The Origin of Species Chapter 9 Part 2 - Hybridism

In which the curious and variable sterility of hybrids and illegitimate unions among plants and animals is examined through careful experimentation and observation, revealing a close parallelism between the reproductive disturbances caused by unnatural conditions and those arising from the mingling of distinct sexual constitutions. We are led to conclude that the mysterious diminution of fertility signals differences confined to sexual elements, while the resemblance and variability of hybrid and mongrel offspring alike soften the boundary once thought to divide species from varieties.