PHYSIOGNOMY OF PLANTS. 9 



the ocean ? The distance from other coasts makes it diffi- 

 cult to determine this question ; but, no sooner is the rock 

 of the newly raised islands in direct contact with the 

 atmosphere, than there is formed on its surface, in our nor- 

 thern countries, a soft silky net-work, appearing to the 

 naked eye as coloured spots and patches. Some of these 

 patches are bordered by single or double raised lines run- 

 ning round their margins; other patches are crossed by 

 similar lines traversing them in various directions. Gra- 

 dually the light colour of the patches becomes darker, the 

 bright yellow which was visible at a distance changes to 

 brown, and the bluish gray of the Leprarias becomes a dusty 

 black. The edges of neighbouring patches approach and 

 run into each other ; and on the dark ground thus formed 

 there appear other lichens, of a circular shape and dazzling 

 whiteness. Thus an organic film or covering establishes 

 itself by successive layers ; and as mankind, in forming 

 settled communities, pass through different stages of civili- 

 sation, so is the gradual propagation and extension of plants 

 connected with determinate physical laws. Lichens form 

 the first covering of the naked rock, where afterwards lofty 

 forest trees rear their airy summits. The successive growth 

 of mosses, grasses, herbaceous plants, and shrubs or bushes, 

 occupies the intervening period of long but undetermined 

 duration. The part which lichens and mosses perform in 

 the northern countries is effected within the tropics by 

 Portulacas, Gomphrenas, and other low and succulent shore 

 plants. The history of the vegetable covering of our planet, 

 and its gradual propagation over the desert crust of the 



