In The Origin of Species (the shortened title of his book from the second edition onward), Charles Darwin devotes his first chapter to the diversity observed in domesticated plants and animals. One might wonder why such a well-traveled naturalist would initially focus on these common, well-known species.
Since his book was written in the 1850s, well before scientists, let alone the general public, had much knowledge of inheritance, Darwin apparently chose this approach to insure that his points were more easily understood, based on the personal experience of his audience. In essence, he uses the chapter to illustrate the diversity of domesticated species and the diversity of individuals within any given species. Placing emphasis on the selective breeding process, engineered by humans, he sets the stage for his theory of natural selection to follow.
Clearly conscious of the potential reaction from other scientists, religious leaders and the public at large, Darwin eases his readers into the concept of evolution, a process that negates the role of a Creator. In the mid 19th Century, that was a courageous undertaking indeed.