In his 1979 book, The Sinking Ark, biologist Norman Myers estimated that (1)_____ of more than 100 human-caused extinctions occur each day, and that one million species (2)_____ by the century"s end. Yet there is little evidence of (3)_____ that number of extinctions. For example, only seven species on the (4)_____ species list have become extinct (5)_____ the list was created in 1973. Bio (6)_____ is an important value, according to many scientists. Nevertheless, the supposed mass extinction rates bandied about are (7)_____ by multiplying (8)_____ by improbables to get imponderables. Many estimates, for instance, rely a great deal on a "species-area (9)_____", which predicts that twice as many species will be found on 100 square miles (10)_____ on ten square miles. The problem is that species am not distributed (11)_____, so how much of a forest am destroyed may be as important as (12)_____. (13)_____, says Ariel Lugo, director of the International Institute of Tropical Forestry in Puerto Rico, "Biologists who predict high (14)_____ rates (15)_____ the resiliency of nature". One of the main muses of extinctions is deforestation. According to the Consultative Group on International Agricultural Research, what destroys tropical trees is not commercial logging, (16)_____ "poor farmers who have no other (17)_____ for feeding their families than slashing and burning a (18)_____ of forest". In countries that practice modern (19)_____ agriculture, forests are in (20)_____ danger. In 1920, U.S. forests covered 732 million acres. Today they cover 737 million.