The
GALE
ENCYCLOPEDIA
of
Science
The
GALE
ENCYCLOPEDIA
of
Science
THIRD EDITION
K. Lee Lerner and
Brenda Wilmoth Lerner,
Editors
Gale Encyclopedia of Science, Third Edition
K. Lee Lerner and Brenda Wilmoth Lerner, Editors
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LIBRARY OF CONGRESS CATALOGING-IN-PUBLICATION DATA
Gale encyclopedia of science / K. Lee Lerner & Brenda Wilmoth Lerner, editors.— 3rd ed. p. cm.
Includes index.
ISBN 0-7876-7554-7 (set) — ISBN 0-7876-7555-5 (v. 1) — ISBN 0-7876-7556-3 (v. 2) — ISBN 0-7876-7557-1 (v. 3) — ISBN 0-7876-7558-X (v. 4) — ISBN 0-7876-7559-8 (v. 5) — ISBN 0-7876-7560-1 (v. 6) 1. Science—Encyclopedias. I. Lerner, K. Lee. II. Lerner, Brenda Wilmoth.
Q121.G37 2004
503—dc22 2003015731
Disclaimer:
CONTENTS
Topic List. . . vii
Organization of the Encyclopedia. . . xxvii
Advisory Board . . . xxix
Contributors . . . xxxi
Entries
Volume 1 (Aardvark–Chaos). . . 1–818
Volume 2 (Charge-coupled device–Eye). . . 819–1572
Volume 3 (Factor–Kuru) . . . 1573–2254
Volume 4 (Lacewings–Pharmacogenetics) . . 2255–3036
Volume 5 (Pheasants–Star). . . 3037–3800
Volume 6 (Star cluster–Zooplankton) . . . 3801–4378
A
Anesthesia
Coffee plant
Formula, structural
Monoculture
Rate
Wren-warblers Wrens
Wrynecks
X
X-ray astronomy X-ray crystallography X rays
Xenogamy
Z
Zebras Zero
Zodiacal light Zoonoses Zooplankton
Y
Y2K Yak Yam Yeast Yellow fever Yew Yttrium
The Gale Encyclopedia of Science, Third Edition
has been designed with ease of use and ready reference in mind.
• Entries are alphabetically arranged across six volumes, in a single sequence, rather than by scientific field • Length of entries varies from short definitions of one or
two paragraphs, to longer, more detailed entries on more complex subjects.
• Longer entries are arranged so that an overview of the subject appears first, followed by a detailed discussion conveniently arranged under subheadings.
• A list of key terms is provided where appropriate to de-fine unfamiliar terms or concepts.
• Bold-faced terms direct the reader to related articles. • Longer entries conclude with a “Resources” section,
which points readers to other helpful materials (includ-ing books, periodicals, and Web sites).
• The author’s name appears at the end of longer entries. His or her affiliation can be found in the “Contributors” section at the front of each volume.
• “See also” references appear at the end of entries to point readers to related entries.
• Cross references placed throughout the encyclopedia direct readers to where information on subjects without their own entries can be found.
• A comprehensive, two-level General Index guides readers to all topics, illustrations, tables, and persons mentioned in the book.
AVAILABLE IN ELECTRONIC FORMATS
ACADEMIC ADVISORS
Marcelo Amar, M.D.
Senior Fellow, Molecular Disease Branch National Institutes of Health (NIH) Bethesda, Maryland
Robert G. Best, Ph.D. Director
Divison of Genetics, Department of Obstetrics and Gynecology
University of South Carolina School of Medicine Columbia, South Carolina
Bryan Bunch Adjunct Instructor
Department of Mathematics Pace University
New York, New York Cynthia V. Burek, Ph.D.
Environment Research Group, Biology Department Chester College
England, UK David Campbell Head
Department of Physics
University of Illinois at Urbana Champaign Urbana, Illinois
Morris Chafetz
Health Education Foundation Washington, DC
Brian Cobb, Ph.D.
Institute for Molecular and Human Genetics Georgetown University
Washington, DC Neil Cumberlidge Professor
Department of Biology
Northern Michigan University Marquette, Michigan
Nicholas Dittert, Ph.D.
Institut Universitaire Européen de la Mer University of Western Brittany
France
William J. Engle. P.E.
Exxon-Mobil Oil Corporation (Rt.) New Orleans, Louisiana
Bill Freedman Professor
Department of Biology and School for Resource and Environmental Studies
Dalhousie University
Halifax, Nova Scotia, Canada Antonio Farina, M.D., Ph.D.
Department of Embryology, Obstetrics, and Gynecology
University of Bologna Bologna, Italy
G. Thomas Farmer, Ph.D., R.G.
Earth & Environmental Sciences Division Los Alamos National Laboratory
Los Alamos, New Mexico Jeffrey C. Hall
Lowell Observatory Flagstaff, Arizona Clayton Harris Associate Professor
Department of Geography and Geology Middle Tennessee State University Murfreesboro, Tennesses
Lyal Harris, Ph.D.
Tectonics Special Research Centre Department of Geology & Geophysics
ADVISORY BOARD
The University of Western Australia Perth, Australia
Edward J. Hollox, Ph.D. Queen’s Medical Centre University of Nottingham Nottingham, England
Brian D. Hoyle, Ph.D. (Microbiology) Microbiologist
Square Rainbow Nova Scotia, Canada Alexander I. Ioffe, Ph.D. Senior Scientist
Geological Institute of the Russian Academy of Sciences
Moscow, Russia Jennifer L. McGrath Northwood High School Nappannee, Indiana David T. King Jr., Ph.D. Professor
Department of Geology Auburn University Auburn, Alabama Danila Morano, M.D.
Department of Embryology, Obstetrics, and Gynecology
University of Bologna Bologna, Italy
Abdel Hakim Ben Nasr, Ph.D. Department of Genetics
Molecular Oncology and Development
Program/Boyer Center for Molecular Medicine Yale University School of Medicine
New Haven, Connecticut William S. Pretzer Curator
Henry Ford Museum and Greenfield Village Dearborn, Michigan
Judyth Sassoon, Ph.D., ARCS
Department of Biology and Biochemistry University of Bath
Bath, England, U.K. Yavor Shopov, Ph.D.
Professor of Geology & Geophysics University of Sofia
Bulgaria
Theodore Snow
Professor, Department of Astrophysical and Planetary Sciences
Fellow, Center for Astrophysics and Space Astronomy University of Colorado at Boulder
Boulder, Colorado
Michael J. Sullivan, M.D., Ph.D., FRACP Cancer Genetics Laboratory
University of Otago Dunedin, New Zealand Constance K. Stein, Ph.D.
