PHYSIOGNOMY OF PLANTS. 231 



island. How are the seeds of plants brought so immediately to 

 these new shores ? by wandering birds, or by the winds and waves of 

 the ocean ? The distance from other coasts makes it difficult to de- 

 termine 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 northern countries, a soft silky net-work, ap- 

 pearing to the naked eye as colored spots and patches. Some of 

 these patches are bordered by single or double raised lines running 

 round their margins; other patches are crossed by similar lines 

 traversing them in various directions. Gradually the light color 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 neighboring patches ap- 

 proach 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 civilization, so is the gradual pro- 

 pagation 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 suc- 

 cessive growth of mosses, grasses, herbaceous plants, and shrubs 

 or bushes, occupies the intervening period of long but undeter- 

 mined duration. The part which lichens and mosses perform 

 in the northern countries is effected within the tropics by Por- 

 tulacas, G-omphrenas, 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 earth, has its epochs, as well 

 as that of the migrations of the animal world. 



Yet although organic life is everywhere diffused, and the organic 

 powers are incessantly at work in reconnecting with each other the 

 elements set free by death or dissolution, the abundance and variety 

 of organized beings, and the rapidity with which they are renewed, 

 differ in different climates. In the cold zones, the activity of organic 

 life undergoes a temporary suspension during a portion of the year 

 by frost; fluidity is an essential condition of life or vital action, and 

 animals and plants, with the exception of mosses and other crypto- 



