and pass on adaptive characteristics.  This explanation requires, first of all, that animals strive to adapt to their environment.  Second, it requires that the animals change physically as a result of their efforts, and third, that they pass on their acquired characteristics to their offspring (Levins and Lewontin, 30).  In contrast, Darwin hypothesized that randomly occurring variation among individual organisms could be preserved or destroyed in the population of organisms through differential reproductive success.  That is, he conjectured that some characteristics are correlated with the ability to produce a greater number of offspring.  These characteristics will tend to increase in a population over time.  Darwin called this mechanism natural selection.  Natural selection bypasses the problematic assumption that individual organisms could alter themselves through deliberate effort.  Yet it can still explain why organisms seem to fit into their natural environment so well (Levins and Lewontin, 1985, 31–34). 