Director of Cytogenetics, Assistant Director of Molecular Diagnostics
SUNY Upstate Medical University Syracuse, New York
Robert Wolke Professor emeritus Department of Chemistry University of Pittsburgh Pittsburgh, Pennsylvania Richard Addison Wood Meteorological Consultant Tucson, Arizona
Diego F. Wyszynski, M.D., Ph.D. Department of Medicine, Epidemiology &
Biostatistics
Boston University School of Medicine Boston, Massachusetts
Rashmi Venkateswaran Undergraduate Lab Coordinator Department of Chemistry University of Ottawa Ottawa, Ontario, Canada
LIBRARIAN ADVISORS
Donna Miller Director
Craig-Moffet County Library Craig, Colorado
Judy Williams Media Center
Greenwich High School Greenwich, Connecticut Carol Wishmeyer
Science and Technology Department Detroit Public Library
Detroit, Michigan
Advisor
y Boar
Nasrine Adibe Professor Emeritus Department of Education Long Island University Westbury, New York Mary D. Albanese Department of English University of Alaska Juneau, Alaska
Margaret Alic Science Writer
Eastsound, Washington
James L. Anderson Soil Science Department University of Minnesota St. Paul, Minnesota
Monica Anderson Science Writer
Hoffman Estates, Illinois
Susan Andrew Teaching Assistant University of Maryland Washington, DC
John Appel Director
Fundación Museo de Ciencia y Tecnología
Popayán, Colombia
David Ball Assistant Professor Department of Chemistry Cleveland State University Cleveland, Ohio
T. Parker Bishop Professor
Middle Grades and Secondary Education
Georgia Southern University Statesboro, Georgia
Carolyn Black Professor
Incarnate Word College San Antonio, Texas
Larry Blaser Science Writer Lebanon, Tennessee
Jean F. Blashfield Science Writer Walworth, Wisconsin
Richard L. Branham Jr. Director
Centro Rigional de
Investigaciones Científicas y Tecnológicas
Mendoza, Argentina
Patricia Braus Editor
American Demographics Rochester, New York
David L. Brock Biology Instructor St. Louis, Missouri
Leona B. Bronstein Chemistry Teacher (retired) East Lansing High School Okemos, Michigan Dana M. Barry
Editor and Technical Writer Center for Advanced Materials
Processing
Clarkston University Potsdam, New York Puja Batra
Department of Zoology Michigan State University East Lansing, Michigan Donald Beaty
Professor Emeritus College of San Mateo San Mateo, California Eugene C. Beckham
Department of Mathematics and Science
Northwood Institute Midland, Michigan Martin Beech Research Associate Department of Astronomy University of Western Ontario London, Ontario, Canada Julie Berwald, Ph.D. (Ocean
Sciences) Austin, Texas Massimo D. Bezoari Associate Professor Department of Chemistry Huntingdon College Montgomery, Alabama John M. Bishop III Translator
New York, New York
Sarah de Forest
David Goings, Ph.D. (Geology) Geologist
Amy Kenyon-Campbell University College of the Fraser
Valley Reuben H. Fleet Space Theater
and Science Center San Diego, California Adrienne Wilmoth Lerner Graduate School of Arts &
G. H. Miller
University of Arkansas at Little Rock
Vita Richman
University of Colorado at Boulder Boulder, Colorado
Department of Earth and Physical Sciences
Policy Analyst, Air Quality Issues U.S. General Accounting Office Raleigh, North Carolina
Laurie Toupin Science Writer
Pepperell, Massachusetts Melvin Tracy
Science Educator Appleton, Wisconsin Karen Trentelman Research Associate Archaeometric Laboratory University of Toronto Toronto, Ontario, Canada Robert K. Tyson Senior Scientist W. J. Schafer Assoc. Jupiter, Florida James Van Allen Professor Emeritus Department of Physics and
Astronomy University of Iowa Iowa City, Iowa Julia M. Van Denack Biology Instructor Silver Lake College Manitowoc, Wisconsin Kurt Vandervoort
Department of Chemistry and Physics
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Pella, Iowa
Frederick R. West Astronomer
Hanover, Pennsylvania Glenn Whiteside Science Writer Wichita, Kansas John C. Whitmer Professor
Department of Chemistry Western Washington University Bellingham, Washington Donald H. Williams Department of Chemistry Hope College
Holland, Michigan Robert L. Wolke Professor Emeritus Department of Chemistry University of Pittsburgh Pittsburgh, Pennsylvania Xiaomei Zhu, Ph.D.
Postdoctoral research associate Immunology Department Chicago Children’s Memorial
Hospital, Northwestern University Medical School Chicago, Illinois
Jim Zurasky Optical Physicist
Nichols Research Corporation Huntsville, Alabama
Rashmi Venkateswaran Undergraduate Lab Coordinator Department of Chemistry University of Ottawa Ottawa, Ontario, Canada R. A. Virkar
Chair
Department of Biological Sciences
Kean College Iselin, New Jersey Kurt C. Wagner Instructor
South Carolina Governor’s School for Science and Technology
Hartsville, South Carolina Cynthia Washam Science Writer Jensen Beach, Florida Terry Watkins Science Writer Indianapolis, Indiana Joseph D. Wassersug Physician
Boca Raton, Florida Tom Watson
Environmental Writer Seattle, Washington Jeffrey Weld
Instructor, Science Department Chair
Pella High School
Factor
In mathematics, to factor a number or algebraic ex-pression is to find parts whose product is the original number or expression. For instance, 12 can be factored into the product 6 2, or 3 4. The expression (x2- 4)
can be factored into the product (x + 2)(x - 2). Factor is also the name given to the parts. We say that 2 and 6 are factors of 12, and (x-2) is a factor of (x2- 4). Thus we
refer to the factors of a product and the product of factors.
The fundamental theoremof arithmeticstates that every positive integer can be expressed as the product of prime factors in essentially a single way. A prime num-ber is a numnum-ber whose only factors are itself and 1 (the first few prime numbersare 1, 2, 3, 5, 7, 11, 13). Inte-gersthat are not prime are called composite. The num-ber 99 is composite because it can be factored into the product 9 11. It can be factored further by noting that 9 is the product 3 3. Thus, 99 can be factored into the product 3 3 11, all of which are prime. By saying “in essentially one way,” it is meant that although the factors of 99 could be arranged into 3 11 3 or 11
3 3, there is no factoring of 99 that includes any primes other than 3 used twice and 11.
Factoring large numbers was once mainly of interest to mathematicians, but today factoring is the basis of the security codes used by computers in military codes and in protecting financial transactions. High-powered com-puters can factor numbers with 50 digits, so these codes must be based on numbers with a hundred or more digits to keep the data secure.
In algebra, it is often useful to factor polynomial expressions (expressions of the type 9x3+ 3x2 or x4
-27xy + 32). For example x2+ 4x + 4 is a polynomial that
can be factored into (x + 2)(x + 2). That this is true can be verified by multiplying the factors together. The de-greeof a polynomial is equal to the largest exponent
that appears in it. Every polynomial of degree n has at most n polynomial factors (though some may contain
complex numbers). For example, the third degree poly-nomial x3+ 6x2+ 11x + 6 can be factored into (x + 3) (x2
+ 3x + 2), and the second factor can be factored again into (x + 2)(x + 1), so that the original polynomial has three factors. This is a form of (or corollary to) the fun-damental theorem of algebra.
