If one quantity increases (or decreases) each time another quantity increases (or decreases), the two quantities are said to vary together. The most common form of this is direct variation in which the ratio of the two amounts is always the same. For example, speed and distance traveled vary directly for a given time. If you travel at 4 mph (6.5 kph) for three hours, you go 12 mi (19.5 km), but at…
Discrimination should be understood as action and therefore distinguished from prejudice, which is a matter of thought, attitude, or belief. Racial discrimination then would consist of social action that produces unjust allocation of valued resources, based on racial categorizations of individuals or groups (Banton; Kairys, 1996). This is the standard definition and still serves in many cases: whe…
Disease can be defined as a change in the body processes that impairs its normal ability to function. Every day the physiology of the human body demands that oxygenation, acidity, salinity, and other functions be maintained within a very narrow spectrum. A deviation from the norm can be brought about by organ failure, toxins, heredity, radiation, or invading bacteria and viruses. Normally the body…
Distance has two different meanings. It is a number used to characterize the shortest length between two geometric figures, and it is the total length of a path. In the first case, the distance between two points is the simplest instance. In the case of parallel lines, the distance between the two lines is the length of a perpendicular segment connecting them. If two figures such as line segments,…
A forest in Homestead, Florida, that was destroyed by Hurricane Hugo. JLM Visuals. Reproduced by permission. In the ecological context, disturbance is regarded as an event of intense environmental stress occurring over a relatively short period of time and causing large changes in the affected ecosystem. Disturbance can result from natural causes or from the activities of humans. Disturbanc…
Diurnal cycles refer to patterns within about a 24-hour period that typically reoccur each day. Most daily cycles are caused by the rotation of Earth, which spins once around its axis about every 24 hours. The term diurnal comes from the Latin word diurnus, meaning daily. Diurnal cycles such as temperature diurnal cycles, diurnal tides, and solar diurnal cycles affect global processes. Diurnal tid…
Diversity, as a word or concept, can apply to rocks, plants, animals, people, systems of law, and much else. In the United States, since the 1970s, its immediate reference, if the word is presented with nothing more to specify it, is to the diversity of races, ethnic groups, and language groups that make the United States possibly the most diverse country in the world. But its import extends far b…
Division is the mathematical operation that is the inverse of multiplication. If one multiplies 47 by 92 then divides by 92, the result is the original 47. In general, (ab)/b = a. Likewise, if one divides first then multiplies, the two operations nullify each other: (a/b)b = a. This latter relationship can be taken as the definition of division: a/b is a number which, when multiplied by b, yields …
DNA, short for deoxyribonucleic acid, is a double-stranded, helical molecule that forms the molecular basis for heredity. For DNA replication to occur, this molecule must first unwind, or "unzip," itself to allow the information-encoding bases to become accessible. The base pairing within DNA is of a complementary nature and, consequently, when the molecule unzips, due to the action …
Deoxyribonucleic acid (DNA) synthesis is a process by which strands of nucleic acids are created. In a cell, DNA synthesis takes place in a process known as replication. Using genetic engineering and enzyme chemistry, scientists have also developed man-made methods for synthesizing DNA. The DNA molecule was discovered by Francis Crick, James Watson, and Maurice Wilkins. In 1953, Watson and Crick u…
DNA technology has revolutionized modern science. Deoxyribonucleic acid (DNA), or an organism's genetic material—inherited from one generation to the next—holds many clues that have unlocked some of the mysteries behind human behavior, disease, evolution, and aging. As technological advances lead to a better understanding of DNA, new DNA-based technologies will emerge. Recent …
The use of a vaccine constructed of a protein has traditionally been to induce the formation of an antibody to the particular protein. Antibodies are crucial to an or ganism's attempt to stop an infection caused by a microorganism. In the early 1990s, scientists observed that plasmid DNA (DNA that is present in bacteria that is not part of the main body of DNA) could affect test animals. Wo…
Dobsonflies are species of medium- to large-sized insects in the order Neuroptera, family Corydalidae. The life cycle of dobsonflies is characterized by a complete metamorphosis, with four developmental stages: egg, larva, pupa, and adult. Adult dobsonflies are usually found near freshwater, especially streams, either resting on vegetation or engaged in an awkward, fluttering flight. Sometimes adu…
Dogwood refers to certain species of trees and shrubs in the dogwood family (Cornaceae). The dogwoods are in the genus Cornus, which mostly occur in temperate and boreal forests of the Northern Hemisphere. Species in the dogwood family have seasonally deciduous foliage. The leaves are simple, usually untoothed, and generally have an opposite arrangement on the twig. The flowers of dogwoods develop…
