Chapter X 



HISTORICAL CAUSES FOR THE PRESENT STRUCTURE 

 OF AREAS AND THE COMPOSITION OF FLORAS 



From the preceding chapters it is clear that in many cases the 

 structure of the areas of species and the composition of floras cannot 

 be explained by existing factors. The present distribution of any given 

 species is a reflection of the geological revolutions and cUmatic changes 

 that have occurred on our globe during the entire period of existence of 

 that species. An elucidation of these great changes in the surface of 

 our planet is the task of historical geology and paleogeography, a task 

 as yet far from fulMment. Consequently, the elucidation of the his- 

 tory of areas, the most difficult task of biogeography, hkewise falls far 

 short of achievement. In the present chapter we can, therefore, do no 

 more than examine the chief theories that have been advanced and point 

 out which give the most plausible and satisfactory explanation of the 

 knotty problems of historical plant geography. 



From very ancient times — at first without adequate foundation and 

 later, with the development of geology and biogeography, on the basis 

 of numerous data — the conviction has been held that the distribution 

 of lands and seas was not always the same as now. For, if it had been, 

 a considerable number of facts, both of a geological and biogeographical 

 nature, would be inexplicable. The sedimentary character of the rocks 

 covering extensive territories on the continents and the finding in these 

 rocks of fossil marine animals testify to the fact that at one time seas 

 covered these parts of the continents. That many islands formerly 

 constituted a part of the mainland is shown by the geological structure 

 of these islands, by their fossil and extant fauna and flora, and by the 

 finding of submerged trees in various straits and channels, e.g., in the 

 English Channel. Furthermore, the outermost edges of continents and 

 islands do not necessarily coincide with their shore lines, as their outer 

 margins often he submerged under so-called "shelf seas". The latter 

 differ in extent, but their boundaries may be ascertained with consider- 

 able precision. The determination of these boundaries gives certain 

 clues to the changes that have taken place in the distribution of lands 

 and seas on the globe and on the probable existence in former times of 

 connections between bodies of land now separated by the sea. Bio- 

 geographical data, in many cases, indicate that these changes occurred 

 at a comparatively recent date. 



I. Theory of Land Bridges. — We have already seen that a con- 

 siderable number of cases of discontinuous areas of plants (and these 

 might be supplemented by an equal number of instances of similarly 

 distributed animals) cannot be regarded as accidental and require ex- 

 planation. Often there seems to be only one possible explanation, viz., 

 that there formerly existed some sort of connection between the iso- 

 lated habitats and that the now discontinuous areas were formerly 

 continuous. Hence, many investigators have assumed that at one time 



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