Chapter 13: Geographical Distribution Continued

Geographical Distribution Continued continues the examination of geographical distribution, focusing first on the unexpected and enormous global range of fresh-water productions despite apparent barriers of land and sea. This wide dispersal is explained by the capacity of these organisms for frequent short migrations, which makes them highly susceptible to accidental, long-distance transport. Evidence for this includes the ability of fish to move through interconnected river systems caused by shifting land levels or floods, the scattering of viable fish ova, and the crucial role of birds in moving smaller forms. For instance, minute, newly hatched fresh-water shells can cling firmly to a duck’s feet, surviving for many hours in damp air, allowing potential travel over hundreds of miles. Furthermore, seeds of aquatic plants are often transported within mud adhering to the feet and beaks of wading birds, or can survive ingestion and subsequent rejection in pellets by fish-eating birds like herons. The chapter then pivots to the specific characteristics of oceanic islands, which possess markedly fewer species than comparable continental areas, but maintain an extremely high proportion of endemic species—those found nowhere else. This pattern supports the view that islands are colonized by occasional immigrants which, facing new competition, undergo modification via natural selection in their isolated environments. The presence or absence of whole classes is heavily dictated by their ability to cross ocean barriers. A striking observation is the general absence of Batrachians (frogs, toads, newts) and terrestrial mammals on truly oceanic islands, as sea-water rapidly kills these animals and their spawn, preventing successful transport. Conversely, aerial mammals like bats are frequently found and are often endemic, because they possess the inherent power to fly across vast marine distances. Critically, the species inhabiting any given island display a close affinity to those found on the nearest mainland, indicating their origin from continental colonists. Within an archipelago—even one where islands are within sight of each other—distinct but closely related species are often found on separate islets. This micro-differentiation is attributed not primarily to physical conditions, but to the varied sets of competitors (struggle for life) present on each island, which favors different varieties. These accumulated facts, demonstrating the importance of migration, subsequent adaptation, and the influence of barriers, strongly support the theory of descent with modification from a single source, which cannot be explained by the alternative doctrine of independent creation.