Summary

In this chapter I have attempted to show that the arrangement of all organic beings throughout all time in groups under groups⁠—that the nature of the relationships by which all living and extinct organisms are united by complex, radiating, and circuitous lines of affinities into a few grand classes⁠—the rules followed and the difficulties encountered by naturalists in their classifications⁠—the value set upon characters, if constant and prevalent, whether of high or of the most trifling importance, or, as with rudimentary organs of no importance⁠—the wide opposition in value between analogical or adaptive characters, and characters of true affinity; and other such rules⁠—all naturally follow if we admit the common parentage of allied forms, together with their modification through variation and natural selection, with the contingencies of extinction and divergence of character. In considering this view of classification, it should be borne in mind that the element of descent has been universally used in ranking together the sexes, ages, dimorphic forms, and acknowledged varieties of the same species, however much they may differ from each other in structure.