Darwin opens by saying that while on board the HMS Beagle as a naturalist in South America he made observations about the animals and the land which formed the foundation for what would become his theory of natural selection. He hopes this theory, which he will develop in this book sheds light on “the mystery of mysteries”, the origin of species.
Darwin tells how, when he returned home in 1837, he thought that he may have something worth investigating so he spends the next years accumulating and reflecting on information. In 1844 he writes a sketch of the conclusions that his research is pointing towards but he holds off expanding his ideas, instead researching further, for another 15 years.
He calls his work an abstract for a larger book which will take many more years to complete. He notes that he felt compelled to publish his work now after reading the manuscript of Alfred Russel Wallace, who had arrived at similar conclusions as Darwin and was about to publish his findings. As an abstract he notes that it may be incomplete, imperfect and at times lacking references, but that any questions which arise will be answered when he publishes his findings in full.
Darwin acknowledges that other naturalists have observed the relations between similar species but have attributed the cause of these changes exclusively to external conditions such as climate and food. He sees these causes as insufficient to account for the great variation in nature. In order to research what other forces could be at work he will look first at his research with domesticated plants and animals even though this was not a common practice of other naturalists.
The introduction closes with Darwin laying out in the arguments that each of the following chapters will explore in more detail. In particular he explains that traits may pass from one generation to the next, that into each generation more individuals will be born than can possibly survive and the ones that do survive will do so because of slight competitive advantages that they inherit and consequently pass on to their offspring. This natural selection is what, over time, causes some species to evolve, others to diverge and become their own unique species and others to become extinct. This being true, all species are in some manner related to each other and not independently created.