The domain of a relation is the set that contains all the first elements, x, from the ordered pairs (x,y) that make up the relation. In mathematics, a relation is defined as a set of ordered pairs (x,y) for which each y depends on x in a predetermined way. If x represents an element from the set X, and y represents an element from the set Y, the Cartesian product of X and Y is the set of all possi…
Domestic donkeys, members of the order Perissodactyla, are large single-hoofed horse-like mammals with elongated heads. Donkeys usually stand between 9.5 and 11 hands high measured at the withers, that is, 38-44 in (95-110 cm) tall. Because of the large amount of interbreeding among different donkey species, donkeys differ markedly in appearance. They can be brown, gray, black, roan (a mixture of …
Dopamine is a neurotransmitter (a chemical used to send signals between nerve cells) in the same family as epinephrine (adrenaline). Dopamine is one of the primary neurotransmitters and it affects motor functions (movement), emotions, learning, and behavior. It was originally identified as the brain chemical associated with pleasure. A decrease in the amount of dopamine in specific sections of the…
Dories are bony fish in the family Zeidae. A dory has an oval body with a back that rises so that the main part of the body is higher than the head. The body itself is relatively thin and compressed and appears oval in side view. Another distinguishing mark of the dories is a dark spot on each side of the body surrounded by a yellow ring. Dories typically are found in the middle depths of the seas…
Dormice are approximately ten species of rodents that make up the family Gliridae. Dormice typically live in trees, bushes, hedgerows, gardens, and rock piles. Dormice have a superficial resemblance to squirrels (family Sciuridae), but they are smaller and differ in many other anatomical and behavioral characters. Dormice have soft fur, and a long, bushy tail. Their forefeet have four digits, the …
New drugs undergo double-blind testing to determine whether they are effective. The test is called double-blind because neither the doctor who is administering the medication nor the patient who is taking it knows whether the patient is getting the experimental drug or a neutral substitute, called a placebo. Getting a new drug approved is a long, complex process in order to ensure the drug is safe…
The double helix refers to DNA's "spiral staircase" structure, consisting of two right-handed helical polynucleotide chains coiled around a central axis. Genes, which are specific regions of DNA, contain the instructions for synthesizing every protein. Because life cannot exist without proteins, the discovery of DNA's structure unveiled the secret of life: protein synth…
Down syndrome is the most common cause of mental retardation. It can be caused by the presence of an extra chromosome. Chromosomes contain sequences of DNA called genes, which represent the genetic information that exists within almost every cell of the body. Twenty-three distinctive pairs, or 46 total chromosomes, are located within the nucleus (central DNA-containing structure) of each cell. Whe…
Dragonflies are large flying insects in the order Odonata. Dragonflies can be as large as 3 in (7.5 cm) in length, with a wing span of up to 8 in (20 cm). The fossilized remains of a huge dragonfly-like insect that had a wingspread of more than 2 ft (70 cm) is known from the Carboniferous period, some 300 million years ago. Dragonflies are very distinctive insects, with large eyes that almost cove…
It is midnight in the desert, and the full moon has just passed its apex. On the sandy ground, staff in hand, guitar and jug by his side, a dark-skinned man is nuzzled by a tawny-maned lion. Is the man dreaming? Are we? Or is this the dream of the artist, Henri Rousseau (1844–1910)? If, as some traditions have it, the Universe was dreamed into existence by its Creator, then it makes perfect…
Since the 1980s new generations of academics, collectors, curators, and enthusiasts have discovered the value of the study of dress as an analytical research tool through which to examine aspects of social and economic history, material culture, cultural and gender studies, art history, anthropology, and sociology. As a consequence, the study of the history of dress has been transformed from its m…
Drift nets are lengthy, free-floating, 26-49 ft (8-15 m) deep nets, each as long as 55 mi (90 km). Drift nets are used to snare fish by their gills in pelagic, open-water situations. Because drift nets are not very selective of species, their use results in a large by-catch of non-target fish, sharks, turtles, seabirds, and marine mammals, which are usually jettisoned, dead, back to the ocean. Dri…
Drongos are 20 species of handsome birds that make up the family of perching birds known as Dicruridae. Drongos occur in Africa, southern and southeastern Asia, and Australasia. Their usual habitats are open forests, savannas, and some types of cultivated areas with trees. Drongos are typically black colored with a beautiful, greenish or purplish iridescence. The wings of these elegant, jay-sized …
