“Natural selection” and “survival of the fittest” 
Spencer emphasized three developmental tendencies shared by societies and organisms: (1) growth in size, (2) increasing complexity of structure, and (3) differentiation of function.
Various schools in social theory (population ecology, functional analysis, and others) have embraced some of the principles of evolutionary biology. However, use of explanations of social change in terms of natural selection (or simply selection) has always been controversial in the social sciences. A major reason would seem to be that it is very difficult to explain and empirically show exactly how selection takes place.
Even so, there is little doubt that selection does play a prominent part in social change, and that thinking in terms of selection and selection processes is valuable as a corrective to thinking that seeks to explain everything in terms of rational agency. The Principles of Sociology is a book that belongs in any library of social classics and still deserves to be read.
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