In general, factoring can be rather difficult. There are some special cases and helpful hints, though, that often make the job easier. For instance, a common factor in each termis immediately factorable; certain common situations occur often and one learns to recognize them, such as x3+ 3x2+ xy = x(x2+ 3x + y). The difference of
two squares is a good example: a2- b2= (a + b)(a - b).
Another common pattern consists of perfect squares of binomial expressions, such as (x + b)2. Any squared
bi-nomial has the form x2+ 2bx + b2. The important things
to note are: (1) the coefficientof x2is always one (2) the
coefficient of x in the middle term is always twice the square root of the last term. Thus x2 + 10x + 25 =
(x+5)2, x2- 6x + 9 = (x-3)2, and so on.
Many practical problems of interest involve polyno-mial equations. A polynopolyno-mial equation of the form ax2+
bx + c = 0 can be solved if the polynomial can be fac-tored. For instance, the equation x2+ x - 2 = 0 can be
written (x + 2)(x - 1) = 0, by factoring the polynomial. Whenever the product of two numbers or expressions is zero, one or the other must be zero. Thus either x + 2 = 0 or x - 1 = 0, meaning that x = -2 and x = 1 are solutions of the equation.
Resources Books
Bittinger, Marvin L, and Davic Ellenbogen. Intermediate Alge-bra: Concepts and Applications.6th ed. Reading, MA: Addison-Wesley Publishing, 2001.
Davison, David M., Marsha Landau, Leah McCracken, Linda Immergut, and Brita and Jean Burr Smith. Arithmetic and Algebra Again.New York: McGraw Hill, 1994.
Larson, Ron. Precalculus.5th ed. New York: Houghton Mifflin College, 2000.
McKeague, Charles P. Intermediate Algebra. 5th ed. Fort Worth: Saunders College Publishing, 1995.
J.R. Maddocks
Factorial
The number n! is the product 1 2 3 4 ...
n, that is, the product of all the natural numbersfrom 1 up to n, including n itself where 1 is a natural number. It is called either “n factorial” or “factorial n.” Thus 5! is the number 1 2 3 4 5, or 120.
Older books sometimes used the symbol In for n factorial, but the numeral followed by an exclamation pointis currently the standard symbol.
Factorials show up in many formulas of statistics, probability,combinatorics,calculus,algebra, and else-where. For example, the formula for the number of per-mutations of n things, taken n at a time, is simply n!. If a singer chooses eight songs for his or her concert, these songs can be presented in 8!, or 40,320 different orders. Similarly the number of combinations of n things r at a time is n! divided by the product r!(n - r)!. Thus the num-ber of different bridge hands that can be dealt is 52! di-vided by 13!39!. This happens to be a verylarge number. When used in conjunction with other operations, as in the formula for combinations, the factorial function takes precedence over addition,subtraction, negation, multiplication, and divisionunless parentheses are used to indicate otherwise. Thus in the expression r!(n - r)!, the subtraction is done first because of the parentheses; then r! and (r - n)! are computed; then the results are multiplied.
As n! has been defined, 0! makes no sense. However, in many formulas, such as the one above, 0! can occur. If one uses this formula to compute the number of combina-tions of 6 things 6 at a time, the formula gives 6! divided by 6!0!. To make formulas like this work, mathematicians have decided to give 0! the value 1. When this is done, one gets 6!/6!, or 1, which is, of course, exactly the num-ber of ways in which one can choose all six things.
As one substitutes increasingly large values for n, the value of n! increases very fast. Ten factorial is more
than three million, and 70! is beyond the capacity of even those calculators which can represent numbers in scientific notation.
This is not necessarily a disadvantage. In the series representation of sine x, which is x/1! - x3/3! + x5/5! -...,
the denominators get large so fast that very few terms of the series are needed to compute a good decimal ap-proximationfor a particular value of sine x.
Fahrenheit
see
Temperature
Falcons
Falcons are birds of preyin the family Falconidae. There are 39 species of true falcons, all in the genus
Falco. Like other species in the order Falconiformes (which also includes hawks,eagles, osprey, and vul-tures), falcons have strong raptorial (or grasping) talons, a hooked beak, extremely acute vision, and a fierce de-meanor. Falcons can be distinguished from other raptors
by the small toothlike serrations (called tomial teeth) on their mandibles and by their specific coloration. They also have distinctive behaviorpatterns, such as killing their preyby a neck-breaking bite, head-bobbing, defe-cating below the perchor nest, and an often swift and direct flight pattern.
Falcons can be found on all continents except
Antarctica. Some species have a very widespread distri-bution. In particular, the peregrine falcon (F. peregrinus) is virtually cosmopolitan, having a number of subspecies, some of them specific to particular ocean-ic islands. Other falcons are much more restrocean-icted in their distribution: for example, the Mauritius kestrel (F. puctatus) only breeds on the islandof Mauritius in the Indian Ocean. At one time, fewer than ten individuals of this endangered speciesremained, although populations have since increased as a result of strict protection and a program of captive breeding and release.
Species of falcons exploit a very wide variety of
habitattypes, ranging from the high arctic tundrato bo-real and temperate forest,prairieand savanna, and tropi-cal forests of all types. Falcons catch their own food. Most species of falcons catch their prey in flight, although kestrels generally seize their food on the ground, often after hovering above. As a group, falcons eat a great range of foods; however, particular species are relatively specific in their feeding, limiting themselves to prey within certain size ranges. The American kestrel (F. sparverius), for ex-ample, eats mostly insects, earthworms, small mammals, and small birds, depending on their seasonal availability.
Factorial
KEY TERMS
. . . .
Falcons
A peregrine falcon taking flight. Photograph by Alan & Sandy Carey. The National Audubon Society Collection/Photo Researchers. Reproduced by permission.
Heavier, more powerful falcons, such as the peregrine, will eat larger species of birds, including ducks, seabirds,
grouse, pigeons, and shorebirds.
The nests of many falcons are rather crudely made, often a mere scrape on a cliff ledge or on the ground. Some species, however, nest in natural cavities or old woodpeck-er holes in trees, as is the case with the Amwoodpeck-erican kestrel. Most kestrels will also use nest boxes provided by humans. Peregrines, which sometimes breed in cities, will nest on ledges on tall buildings, a type of artificial cliff.
The courtshipdisplays of falcons can be impressive, in some cases involving spectacular aerial displays and acrobatics. Those of the peregrine are most famous. To impress a female (properly called a falcon), the male bird (called a tiercel) will swoop down from great heights at speeds as high as 217 MPH (350 km/h) and will execute rolls and other maneuvers, including midair exchanges of
food with its intended mate. Although this species under-takes long-distance seasonal migrations, the birds return to the same nesting locale and, if possible, will mate with the same partner each year. Incubation of falcon eggs does not begin until the entire clutch is laid, so all young birds in a nest are about the same size. This is different from many other birds of prey, which incubate as soon as the first egg is laid, resulting in a great size range of young birds in the nest. In falcons, the female (which is always larger than the male) does most of the incubating, while the tiercel forages widely for food.