Throughout the last century, the fruit fly Drosophila melanogaster, has been the workhorse for genetic studies in eukaryotes. These studies provided the basis of much of scientists' understanding of fundamental aspects of eukaryotic genetics. Cloned fruit fly genes have led to the identification of mammalian cognates. Discoveries have shown that the conservation between the fruit fly and ma…
Drought is characterized by low precipitation compared to the normal amount for the region, low humidity, high temperatures, and/or high wind velocities. When these conditions occur over an extended period of time, drought causes low water supplies that are inadequate to support the demands of plants, animals, and people. Drought is a temporary condition that occurs in moist climates. This is in c…
Dualism is a doctrine positing two equally powerful and antagonistic metaphysical principles, which are constitutive of the world and must explain our experience of the world. They are often conceived as dichotomies, such as good and evil, light and darkness, attraction and repulsion, or love and strife. Earlier forms of dualism can be traced in ancient Egyptian religion, with the contest between …
Ducks are waterfowl in the order Anseriformes, in the family Anatidae, which also includes geese and swans. Ducks occur on all continents except Antarctica, and are widespread in many types of aquatic habitats. Almost all ducks breed in freshwater habitats, especially shallow lakes, marshes, and swamps. Most species of ducks also winter in these habitats, sometimes additionally using grain fields …
Duckweeds are small, floating to slightly submerged species of flowering plants in the genus Lemna. The simple body is leaf-like, generally flat on top and convex below, lacks stems or leaves, is oval to tear-dropped in shape, and has one unbranched root that lacks vascular (conducting) tissue. The upper surface of the plant is covered with waxy compounds so as to shed water. Duckweeds are abundan…
Duikers are small African antelopes in the large family of Bovidae. This family of hoofed animals includes antelope, gazelles, cattle, sheep, and goats. Like all bovids, duikers have even-toed hooves, horns, and a four-chambered stomach structure that allows them to digest a diet of plants. Duikers are found throughout sub-Saharan Africa. These small antelopes range in size from 22 in (55 cm) to a…
A dune is a wind-blown pile of sand. Over time, dunes become well-sorted deposits of materials by wind Dunes form different characteristic shapes depending on the amount of sand, amount of moisture, and the strength and prevailing direction of the wind (i.e., windward to leeward). Illustration by Argosy. The Gale Group. or water that take on a characteristic shape and that retain that genera…
Along with squaring the circle and trisecting an angle, duplication of a cube is considered one of the three "unsolvable" problems of mathematical antiquity. According to tradition, the problem of duplication of the cube arose when the Greeks of Athens sought the assistance of the oracle at Delos in order to gain relief from a devasting epidemic. The oracle told them that to do so th…
A dust devil is a relatively small, rapidly rotating wind that stirs up dust, sand, leaves, and other material as it moves across the ground. Dust devils are also known as whirlwinds or, especially in Australia, willywillys. In most cases, dust devils are no more than 10 ft (3 m) in width and less than 300 ft (100 m) in height. Dust devils form most commonly on hot dry days in arid regions such as…
In 1995, Philips and Sony introduced the digital video disc (DVD), which had the same dimensions as a standard compact disk (CD), but was able to store up to 4.7 gigabytes of data, such as high-definition digital video files. This is more than three times the capacity of a CD. DVD players use a higher-power laser than that used for CDs, which enables smaller pits (0.4 micrometre) and separation tr…
These small antelopes belong to the ruminant family Bovidae, and are grouped with the gazelles in the subfamily Antilopinae. The 13 species of dwarf antelopes are in the tribe Neotragini. Dwarf antelopes range from extremely small (3.3-4.4 lb or 1.5-2 kg) hare-sized royal antelopes and dik-diks to the medium-sized oribi and beira weighing from 30-50 lb (10-25 kg). Dwarf antelopes engage in territo…
Color scientists use the term "colorant" for the entire spectrum of coloring materials, including dyes and pigments. While both dyes and pigments are sources of color, they are different from one another. Pigments are particles of color that are insoluble in water, oils, and resins. They need a binder or to be suspended in a dispersing agent to impart or spread their color. Dyes are …
Dyslexia is a disorder that falls under the broad category of learning disabilities. It is often described as a neurological syndrome in which otherwise normal people have difficulty reading and writing. Frequently, dyslexia is defined by what it is not—dyslexia is not mental retardation, a psychiatric or emotional disorder, or a vision problem. Dyslexia is not caused by poverty, psychologi…
Dysplasia is a combination of two Greek words; dys, which means difficult or disordered; and plassein, to form. In other words, dysplasia is the abnormal or disordered formation of certain structures. In medicine, dysplasia refers to cells that have acquired an abnormality in their form, size, or orientation with respect to each other. Dysplasia may occur as the result of any number of stimuli. Su…