Chapter Three The Invasion of Continents
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When contemplating the invasion of continents and islands and seas by plants and animals and their microscopic parasites, one’s impression is of dislocation, unexpected consequences, an increase in the complexity of ecosystems already difficult enough to understand let alone control, and the piling up of new human difficulties. These difficulties have mounted especially in the last 150 years, and they have had to be met by means of a series of fairly hasty and temporary measures of relief that are only here and there supported by fundamental research on populations, or even a systematic record of events. Indeed it is easy to feel like Edward Gibbon, who wrote at the end of The Decline and Fall of the Roman Empire: ‘The historian may applaud the importance and variety of his subject; but, while he is conscious of his own imperfections, he must often accuse the deficiency of his materials.’ This is not, however, to criticize the biological workers who have had to grapple with an unending string of unforeseen emergencies with the scanty means at hand; and there are a certain number of remarkably fine and carefully compiled histories of invasions, notably by the various branches of the United States Department of Agriculture, who were the first to bring some sort of method and order into this field. In the present chapter it will only be possible to select a few examples, and these are not so much chosen for their economic or medical or veterinary importance, as to illustrate the ideas of this book, or because they happen to have good maps of the invasions of continents by foreign species.