196 HEREDITY AND EVOLUTION IN PLANTS 



involve the entire area of distribution of the species con- 

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

 following nine factors (paragraphs 4-12) 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 destroyd 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, Elodea, Vallisneria), and seeds readily 

 transported 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 hfe over a 

 radius of many miles. In the states of Washington, 

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

 sands of square miles, have, during a previous geological 

 age, been poured out over the surface, forming a wide 

 plateau. 



A great volcanic eruption in Alaska, in prehistoric 

 times, covered an area of over 140 square miles with a 

 deposit of ash and pumice varying in thickness from a 



