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 ; hydrophy tic 
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 eruptions, 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 