Falcons
An American kestrel (Falco sparverius) at the Arizona Sonora Desert Museum, Arizona. Not much larger than a blue jay, the kestrel is the smallest of the North American falcons.Potograph by Robert J. Huffman. Field Mark Publications. Reproduced by permission.
major pests—dangerous predators of game birds. As a result, falcons, especially peregrines, were killed in large numbers by professional gamekeepers and hunters. For-tunately, this practice ended, and falcons are now rarely hunted by humans. However, young falcons are still taken from wild nests, often illegally, for use in falconry.
Falconry is a sport with a three-thousand-year histo-ry, in which falcons are free-flown to catch and kill game birds, such as grouse, ptarmigan,pheasants, and ducks. Falcons are rather wild birds, however, and they must be well trained or they may not return to the falconer’s hand. Because of their power, speed, and fierce and inde-pendent demeanor, the most highly prized species in fal-conry are the largest, most robust falcons, especially the gyrfalcon and the peregrine.
Some birds trained in falconry are not only used for sport. Falcons are also used in some places to drive birds such as gullsaway from airports, to help prevent poten-tially catastrophic collisions with aircraft.
Some species of falcons have suffered considerable damage from the widespread usage of certain types of in-secticides. Most harmful has been the use of persistent streakings of guano and rock lichensgrowing in a
fertil-ized zone extending several meters beneath the nest. De-pending on the nearby habitat, gyrfalcons may feed on ptarmigan, seabirds, or small migratory birds such as buntings and shorebirds.
Other familiar falcons of North America include the
prairie falcon(F. mexicanus), which ranges widely in open habitats of the southwestern region, and the merlin or pigeon hawk (F. columbarius), which breeds in boreal and subarctic habitats and winters in the southern part of the continentand Central America.
Interaction of falcons with humans
Falcons fascinate many people, largely because of their fierce, predatory behavior. As a result, sightings of falcons are considered to be exceptional events for bird watchers and many other people. Some species of fal-cons, such as kestrels, are also beneficial to humans be-cause they eat large numbers of mice,grasshoppers, and locusts that are potential agricultural pests.
bioaccumulating chlorinated-hydrocarbon insecticides, such as DDT and dieldrin. These and other related chemi-cals (such as polychlorinated biphenyls, or PCBs) have caused the collapse of populations of peregrines and other species of birds. For example, populations of the most widespread subspecies of the peregrine falcon in North America (F. peregrinus anatum) were widely de-stroyed by these toxic exposures, and the northern sub-species (F. p. tundrius) suffered large declines. However, because of restrictions in the use of these chemicals since the 1970s, they now have less of an effect on falcons and other birds. In fact, some breeding and migratory popula-tions of peregrine falcons in North America have signifi-cantly increased since the late 1970s. This recovery has been aided by large captive breeding programs in the United States and Canada aimed at releasing these birds into formerly occupied or underpopulated habitats.
Still, the populations of many species of falcons is greatly reduced, and some species are threatened or en-dangered. Protecting these species would best be accom-plished by ensuring that extensive tracts of appropriate natural habitat always remain available for falcons and other wildlife. However, in more acute cases, expensive management of the habitat and populations of falcons is necessary to protect these fascinating birds.
Current status of North American falcons
• Aplomado falcon (Falco femoralis). Endangered (sub)Species. Has been reintroduced in Texas. Decline in population is thought to have been due to agricultural expansion and to eggshell thinning resulting from the use of pesticides. Now considered a Southwestern stray. • Collared forest falcon (Micrastur semitorquatus).
Southwestern stray.
• Peregrine falcon (Falco peregrinus). Pesticides and PCB poisoning caused widespread reproductive failure from the 1940s to 1970s, causing species to disappear from many of the former nesting grounds. It has since been reintroduced in many areas, and appears to be doing well locally.
• Prairie falcon (Falco mexicanus). Species has experi-enced some eggshell thinning and mercury poisoning (mainly built up from feeding on the seed-eating Horned lark). Has declined in some areas (including Utah, western Canada, and agricultural regions of Cali-fornia), but the current population appears stable. • American kestrel (Falco sparverius). Decline in
popu-lation in the northeast in recent years, but otherwise the population appears stable. Nest boxes have helped maintain populations in some areas.
• Gyrfalcon (Falco rusticolus). Rare. Has declined in parts of Arctic Europe, but appears stable in North America. Illegal poaching for falconry may be a prob-lem in some areas, but fortunately most nest sites are out of range of human disturbance.
• Merlin (Falco columbarius). There were earlier indica-tions that this bird was experiencing adverse effects from the use of pesticides in eastern Canada, and from mercury buildup in western Canada. Numbers now ap-pear to be increasing in the northern prairies, and to be remaining stale elsewhere.
• Crested caracara (Polyborus plancus). Has declined due to loss of habitat due to agricultural expansion and hunting. There has been some evidence of an increase in population in Texas. The population on Guadalupe Island, Mexico, became extinct in 1900.
Resources Books
Cade, T.J. The Falcons of the World.Ithaca, NY: Cornell Uni-versity Press, 1982.
Ehrlich, Paul R., David S. Dobkin, and Darryl Wheye. The Birder’s Handbook. New York: Simon & Schuster Inc., 1988.
Freedman, B. Environmental Ecology.2nd ed. San Diego: Aca-demic Press, 1994.
Peterson, Roger Tory. North American Birds. Houghton Miflin Interactive (CD-ROM), Somerville, MA: Houghton Mi-flin, 1995.
Randall Frost
Faraday effect
The Faraday effect is manifest when a changing magnetic field induces an electric field. Hence the ef-fect is also known as “induction.” It is most simply ex-emplified by a loop of wire and a bar magnet. If one moves the magnet through the loop of wire, the chang-ing magnetic field within the loop gives rise to an elec-trical current in the wire. The current is larger for stronger magnets, and it can also be augmented by moving the magnet more quickly. In other words, the size of the electric field created depends directly on the rateat which the magnetic field changes. In principal, by moving a very strong magnet quickly enough, the induced current could illuminate a common lightbulb. To really understand the effect, note that the bulb would only be lit as long as the magnet was moving. As soon as a magnetic field quits changing, the Faraday ef-fect disappears.
Far
ada
Many useful devices exploit the Faraday effect. Most notably, an electric generatorrelies on it to derive electricityfrom mechanical motion. A generator uses the energy of burning fossil fuels, for example (or falling waterin the case of a hydroelectric plant), to ro-tate a loop of wire between two magnets. Since the loop spins, it perceives that the magnetic field is changing and, via the Faraday effect, yields electricity which can then be sent out to traffic lights,radioalarm clocks, hair dryers, et cetera.
