Chapter 12: Geographical Distribution

Geographical Distribution highlights that areas with closely parallel conditions, such as the New World and the Old World, or Australia, South Africa, and South America in the same latitudes, possess vastly dissimilar living productions. A key finding is the crucial role of barriers—including wide oceans separating continents, lofty mountain ranges, and large deserts—in creating differences between regional productions by preventing free migration. Conversely, a deep organic bond, attributed to inheritance, explains the affinity among distinct yet closely related species found across different points of the same continent, suggesting they share a common ancestor specific to that landmass. This evidence supports the concept that each species originated in a single area and subsequently migrated as far as conditions permitted, rather than being created simultaneously at multiple points. The mechanism of dispersal is explored, noting that changes in land level and climate, such as the submergence of an isthmus, have historically allowed faunas to blend. Beyond these geographical shifts, numerous occasional means facilitate transport across barriers; experiments demonstrated that many seeds can retain their germination power after prolonged immersion in sea-water, especially when dried or protected within floating fruit, wood, or stones embedded in tree roots. Furthermore, living agents like birds transport seeds, sometimes in their crops or intestines, or accidentally embedded in dirt adhering to their feet or beaks. This movement, combined with transport via fish and even swarms of locusts, helps explain migration across moderate tracts of sea. The perplexing observation of identical plant species living on widely separated mountain summits and in arctic regions is resolved by the geological evidence of the Glacial period. During the maximum cold, Arctic forms and Alpine species descended and spread across the temperate lowlands. As the climate warmed, these forms retreated northward or ascended mountain slopes, leaving isolated, identical populations stranded on distant peaks and arctic lands. Looking further back, the uniformity of sub-arctic and temperate productions prior to the Glacial period is accounted for by a formerly warmer Pliocene era, during which continuous circumpolar land allowed for the intermigration of species across the northern hemisphere. Finally, this distribution pattern is illuminated by Mr. Croll’s theory that Glacial periods alternate between hemispheres; when the Northern Hemisphere was cold, the Southern was warmer, allowing temperate species from both regions to temporarily cross or survive on equatorial mountains. Because northern forms originated in a larger land area, they are hypothesized to have achieved a higher degree of modification and dominance, resulting in a greater preponderance of northern species migrating south compared to southern species migrating north.