Chapter 5: Laws of Variation

Instead, the narrative establishes that variability is deeply connected to the environmental conditions an organism experiences over many generations, acting both directly on the physical structure and indirectly through the reproductive system. A central theme is the influence of habit, specifically the increased use or disuse of physical parts; organs that are frequently utilized tend to strengthen and enlarge, while those neglected—such as the wings of island-dwelling beetles or the eyes of subterranean animals—gradually diminish, often aided by the efficiency of natural selection which preserves energy by reducing superfluous structures. The text also details the phenomenon of correlated variation, where different parts of an organism are so biologically linked that a modification in one area inevitably triggers changes elsewhere, even if those secondary changes offer no immediate functional advantage. Furthermore, the discussion highlights that specific characters, which distinguish one species from another, are inherently more prone to fluctuation than the stable generic characters shared by an entire group, largely because these specific traits have undergone more recent evolutionary modification. This variability is also notably high in secondary sexual characteristics and rudimentary organs, which are no longer strictly regulated by the pressures of survival. By examining patterns of analogous variation and the occasional "reversion" to ancestral traits—such as the appearance of stripes on various equine species—the chapter provides compelling evidence for common descent, suggesting that modern species retain a latent genetic memory of their shared progenitors. Ultimately, the laws of variation demonstrate that while the precise causes of individual differences remain largely mysterious, the steady accumulation of beneficial modifications through natural selection remains the primary architect of the harmonious complexity found throughout the natural world.