Michael Faraday discovered the effect in 1831 at the Royal Institution Laboratories in London. When he pow-ered up an electromagnet, a nearby coil of wire (in no way physically connected to the magnet) registered a sizable but brief current. While the electromagnet remained on, no further current could be detected in the nearby coil. However, when he turned his magnet off he again ob-served a short-lived burst of electrical activity in the other-wise dormant coil. He reasoned that by turning the elec-tromagnet on and off, he had created abrupt changes in the magnetic field inside the coil and that these changes had, in turn, created the fleeting electric current. For Faraday, this discovery carved both prestige in the physics commu-nity and, moreover, a place in scientific history. Neither of these was a small feat because his fellow physicists con-sidered his educational background to be inferior. He lacked any formal learning of mathematics, and his train-ing in chemistry was (in the eyes of his colleagues) a questionable preparation for his career as a physicist.
Farm machinery
see
Agricultural machines
Fat
A fat is a solid triester of glycerol. It is formed when a moleculeof glycerol, an alcoholwith three hydroxyl groups, reacts with three molecules of fatty acids. A fatty acid is a long-chain aliphatic carboxylic acid. The more correct name for a fat is a triglyceride.
The three fatty acid fragments in a fat may be all the same (a simple triglyceride) or they may be different from each other (a mixed triglyceride). The fat known as glyceryl tripalmitate, for example, is formed when a molecule of glycerol reacts with three molecules of palmitic acid. Glyceryl palmitate distearate, on the other hand, is produced in the reaction between one molecule of glycerol, one molecule of palmitic acid and two mole-cules of stearic acid.
Fats and oils are closely related to each other in that both are triesters of glycerol. The two families differ
from each other, however, in that fats are solid and oils are liquid. The difference in physical state between the two families reflects differences in the fatty acids of which they are made. Fats contain a larger fraction of saturated fatty acid fragments and have, therefore, higher melting points. Oils contain a larger fraction of unsatu-rated fatty acid fragments and have, as a result, lower melting points.
As an example, beef tallow contains about 56% sat-urated fatty acid fragments and about 44% unsatsat-urated fatty acid fragments. In comparison, corn oil contains about 13% saturated fatty acid fragments and 87% unsat-urated fatty acid fragments.
Both fats and oils belong to the family of biochemi-cals known as the lipids. The common characteristics that all lipids share with each other is that they tend to be insoluble in water, but soluble in organic solvents such as ether, alcohol,benzene, and carbon tetrachloride.
Fats are an important constituent of animalbodies where they have four main functions. First, they are a source of energyfor metabolism. Although carbohy-drates are often regarded as the primary source of energy in an organism, fats actually provide more than twice as much energy per calories as do carbohydrates.
Fats also provide insulation for the body, protecting against excessive heat losses to the environment. Third, fats act as a protective cushion around bones and organs. Finally, fats store certain vitamins, such as vitamins A, D, E, and K, which are not soluble in water but are solu-ble in fats and oils.
Animal bodies are able to synthesize the fats they need from the foods that make up their diets. Among hu-mans, 25-50% of the typical diet may consist of fats and oils. In general, a healthful diet is thought to be one that contains a smaller, rather than larger, proportion of fats.
The main use of fats commercially is in the production of soaps and other cleaning products. When a fat is boiled in water in the presence of a base such as sodium hydrox-ide, the fat breaks down into glycerol and fatty acids. The sodium saltof fatty acids formed in this process is the product known as soap. The process of making soap from a fatty material is known as saponification.
See also Lipid.
Fatty acids
A fatty acid is a combination of a chain of carbonand hydrogen atoms, known as a hydrocarbon, and a
particu-Fa
lar acid group (-COOH). Three fatty-acid molecules com-bined with a glycerolform a triglyceride fator oil.
While several varieties of fatty acid occur in nature, all belong in one of two categories—saturated or unsatu-rated. In a saturated fatty-acid molecule, all the carbon atoms in the chain are attached to two hydrogen atoms, the maximum amount. All the bonds between the carbon atoms in the chain are single electronbonds. An exam-ple of fat made of saturated fatty acids is butter.
Unsaturated fatty-acid molecules have one or more carbon atoms with only a single hydrogen atom attached. In these chains, one or more bonds between the carbon atoms are double. A molecule with one double bond is called monounsaturated, and two or more double bonds is called polyunsaturated. An example of unsaturated fat is vegetable oil.
Generally, fats consisting of saturated fatty acids are solid, and those made up of unsaturated molecules are liquid. An unsaturated fatty acid may be converted into saturated through a process called hydrogenation. While most modern diets are aimed at the reduction of fatty acids (fats), it is important to recognize that several of them, such as oleic, butyric, and palmitic acid, are im-portant parts of the human diet. Another, linoleic acid, is absolutely essential to human life. It is an important part of a vital chemical reaction in the body, and is obtained solely through ingestion. It is found in corn,soybean, and peanut oils.
Recently, concern about the amount of trans fatty acids present in food has caused debate. Trans fatty acids are formed during the process of partial hydrogenation of unsaturated fatty acids (like vegetable oil) into mar-garine and vegetable shortening. Some research suggests that levels of trans fatty acids can alter the amount of cholesterolfound in blood, which can be a significant risk to people suffering from high cholesterol levels and heart disease. In addition to being found in margarine, trans fatty acids are also found naturally in small quanti-ties in beef, pork, lamb, and milk. There is conflicting evidence, however, of the dangers of trans fatty acids in daily diets. Generally, it is recommended to limit the total daily amount of fat eaten, rather than focusing sole-ly on trans fatty acid consumption.
See also Carboxylic acids.
Fault
A fault is a geologic term describing a fracture at which two bodies of rock have been displaced relative to each other. Bedrockfaults are those in which bodies of
rock meet; small, local movements may occur on bedrock faults. Much larger movements or displace-ments occur along Faults where plates of Earth’s crust abut each other. Faults may be inches (centimeters) to hundreds of miles (kilometers) in length, and movements or displacements have the same range in length. Major fault systems are typically found where plates meet; for example, the San Andreas Fault in California, is really a fault system including many smaller faults that branch off of the main trace of the San Andreas as well as faults that parallelthe main fault. It may be more accurate to call these systems “fault zones” or “fault belts” that con-tain known and unknown faults. The Northridge earth-quakein the Los Angeles, California, area in January 1994, occurred along a thrust fault that had not previous-ly been known but is within the San Andreas zone. A fault zone may be hundreds of feet (meters) wide and each has a unique character; some include countless faults and others have very few.
Plate tectonics
To understand faults, it is helpful to understand plate tectonics. Earth’s crust is not a solid skin. Instead, it is made up of huge blocks of rock that fit together to form the entire surface of the planet, including the conti-nents or land masses and the floors of the oceans. Scien-tists believe the crust is composed of about 12 of these plates. Each plate is relatively rigid, and, where the plates meet, they can spread apart, grind against each other, or ride one over the other in a process called sub-duction. Spreading plates most commonly occur in the oceans in the phenomenon known as sea-floor spreading; when plates spread within land masses, they create huge valleys called rifts. The process of plates grinding to-gether causes near-surface earthquakes, and the collision and subduction of plates causes the most intense earth-quakes much deeper in the crust.
