But this principle, if sound, must be carried farther, and be applied to plants also. There are not wanting indications that this may be successfully done; and it seems not improbable, that the reason why botanists have hitherto failed to determine, with any unanimity, which are the most natural phytological regions, and to work out any connected theory of the migrations of plants, is, because they have not been furnished with the clue to the past changes of the great land masses, which could only be arrived at by such an examination of the past and present distribution of the higher animals as has been here attempted. The difficulties in the way of the study of the distribution of plants, from this point of view, will be undoubtedly very great; owing to the unusual facilities for distribution many of them possess, and the absence of any group which might take the place of the mammalia among animals, and serve as a guide and standard for the rest. We cannot expect the regions to be so well defined in the case of plants as in that of animals; and there are sure to be many anomalies and discrepancies, which will require long study to unravel. The Six Great Regions here adopted, are however, as a whole, very well characterised by their vegetable forms. The floras of tropical America, of Australia, of South Africa, and of Indo-Malaya, stand out with as much individuality as do the faunas; while the plants of the Palæarctic and Nearctic regions, exhibit resemblances and diversities, of a character not unlike those found among the animals. This is not a mere question of applying to the vegetable kingdom a series of arbitrary divisions of the earth which have been {163}found useful to zoologists; for it really involves a fundamental problem in the theory of evolution. The question we have to answer, is, firstly--whether the distribution of plants is, like that of animals, mainly and primarily dependent on the past revolutions of the earth's surface; or, whether other, and altogether distinct causes, have had a preponderating influence in determining the range and limits of vegetable forms; and, secondly--whether those revolutions have been, in their general outlines, correctly interpreted by means of a study of the distribution and affinities of the higher animals. The first question is one for botanists alone to answer; but, on the second point, the author ventures to hope for an affirmative reply, from such of his readers as will weigh carefully the facts and arguments he has adduced.