We have learned about "Variation under Nature" and "The Struggle for Existence" in the last two chapters, so now we can look into the combined effect of this two forces "Natural Selection".

Chapter 3 introduces the mechanism Natural Selection uses to create new species and that is the Struggle for Existence among organisms.

Chapter 1 talked about how variation in domestic animals exists and how it led to the existence of new domesticated species. Chapter 2 does the same for variation under Nature.

He specifically mentions that this is not a complete representation of his work, he says that the book is an abstract he decided to publish early.