The engine driving the movement of the plates orig-inates deep in the earth. The mantle, a zone underlying the crust, is very dense rock that is almost liquid. Deeper still is Earth’s core, which is molten rock. Because it is fluid, the core moves constantly. The mantle responds to this, as well as to centrifugal forcecaused by the rota-tionof Earth on its axis and to the force of gravity. The slower motions of the mantle pulse through the thin crust, causing earthquakes, volcanic activity, and the movement of tectonic plates. Together, the pulses caused by the heatengine inside Earth result in over a million earthquakes per year that can be detected by instruments. Only one third of these can be felt by humans, most of which are very small and do not cause any damage. About 100–200 earthquakes per year cause some dam-age, and one or two per year are catastrophic.
History of our understanding of faults
In the history of the study of faults, Robert Mallet, an Irish engineer, was the first to believe that simple mechan-ics of the earth’s crust cause earthquakes. Until 1859, when he proposed his theory, earthquakes were believed to be caused by huge explosions deep within the earth, and the origin of these explosions was never questioned. Mallet knew that iron, which appears indestructible, rup-tures under extreme stress, and Mallet theorized that earthquakes are caused “either by the sudden flexure and constraint of the elastic materials forming a portion of the earth’s crust, or by their giving way and become frac-tured.” Mallet was not supported, primarily because he was not a scholar and lived in Ireland where earthquakes seldom occur. In 1891, however, Professor Bunjiro Koto, a Japanese specialist in seismology, or the study of earth-quakes, endorsed Mallet’s theory. After the Mino-Iwari earthquake, which occurred along a remarkably clear fault line crossing the islandof Honshu, he said the shaking earth caused quakes and not the other way around. Harry Fielding Reid, an American scientist, was the first to relate the stresses along faults to tectonic plate boundaries after the 1906 Great San Francisco Earthquake.
Types of faults
Faults themselves do not cause earthquakes; instead, they are the lines at which plates meet. When the plates press together (compress) or pull apart (are in tension), earthquakes occur. The fault line is essentially a stress concentration. If a rubber band is cut partially through then pulled, the rubber band is most likely to break at the cut (the stress concentration). Similarly, the “break” (stress release or earthquake) occurs along a fault when
the plates or rock bodies that meet at the fault press to-gether or pull apart.
Movement along a fault can be vertical (up and down, changing the surface elevation), horizontal (flat at the surface but with one side moving relative to the other), or a combination of motions that inclines at any angle. The angle of inclination of the fault plane mea-sured from the horizontal is called the dip of the fault plane. This movement occurs along a fault surface or fault plane. Any relative vertical motionwill produce a hanging wall and a footwall. The hanging wall is the block that rests upon the fault plane, and the footwall is the block upon which you would stand if you were to walk on the fault plane.
Dip-slip faults are those in which the primary mo-tion is parallel to the dip of the fault plane. A normal fault is a dip-slip fault produced by tension that stretches or thins Earth’s crust. At a normal fault, the hanging wall moves downward relative to the footwall. Two normal faults are often separated by blocks of rock or land creat-ed by the thinning of the crust. When such a block drops down relative to two normal faults dipping toward each other, the block is called a graben. The huge troughs or
Fault
Figure 1. Normal fault striking north. The solid square repre-sents the slip vector showing the motion of block A relative to block B.Illustration by Hans & Cassidy. Courtesy of Gale Group.
rift valleys created as plates move apart from each other are grabens. The Rhine Valley of Germany is a graben. An extreme example is the Atlantic Ocean; over 250 mil-lion years ago,North Americaand Africawere a single mass of land that slowly split apart and moved away from each other (a process called divergence), creating a huge graben that became the Atlantic Ocean basin. Two normal faults dipping away from each other can create an uplifted block between them that is called a horst. Horsts look like raised plateaus instead of sunken val-leys. If the block between normal faults tilts from one side to the other, it is called a tilted fault block.
A reverse fault is another type of dip-slip fault caused by compression of two plates or masses in the horizontal direction that shortens or contracts the earth’s surface. When two crustal masses butt into each other at a reverse fault, the easiest path of movement is upward. The hanging wall moves up relative to the footwall. When the dip is less than (flatter than) 45°, the fault is termed a thrust fault, which looks much like a ramp. When the angle of dip is much less than 45° and the total movement or displacement is large, the thrust fault is called an overthrust fault. In terms of plate movement, the footwall is slipping underneath the hanging wall in a process called subduction.
Strike-slip faults are caused by shear (side-by-side) stress, resulting in a horizontal direction, parallel to the nearly vertical fault plane. Strike-slip faults are common in the sea floor and create the extensive offsets mapped along the mid-oceanic ridges. The San Andreas Fault is perhaps the best-known strike-slip fault, and, because much of its length crosses land, its offsets are easily ob-served. Strike-slip faults have many other names includ-ing lateral, transcurrent, and wrench faults. Strike-slip
faults located along mid-oceanic ridges are called trans-form faults. As the sea floor spreads, new crust is trans-formed by magma(molten rock) that flows up through the break in the crust. This new crust moves away from the ridge, and the plane between the new crust and the older ridge is the transform fault.
Relative fault movement is difficult to measure be-cause no point on the earth’s surface, including sea level is fixed or absolute. Geologists usually measure displace-ment by relative movedisplace-ment of markers that include veins or dikes in the rock. Sedimentary rocklayers are espe-cially helpful in measuring relative uplift over time. Faults also produce rotational movements in which the blocks rotate relative to each other; some sedimentary stratahave been rotated completely upside down by fault movements. These beds can also be warped, bent, or fold-ed as the comparatively soft rock tries to resist compres-sional forces and frictioncaused by slippage along the fault. Geologists look for many other kinds of evidence of fault activity such as slickensides, which are polished or scratched fault-plane walls, or fault gouge, which is clayey, fine-grained crushed rock caused by compression. Coarse-grained fault gouge is called fault breccia.
Mountain-building by small movements along faults
Compression of land masses along faults has built some of the great mountain ranges of the world. Moun-tain-building fault movements are extremely slow, but, over a long time, they can cause displacements of thou-sands of feet (meters). Examples of mountain ranges that have been raised by cumulative lifting along faults are the Wasatch Range in Utah, the uplifting of layer upon layer of sedimentary rocksthat form the eastern front of
Fault
Figure 3. Thrust fault striking north. The solid square repre-sents the slip vector showing the motion of block A relative to block B. Illustration by Hans & Cassidy. Courtesy of Gale Group.
the Rocky Mountains in Wyoming and Montana, the large thrust faults that formed the Ridge and Valley Province of the Appalachian Mountains in Virginia and Tennessee, and the Himalayas (including Mount Everest and several of the other tallest mountains in the world) that are continuing to be pushed upward as the tectonic plate bearing the Indian Subcontinent collides with the Eurasian plate. Tension along smaller faults has created the mountain ranges that bracket the Great Basin of Nevada and Utah. These mountains may have been formed by the hanging walls of the many local faults that slid downward by thousands of feet (meters) until they became valley floors.
