io PLANT GEOGRAPHY 



of the Tropic of Cancer, Madagascar, Arabia, Southern 

 India, Australia, and Tasmania in another vast conti- 

 nental area known as Gondwanaland. Possibly the 

 rigidity of these two ancient land -masses in the shrinking 

 crust of the earth may have produced the folding of the 

 more sedimentary rocks of the area between them, 

 giving rise to those systems of " fold -mountains " which 

 form the axes of our existing continents. Although 

 Gondwanaland seems to have been broken up by the 

 subsidence of the areas now occupied by the South 

 Atlantic and Indian Oceans, a possible continuation 

 to a later period of some southern connection between 

 South America and Australasia, perhaps by way of the 

 Antarctic Continent, may serve to explain some diffi- 

 culties in the present distribution of plants and animals. 



Such a minor difference in the distribution of land 

 and water as the union of the British Isles to the Con- 

 tinent of Europe both eastward and south-westward, 

 involving as it does but a slight difference in the relative 

 levels of land and ocean, has certainly occurred within 

 far more recent times than the differences just referred to. 



Though most of our existing species of plants, and 

 even many larger groups, are of so modern a date, 

 geologically speaking, as to have originated independ- 

 ently of the more ancient distributions of land and 

 water, we may trace the influence of these last in the 

 origin and dispersion of some of the main divisions 

 of the plant- world. As we shall see, however, it is 

 extremely difficult to do this with any certainty. 



CHAPTER II 



CLIMATE IN THE PAST 



THE term climate referred originally to what we term 

 the sun's altitude, or the angle at which the sun's rays 

 fall upon various parts of the earth's surface. The 

 earth's axis of rotation, the line joining the North and 

 South Poles, is inclined a^J towards the plane of its 

 orbit or path round the sun, which plane is termed the 

 ecliptic, i.e. it makes with that plane an angle of 66. 

 As a consequence of this inclination the sun is vertical 



