590 STRUCTURE AND LIFE HISTORIES 



involve the entire area of distribution of the species con- 

 cerned, the species will, obviously, become extinct. The 

 following seven factors are specific instances of this. 



4. Diminished water supply. Aquatic plants may be 

 destroyed by the draining of a pond or lake; hydrophytic 

 forms by the drying up of a swamp. Sometimes forms 

 suited to conditions of moderate water supply {hydro- 

 phytes) are destroyed by the conversion of wide areas into 

 desert regions, as has doubtless occurred. If such changes 

 are gradual, resting spores {e.g., Spirogyra), winter buds 

 {e.g., Utricularia, and eel-grass), and seeds readily trans- 

 ported by wind {e.g., cat-tail) enable the species to become 

 reestablished in a new location, but not so when the 

 changes are too abrupt, or cover too wide an area. 



5. Temperature changes, when too abrupt, too extreme, 

 or too long continued. When the continental ice-sheet 

 advanced southward during the glacial period, many 

 forms, adapted only to temperate conditions, became ex- 

 tinct. Fossils of extinct tropical plants are found in 

 Greenland, which is now undergoing a glacial period. 



6. Volcanic eruptio7is, such, for example, as those of 

 Mount Pelee, which occurred in 1902, on the island of 

 Martinique, W. I., often destroy all signs of life over a 

 radius of many miles. In the states of Washington, 

 Oregon, and Idaho floods of molten lava, covering thous- 

 ands of square miles, have been poured out over the sur- 

 face, forming a wide plateau. It is almost certain that 

 many species of plants and animals have become extinct 

 by such agencies. Not only the lava, but poisonous 

 gases that fill the air during volcanic eruptions, are fatal 

 to plant life. 



7. Encroachment of salt water in coastal regions, caused 



