FOREST SOCIETIES. 541 



not exceed 500. The trees range in height from 250 to 360 

 feet, and the larger ones from 20 to 35 feet in diameter, while 

 the bark of the larger trees is 2 feet thick. They occur at an 

 elevation of 4500 to 7000 feet. They do not occur alone, but 

 the other principal trees of the forest mixed with them are the 

 pitch-pine, sugar-pine, Douglas spruce, white fir, and bastard 

 cedar. 



1022. Geological history of the Big tree. Evideri^ of fossil 

 specimens collected in Alaska, British America, Iceland, Green- 

 land, Spitzbergen, Europe, and the Rocky Mountains in the 

 United States shows that several species of the Sequoia existed in 

 the Miocene period, and one of these, the Big tree, must have 

 grown in Greenland as well as in the lower latitude of Europe. 

 During glacial times they were destroyed in Europe, probably 

 because in their southward migration they could not pass the 

 barrier of transcontinental mountain chains in southern Europe, 

 while they were permitted in North America to migrate south 

 of the extent of the cold wave and thus were saved from extinc- 

 tion. 



1023. Preservation of the Big trees. The greater part of the 

 Big trees are owned by private interests or by lumbering com- 

 panies, but the U. S. government owns and controls in large part 

 two large areas within the Sequoia and General Grant National 

 Park, while the State of California owns one tract, the Mariposa 

 Grove, an area of about two miles square, which is held as a State 

 park. 



IV. Tropical Forests. 



1024. Kinds of tropical woods. According to Schimper the 

 woods in the tropics are divided into three groups: ist. The ever- 

 green forest, which occurs in the areas of great rainfall where there 

 is no dry period. The woods are hygrophile in character, are 

 at least 30 meters high, and usually much higher. Besides the 

 forest trees there are numerous stout lianas, i.e., climbing trees 

 and vines as well as woody and herbaceous epiphytes. 2d. Mon- 