Earthquake generation by large, sudden movements along faults
The majority of fault motion are slow and creeping movements, unlikely to be felt by humans at ground sur-face. Some movements occur as rapid spasms that hap-pen in a few seconds and can cause ground displace-ments of inches or feet (centimeters or meters). These
movements are resisted by friction along the two faces of the fault plane until the tensional, compressional or shear stress exceeds the frictional force. Earthquakes are caused by these sudden jumps or spasms. Severe shaking can result, and ground rupture can create fault scarps.
Famous or infamous faults
The San Andreas Fault
The San Andreas Fault may well be the best known fault in the world. It marks a major fracture in the Earth’s crust, passing from Southern through Northern California for a length of about 650 mi (1,050 km) and then travers-ing under part of the northern Pacific Ocean. The San An-dreas does mark a plate boundary between the Northern Pacific and North American plates, and, because this transform fault extends to the surface in a heavily popu-lated area, movement along the fault causes major earth-quakes. The forces that cause these movements are the same ones responsible for continental drift. The Great San Francisco Earthquake of 1906 occurred along the main San Andreas, and the Loma Prieta earthquake of 1989 was caused by movement on a branch of the San Andreas. The motion of the Northern Pacific plate as it grinds past the North American plate causes strike-slip fault movements. The plate is moving at an average of about 0.4-in (1 cm) per year, but its speed accelerated during the 1900s to between 1.6–2.4 in (4–6 cm) per year as it pushes Los Angeles northward toward San Francis-co. Much more rapid jumps occur during earthquakes; in 1906, movements as great as 21 ft (6.4 m) were measured in some locations along the San Andreas Fault.
The San Andreas Fault is infamous for another rea-son. The major cities of California including Los Ange-les, Oakland, San Jose, and San Francisco, home to mil-lions of people, straddle this fault zone. Such develop-ment in this and other parts of the world puts many at risk of the devastation of major fault movements. Sudden fault movements fill the headlines for weeks, but, over the course of geologic time, they are relatively rare so the chances to study them and their effects are limited. Simi-larly, our knowledge and ability to predict fault motions and to evacuate citizens suffers. An estimated 100 million Americans live on or near an active earthquake fault.
The New Madrid Fault is more properly called a seismic zone because it is a large fracture zone within a tectonic plate. It is a failed rift zone; had it developed like the East African Rift Valley, it would have eventual-ly split the North American continent into two parts. The zone crosses the mid-section of the United States, passing through Missouri, Arkansas, Tennessee, and Kentucky in the center of the North American Plate. The zone is about 190 mi (300 km) long and 45 mi (70 km)
Fault
wide, and it lies very deep below the surface. The zone is covered by alluvial material (soiland rock carried and deposited by water) from the Mississippi, Ohio, and Missouri rivers; because this alluvial material is soft and unstable, movement within the fracture zone transmits easily to the surface and is felt over a broad area.
On December 16, 1811, and January 23 and Febru-ary 7, 1812, three earthquakes estimated to have mea-sured greater than magnitude 8.0 on the Richter scale had their epicenters near the town of New Madrid, Missouri, then part of the American Frontier. An area of 3,000–5,000 sq mi (7,800–13,000 sq km) was scarred by landslides, fissures, heaved-up land, leveled forests, and lakes, swamps, and rivers that were destroyed, rerouted, or created. These earthquakes were felt as far away as the East Coast, north into Canada, and south to New Orleans. On January 16, 1995, the city of Kobe, Japan was struck by a magnitude 7.2 earthquake that killed more than 4,000 people and left almost 275,000 homeless. Like the California cities along the San Andreas, Kobe is a port city, so the earthquake also caused tremendous
losses to the economy of the region. Also like Oakland and San Francisco, California, Kobe is located next to a deep bay. Osaka Bay is encircled by a host of faults and fault zones with complicated relationships. The Nojima Fault on Awaji Island appears to have been the fault that hosted the Hyogogen-Nambu Earthquake of 1995. The North American Plate, Pacific Plate, Eurasian Plate, and Philippine Sea Plate all impact each other near the is-lands united as Japan. Thick, relatively young deposits of alluvial soil overly the faults that pass under Osaka Bay; these amplified the earth’s movements along the fault in this highly populated area.
Earthquakes caused by human activities
Although the most devastating earthquakes occur in nature, humans have been able to learn more about faults and earthquake mechanisms since we have had the power to produce earthquakes ourselves. Nuclear weapons test-ing in the desertnear Los Alamos, New Mexico, was the first known human activity to produce measurable earth-quakes that were found to propagate along existing faults.
Fault
Continental drift—A theory that explained the rel-ative positions and shapes of the continents, and other geologic phenomena, by lateral movement of the continents. This was the precursor to plate tectonic theory.
Core—The molten center of the earth.
Crust—The outermost layer of the earth, situated over the mantle and divided into continental and oceanic crust.
Dip—The angle of inclination (measured from the horizontal) of faults and fractures in rock.
Footwall—The block of rock situated beneath the fault plane.
Graben—A block of land that has dropped down be-tween the two sides of a fault to form a deep valley. Hanging wall—The block of rock that overlies the fault plane.
Horst—A block of land that has been pushed up between the two sides of a fault to form a raised plain or plateau.
Mantle—The middle layer of the earth that wraps around the core and is covered by the crust. The mantle consists of semi-solid, partially melted rock. Normal fault—A fault in which tension is the
pri-mary force and the footwall moves up relative to the hanging wall.
Plate tectonics—The theory, now widely accepted, that the crust of the earth consists of about twelve massive plates that are in motion due to heat and motion within the earth.
Reverse fault—A fault resulting from compression-al forces and the hanging wcompression-all moves up relative the footwall.
Seismic gap—A length of a fault, known to be his-torically active, that has not experienced an earth-quake recently and may be storing strains that will be released as earthquake energy.
Strike-slip fault—A fault at which two plates or rock masses meet and move lateral or horizontally along the fault line and parallel to the compres-sion.
Subduction—In plate tectonics, the movement of one plate down into the mantle where the rock melts and becomes magma source material for new rock.
Thrust fault—A low-angle reverse fault in which the dip of the fault plane is 45° or less and dis-placement is primarily horizontal.
KEY TERMS
Our ability to build major damsthat retain huge quanti-ties of water has also generated earthquakes by so-called “hydrofracturing,” in which the weight of the water stresses fractures in the underlying rock. Pumping of oil and natural gasfrom deep wells and the disposal of liq-uid wastes through injection wells have also produced small motions along faults and fractures.
