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Published online by Cambridge University Press: 31 May 2012
The above title, which forms the subject-matter of the present article, is one that has claimed the attention of the most eminent naturalists of all time. Although much has been said and written upon the subject, still there is room for more. As science advances in its onward march new facts are developed ; some of these have a tendency to subvert long established principles, others to conErm pre-existent notions.