In this chapter Darwin tackles the issue of reduced hybrid viability, believing that sterility was a by-product of the varied characteristics gained from two different species. To do so he mainly focuses on experiments done on plants and animals and the varied degrees of fertility of hybrids.
In a previous post I discussed wild hybridization and the ability of hybrids to survive in conditions that neither of their parent species thrive in. Many of Darwin's examples are of the intercrossing of closely related species, such as variety of cattle and geese. Darwin believed that the sterility of hybrids was caused by the incompatible crosses of reproductive organs. Today we know that sterility can arise through an incompatible number of chromosomes, a prominent example being mules. Horses and donkeys have different numbers of chromosomes and so the resulting Mule has an unpaired chromosome which leads to difficulty creating a viable embryo.
Darwin made a distinction between hybrids of distinct species and what he called "mongrels" or the offspring of distinct varieties of the same species. This difference leads to a powerful closing statement: "There is no fundamental distinction between species and varieties," (Darwin, 268) based on the observation that mongrels often have increased fertility.
The chapter is difficult to summarize without taking into account modern knowledge of hybridization. Darwin had no direct knowledge of pre and post zygotic barriers, but he did guess at the fact that some reproductive systems in hybrids were non-viable because the parent species were too incompatible.