8 PLANT GEOGRAPHY 



great degree antipodal to one another, the Arctic Ocean 

 corresponding to the Antarctic Continent, Europe and 

 Africa to the South and North Pacific, North America 

 to .the South Indian Ocean, and Australia to the North 

 Atlantic. These salient facts of distribution agree well 

 with an interesting hypothesis of the origin of the 

 surface features of our globe, the latter part of which 

 was first put forward by an American writer, Mr. W. 

 Lowthian Green. 



Many facts in astronomy, and especially the results 

 of the examination of the light of heavenly bodies with 

 the spectroscope, lend support to what is known as the 

 nebular hypothesis of the origin of our earth in common 

 with the rest of the solar system. According to this 

 hypothesis the solar system has resulted from the 

 gradual cooling and condensation of a nebula composed 

 either of incandescent gas, or, more probably, of swarms 

 of meteorites constantly colliding with one another and 

 passing into a vaporous condition. This nebula, con- 

 densing towards its centre, would throw off, or leave 

 behind, successive rings of progressively heavier matter 

 which, by disruption, have condensed into planets, of 

 which our earth is one. Similarly, it is thought, the 

 rupture of each such planetary ring would so raise the 

 temperature of the resultant planet as to vaporise it, 

 and allow the vapours to arrange themselves and 

 condense in successive shells of densities increasing 

 towards its centre. Lowthian Green's tetrahedral 

 theory is that the earth, originating in the manner just 

 indicated, would be approximately spherical, the sphere 

 being the geometrical form which combines the maximum 

 of volume with the minimum of surface; and that, in 

 further contraction on cooling, it would tend towards 

 that geometrical form which combines the maximum 

 of surface with the minimum of volume, viz. the tetra- 

 hedron. The tetrahedron is enclosed by four equal and 

 similar triangles; and the theory associates the four 

 oceans with the four faces of this form, and the circum- 

 polar ring of land and the three pairs of continents with 

 its edges, the face represented by the Arctic Ocean 

 having as its antipodes the solid angle represented by 

 the Antarctic Continent. The spherical earth is, in fact, 

 supposed to have undergone a tetrahedral deformation, 

 sagging on the four sides, or faces, which have produced 