Advances in fault studies
Our understanding of how faults move has improved greatly with modern technology and mapping. Laser survey equipment and satellitephotogrammetry (mea-surements made with highly accurate photographs) have helped measure minute movements on faults that may in-dicate significant patterns and imminent earthquakes. Seismic gaps have been identified along plate bound-aries. Through detailed mapping of tiny earthquakes, zones where strains in the earth have been relieved are identified; similarly, seismic gap areas without those strain-relieving motions are studied as the most likely zones of origin of coming earthquakes.
Resources Books
Erickson, Jon. “Quakes, Eruptions, and Other Geologic Cata-clysms.”The Changing Earth Series.New York: Facts on File, 1994.
Halacy, D. S., Jr. Earthquakes: A Natural History.Indianapolis, IN: The Bobbs-Merrill Company, Inc., 1974.
Keller, Edward. Environmental Geology.Upper Saddle River, NJ: Prentice-Hall, Inc., 2000.
Japanese Geotechnical Society. Soils and Foundations: Special Issue on Geotechnical Aspects of the January 17, 1995, Hyogoken-Nambu Earthquake.Tokyo: Japanese Geotech-nical Society, January 1996.
Verney, Peter. The Earthquake Handbook.New York: Padding-ton Press Ltd., 1970.
Walker, Bryce and the Editors of Time-Life Books. Planet Earth: Earthquake.Alexandria, VA: Time-Life Books, 1982.
Gillian S. Holmes
Fauna
Fauna is a generic term for the list of animalspecies occurring in a particular, large region. Fauna can refer to a prehistoric collection of animals, as might be inferred from the fossil record, or to a modern assemblage of species living in a region. The botanical analogue is known as flora. More locally, a faunation refers to the communities of individuals of the various animal species
and occurring in a particular place. Because many zoolo-gists are specialized in the animals they study, faunas are often considered on the basis of systematic groups, as is the case of bird species (avifauna) or reptilesand am-phibians(herpetofauna).
A faunal region is a zoogeographic designation of large zones containing distinct assemblages of species that are more-or-less spatially isolated from other provinces by physical barriers to migration, such as a large body of water, a mountain range, or extensive desert. Faunal provinces are less distinct sub-units of faunal regions. These various designations are typically separated by zones of rapid transition in species types.
In the Americas, for example, there are two major faunal regions, with a zone of rapid transition occurring in Central America. The South American zoofauna in-cludes many species and even families that do not occur naturally in North America, and vice versa. The South and North American faunal regions are divided by the narrow Isthmus of Panama, which has been submerged by oceanic waters at various times in the geological past, or has otherwise presented a significant barrier to the mi-gration of many species of animals. However, during pe-riods in the past when animals were able to pass through this barrier, significant mixtures of the two faunas oc-curred. Lingering evidence of relatively recent episodes of prehistoric faunal blending include the presence of the opossum (Didelphis virginiana) and California condor (Gymnogyps californianus) in North America, and white-tailed deer(Odocoileus virginianus) and cougar (Felis concolor) in South America.
Another famous faunal transition is known as Wal-lace’s Line, after the nineteenth century naturalist who first identified it, A. R. Wallace (he was also with Charles Darwin, the co-publisher of the theory of evolu-tionby natural selection). Wallace’s Line runs through the deepwater oceanic straits that separate Java, Borneo, and the Philippines and Southeast Asiamore generally to the north, from Sulawesi, New Guinea, and Australia to the south. The most extraordinary faunistic difference across Wallace’s Line is the prominence of marsupial an-imals in the south, but there are also other important dis-similarities.
One of the most famous faunal assemblages in the fossil record is that of the Burgess Shale of southeastern British Columbia. This remarkable fauna includes 15-20 extinct phyla of metazoan animals that existed during an evolutionary radiationin the early Cambrian era, about 570 million years ago. Most of the phyla of the Cambri-an marine fauna are now extinct, but all of these lost Cambri- ani-mals represented innovative and fantastic experiments in the form and function of the invertebrate body plan (and
Feather stars
Fax machine. Photograph by David Young-Wolff. PhotoEdit. Reproduced by permission.
also undoubtedly, in invertebrate physiology,behavior, and ecology, although these cannot be inferred from the fossil record).
Faunal dating
see
Dating techniques
Fax machine
The facsimile, or fax, machine is both a transmitting and receiving device that “reads” text, maps, pho-tographs, fingerprints, and graphics and communicates via telephoneline. Since 1980s, fax machines have un-dergone rapid development and refinement and are now indispensable communication aids for news services, businesses, government agencies, and individuals.
The fax was invented by Alexander Bain of Scotland in 1842. His crude device, along with scanning systems invented by Frederick Bakewell in 1848, evolved into several modern versions. In 1869 a Frenchman, Ludovic d’Arlincourt, synchronized transmitters and receivers with tuning forks and thus aided further developments. In 1924, faxes were first used to transmit wire photos from Cleveland to New York, a boom to the newspaper industry. Two years later, RCA inaugurated a trans-At-lantic radiophoto service for businesses.
The use of faxes, and fax technology itself, remained comparatively limited until the mid-1980s. By that time, models either required an electrolytic or photosensitive paper, which changed color when current passed through it; or thermal paper, a material coated with color-less dye, which became visible upon contact with a toner. Updated models from the 1990s employ plain paper (which, unlike thermal paper, avoids curling) and are pre-ferred for their superior reproduction. Another improve-ment is the invention of a scrambler, an encoder that al-lows the sender to secure secrecy for documents, particu-larly those deriving from highly sensitive government projects or secret industrial or business dealings.
Some fax machines are incorporated into telephone units; others stand alone; and still others are part of per-sonal computers. These last models contain a fax board, an electronic circuit that allows the computer to receive messages. In the most common models, the user inserts the material to be transmitted into a slot, then makes a telephone connection with another facsimile machine. When the number is dialed, the two machines make elec-tronic connection. A rotating drum advances the original before an optical scanner. The scanner reads the original document either in horizontal rows or vertical columns and converts the printed image into a pattern of several
million tiny electronic signals, or pixels, per page. The facsimile machine can adjust the number of pixels so that the sender can control the sharpness and quality of the transmission. Within seconds, the encoded pattern is con-verted into electric currentby a photoelectric cell, then travel via telegraphor telephone wires to the receiving fax, which is synchronized to accept the signal and pro-duce an exact replica of the original by reverse process.
Feather stars
Feather stars, or comatulids, are echinoderms that belong to the class Crinoidea (phylum Echinodermata) which they share with the sea lilies. Unlike the latter group, however, feather stars are not obliged to remain in one place; instead they can swim or even crawl over short distances before attaching themselves to some sup-port. Swimming movements are achieved by waving the arms up and down in a slow, controlled manner. Feather stars are widely distributed throughout tropical and warm-temperate waters, with the main center of their distribution being focused on the Indo-Pacific region. An estimated 550 speciesare known.
