LIFE. 609 



After discussing these subjects, some of the methods of contributing 

 to rock-formations are mentioned under the heads, — 



3. Methods of fossilization and concretion. 



4. Examples of the formation of strata through the agency of Life. 



1. Geographical Distribution. 



The subject of the geographical distribution of plants and animals, 

 though highly important in this connection, cannot be satisfactorily 

 treated in a brief chapter ; and the student is therefore referred to 

 treatises on this branch of science. Its general principles and bearing 

 are all that can here be explained. 



A. The distribution of terrestrial plants and animals is limited by 

 different causes. 



1. Climate. — The temperature to which each is adapted in its na- 

 ture determines, within certain limits, its position in the zones between 

 the equator and the poles, and also, under any zone, its special alti- 

 tude, between the level of the sea and the height of perpetual snow. 



Meyen divides heights, under the equator, from the sea to the level of 16,200 feet, - 

 that of perpetual snow, — into eight zones or regions, — beginning below, naming them 

 from the characteristic plants: — 



Feet. 



1. Palms and Bananas, 



2. Tree-ferns and Firs 2,020 



3. Myrtles and Laurels, 4,050 



4. Evergreen dicotyledonous trees, .... 6,120 



5. European dicotyledonous trees, .... 8,100 



6. Pines, 10,140 



7. Rhododendrons, 12,150 



8. Alpine plants 14,170 



The corresponding zones in latitude, at the sea level, — setting aside variations from 

 special currents, are, — 



1. Equatorial, . . Lat. 0°-15° 5. Cold-temperate, . Lat. 45°-58° 



2. Tropical, . . . 15°-23° 6. Subarctic, . . . 58°-66° 



3. Subtropical, . . . 23°-34° 7. Arctic 66°-78° 



4. Warm-temperate, . 34°-45° 8. Polar, .... 78°-88° 

 Beyond 88°, vegetation is supposed to be at present wanting. 



Temperature, during the period of flowering and fruiting of plants, 

 and during the reproductive period of animals, often determines their 

 geographical limits. 



Again, the amount of moisture for which a species is made deter- 

 mines its position in either a moist or an arid region. 



Each continent has its own characteristic climate, arising mainly 

 out of its special combination of these two elements, temperature and 

 moisture ; and this is one source of the great diversity of life among 

 the continents. Another point in which the climate of continents 

 differs is the limit of extreme heat and cold. For example, North 



