 Chapter 8 of the Origin of Species titled “Hybridism” is devoted to discussing whether hybrid sterility serves as an adequate criterion for distinguishing species from varieties. Such naturalists as John Ray and Buffon held that hybrid sterility marked the species/variety boundary. They believed that offspring from parents of different species are sterile, whereas offspring from parents of different varieties of the same species are fertile. Much of Darwin’s chapter on hybridism is dedicated to providing counterexamples to the claim that hybrid sterility marks a distinction between species and varieties. In the end, Darwin rejects hybrid sterility as a criterion for distinguishing species and varieties. He writes, “It can thus be shown that neither sterility nor fertility affords a clear distinction between species and varieties” (1859 [1964], 248).