Chapter 6: Difficulties of the Theory

In this pivotal chapter, the author addresses the primary challenges and objections raised against the theory of descent with modification, focusing on four major areas: the rarity of transitional forms, the origin of organisms with unique habits or structures, the development of highly complex organs, and the nature of sterility in hybrids. To explain why the natural world does not appear as a chaotic blend of intermediate varieties, the text posits that natural selection is an inherently exclusionary process where new, improved forms inevitably displace and exterminate their less-adapted ancestors and transitional relatives. The scarcity of these links in the fossil record is attributed to an inherently imperfect geological record, while their rarity in living populations is explained by the fact that intermediate varieties typically inhabit smaller, narrow transition zones where they are more susceptible to extinction compared to the larger populations they connect. The chapter further explores how dramatic shifts in life habits—such as a terrestrial animal becoming aquatic or a quadruped developing flight—are possible through gradual modifications, using examples like the webbed feet of upland geese or the gliding membranes of squirrels to show that structure does not always perfectly align with current habits due to the influence of inheritance. A significant portion of the discussion is dedicated to "organs of extreme perfection," specifically the eye; it is argued that if a series of graduated improvements from a simple light-sensitive nerve to a complex optical lens can be shown to exist, the formation of such an instrument through natural selection is logically sound. The concept of functional conversion is also highlighted, demonstrating how organs like the swim-bladder in fish can be repurposed into lungs for respiration. By distinguishing between analogous structures, which serve similar functions but evolve independently, and homologous structures, which reveal shared ancestry, the author reinforces that nature acts through "short and sure, though slow steps" rather than sudden leaps. Ultimately, the chapter asserts that all organic beings are governed by the law of Unity of Type, which is explained by shared descent, and the law of Conditions of Existence, which is managed through the ongoing pressure of natural selection.