Darwin closed this chapter on biogeography with a discussion of how past glacial episodes affected the distribution of plants and animals. This section of the Origin is significant because the fact of past glacial episodes was just being worked out by contemporaries of Darwin. The idea that alpine valley glaciers and vast continental ice sheets once covered portions of the Earth currently not under ice was still gaining acceptance. Much to his credit, Darwin was an early advocate of the new glacial theory.
Glacial theory made sense to Darwin because it helped him to make sense of modern day biogeographic patterns, like alpine plants and animals occupying widely-separated mountain summits. Darwin could explain their dispersal from a common origin, migrating up and down mountain slopes following temperature zones as ice sheets waxed and waned.
Photo credit: http://www.scotland.gov.uk/Resource/Img/47121/0021354.jpg
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