Why were later 19th century social thinkers attracted to the idea of 'evolution'?

The idea of evolution had two broad attractions for social thinkers, one concerning its intellectual history, and the other concerning the political and social implications of its conclusions. In the former category, evolution provided the relatively new subject of sociology with a way to 'find its feet', while lending the factually-based credibility of the natural sciences to social science. As for the specific conclusions which social thinkers drew from the concept of 'evolution', the biological …

…were also popular among academics, appearing to justify both the withdrawal of support for the poor and the continuation of colonial expansion. MacRae's conclusion is perhaps the most appropriate response here, making the acute point that although in biology Darwinism is useful as a theory, in sociology the principle might just 'explain too much' - it can be applied to so many different situations where it either does not hold water or is simply unpalatable.