From Today In Science History:
In 1868, Charles Darwin's book - Variation of Animals and Plants under Domestication - was published. He was 58. It is probably the second in importance of all his works.
This was a follow-up work, written in response to criticisms that his
theory of evolution was unsubstantiated. Darwin here supports his views
via analysis of various aspects of plant and animal life, including an
inventory of varieties and their physical and behavioral
characteristics, and an investigation of the impact of a species'
surrounding environment and the effect of both natural and forced
changes in this environment.