 The conceptual synthesis of chapter four also introduced discussions of such matters as the conditions under which natural selection most optimally worked, the role of isolation, the causes of the extinction of species, and the principle of divergence. Many of these points were made through the imaginative use of “thought experiments” in which Darwin constructed possible scenarios through which natural selection could bring about substantial change. Although these were not regarded by his critics as empirical evidence for his claims, it has been argued that these assisted Darwin in satisfying certain criteria of adequacy as set forth by John Herschel’s methodological canons (Lennox 2005). One prominent way Darwin captured the complexity of this process is reflected in the single diagram to appear in all the editions of the Origin (Velasco 2013). In this illustration was summarized the image of gradual change from common ancestral points, the depiction of the frequent extinction of most lineages, the general tendency of populations to diverge and fragment under the pressure of population increase, and a way of envisioning relations of taxonomic affinity to time. It also depicted the persistence of some forms that are unchanged over long geological periods in which stable conditions prevail.