Chapter 9: Hybridism

A central argument presented is that sterility is not a specially endowed quality designed to keep species separate, but rather an incidental result of physiological differences in their reproductive systems. To support this, a crucial distinction is made between the difficulty of effecting a first cross and the potential impotence of the resulting hybrid offspring. While many naturalists historically viewed universal sterility as a definitive barrier between species, evidence from renowned observers like Gärtner and Kölreuter reveals that fertility exists on a graduated scale, ranging from absolute zero to perfect productivity. The text explores the concept of systematic affinity, noting that while closely related forms often cross more easily, this is not an absolute law, as evidenced by reciprocal crosses where the success of an union depends entirely on which species acts as the male or female parent. Furthermore, the chapter draws a striking parallel between the laws of hybridization and the laws of grafting in plants; just as the ability of one tree to be grafted onto another is an incidental consequence of their vegetative systems, the ability of species to cross is an incidental consequence of their reproductive constitutions. The author argues that natural selection could not have produced sterility, as it offers no direct advantage to the individual and frequently appears in species that inhabit separate regions and never interact in nature. The study of dimorphic and trimorphic plants further complicates the traditional view, showing that different forms of a single species can exhibit mutual sterility similar to that of distinct species, suggesting that reproductive barriers are not synonymous with specific distinction. Finally, the chapter compares hybrids to mongrels (the offspring of crossed varieties), noting that while varieties are generally fertile, the occasional appearance of lessened fertility in certain crosses suggests that varieties and species are governed by the same underlying biological principles. Ultimately, the similarities in how hybrids and mongrels inherit traits and vary over generations reinforce the theory that species are simply strongly marked and permanent varieties.