The Origin of Species · Chapter
Alternate Glacial Periods in the North and South explained
Darwin extends Forbes's view of glacial effects to both hemispheres, using evidence from Europe, Asia, New Zealand, Australia, and the Americas to show that glacial periods were recent and extensive.
What happens
Darwin extends Forbes's view of glacial effects to both hemispheres, using evidence from Europe, Asia, New Zealand, Australia, and the Americas to show that glacial periods were recent and extensive. He invokes Croll's theory that cold periods in one hemisphere coincide with warmer periods in the other, explaining the distribution of identical and allied species across temperate zones and tropical mountains. Northern forms, being more dominant due to larger land areas and stronger competition, have migrated southward more successfully, leaving a 'living drift' on mountain summits as a record of past climates.
Themes in this chapter
Geographical Distribution and Barriers
The chapter explores how glacial periods and alternating climates have shaped the distribution of species across hemispheres, with mountains acting as islands and barriers to migration.
Natural Selection and the Struggle for Existence
Northern forms are described as more dominant due to natural selection and competition, enabling them to outcompete southern forms during glacial intermingling.
Extinction and the Succession of Forms
The chapter discusses how endemic Alpine forms have yielded to more dominant northern forms, and how naturalized species can lead to extinction of natives on islands and mountains.
Characters to notice
- Dr. Hooker
Cited for observations on identical species in America and Australia, on moraines in the Atlas range and Sikkim, and on Australian forms naturalizing in the Neilgherrie Mountains.
- De Candolle
Mentioned for his insistence on the preponderant migration from north to south in Australia.
- Sir Charles Lyell
Referenced for speculating on the effects of great alternations of climate on geographical distribution and for his work on the relative position of land and water during glacial periods.
- Mr. Croll
His memoirs on the eccentricity of the earth's orbit and its influence on oceanic currents and glacial periods are central to the chapter's argument.
- Edward Forbes
His view on glacial effects is extended by Darwin, and his observation of isolated Arctic productions in northern temperate seas is cited.
Key passages
“Thus, we should have some few species identically the same in the northern and southern temperate zones and on the mountains of the intermediate tropical regions.”
This process would result in a small number of identical species appearing in both temperate zones and on tropical mountains.
Darwin summarizes the expected outcome of glacial migration and mixing.
“I suspect that this preponderant migration from the north to the south is due to the greater extent of land in the north, and to the northern forms having existed in their own homes in greater numbers, and having consequently been advanced through natural selection and competition to a higher stage of perfection, or dominating power, than the southern forms.”
The greater land area and higher population of northern species have allowed them to evolve greater competitive strength through natural selection, explaining their dominant southward migration.
Darwin explains the asymmetry in migration patterns between hemispheres.
“Mountains are islands on the land; and their inhabitants have yielded to those produced within the larger areas of the north, just in the same way as the inhabitants of real islands have everywhere yielded and are still yielding to continental forms naturalised through man’s agency.”
Mountain species, like island species, are being replaced by more competitive forms from larger northern landmasses, similar to how human-introduced species outcompete native island species.
Darwin draws an analogy between mountain and island biogeography.
“The living waters have flowed during one period from the north and during another from the south, and in both cases have reached the equator; but the stream of life has flowed with greater force from the north than in the opposite direction, and has consequently more freely inundated the south.”
Life has spread alternately from north and south to the equator, but the northern flow has been stronger, leading to greater colonization of the south.
Darwin uses a metaphor to describe the asymmetric dispersal of species due to glacial cycles.
