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The Quaternary period saw the global temperature continue to fall, culminating in what is known as the Ice Age (see Figure 28). Actually there is not one Ice Age but a whole series of glacial periods (principally in the Northern Hemisphere) interspersed by warm, even tropical, inter-glacials. And even the glacial periods themselves are divided into colder phases and temperate interstitials. This marked climatic oscillation is superimposed on the long-term global cooling that began in the mid-Tertiary. The most recent cycle ended only about 12,000 years ago. Plants in this period
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