A heterotroph is a creature that must ingest biomass to obtain its energy and nutrition. In direct contrast, autotrophs are capable of assimilating diffuse, inorganic energy and materials and using these to synthesize biochemicals. Green plants, for example, use sunlight and simple inorganic molecules to photosynthesize organic matter. All heterotrophs have an absolute dependence on the biological…
Hibernation is a state of inactivity, or torpor, in which an animal's heart rate, body temperature, and breathing rate are decreased in order to conserve energy through the cold months of winter. A similar state, known as estivation, occurs in some desert animals during the dry months of summer. Hibernation is an important adaptation to harsh climates, because when food is scarce, an animal…
Early mountaineers from India named the Himalayas "snow abode" based on two Sanskrit words hima and laya. These early climbers were attracted to the mountains by the same features that, today, challenge climbers from all over the world. The range includes the highest peaks in the world, notably Mount Everest; glaciated valleys and snow that never melts create unsurpassable vistas; an…
Histamines are chemicals released by cells of the immune system during the inflammatory response, which is one of the body's defenses against infection. For instance, the inflammatory response helps neutralize bacteria that enter the body when the skin is accidentally cut with a knife. In addition, the sneezing, runny nose, and itchy eyes of allergies are actually "small-scale"…
Historical geology is the study of changes in Earth and its life forms over time. It includes sub-disciplines such as paleontology, paleoclimatology, and paleoseismology. In addition to providing a scientific basis for understanding the evolution of Earth over time, historical geology provides important information about ancient climate changes, volcanic eruptions, and earthquakes that can be used…
Historicism (German Historismus, French historisme, Italian storicismo) is a term of Romantic origins associated first with the German Historical Schools and then more generally with historical method as applied to all the arts and sciences and to human life. "Historicism" appeared first in a fragment of Novalis, who contrasted it with other methods (chemical, mathematical, artistic,…
Genetics as a discipline is young, but the concept that forms its subject—inheritance—stretches back in time. The word has been formed from the adjective genetic, found in the sciences of the nineteenth and twentieth centuries—for example, biogenetic law, genetic affinity, genetic psychology—and meaning, according to the Oxford English Dictionary, "pertaining to,…
The key to the conceptualization of history is the effort to essentialize the past, first in a Herodotean or Thucydidean narrative, and then in more rational, conjectural, philosophical, and religious ways. The "past" became (in Arnold Toynbee's phrase) an "intelligible field of study," and anthropomorphism as well as religious beliefs and ethnic prejudices came …
The hoatzin (Opisthocomus hoazin) is one of the world's most peculiar bird species. It is the sole member of its family, Opisthocomidae. It is peculiar enough to have defied taxonomists' best efforts for years. This bird lives only in the rainforests of northern South America. Its feathers are dark brown on the back and lighter below, and chestnut-colored on its sides. The skin aroun…
Hodgkin's disease is a type of cancer involving tissues of the lymphatic system. The lymphatic system is a network of organs, tissues, and ducts in the human body. The lymphatic system maintains the fluid balance in the body by coordinating the draining of fluid from cells and tissues back into the bloodstream. Also, the lymphatic system aids in fighting infections caused by microorganisms,…
Holography is defined as a method of producing a three-dimensional (3-D) impression of an object. The recording and the image it brings to life are each referred to as holograms. This impression is taken by splitting a beam of coherent (that is, uniform over distance as well as over time) radiation along two paths. One is known and stays undisturbed, to act as a reference. Another strikes the obje…
Homeostasis (a Greek term meaning same state), is the maintenance of constant conditions in the internal environment of the body despite large swings in the external environment. Functions such as blood pressure, body temperature, respiration rate, and blood glucose levels are maintained within a range of normal values around a set point despite constantly changing external conditions. For instanc…
Honeycreepers are 14 living species of birds in the family Drepanididae, which occur only on the Hawaiian and Laysan Islands and nearby islands in the equatorial Pacific Ocean. Unfortunately, a further eight species of honeycreepers have recently become extinct as a result of ecological changes that humans have caused to the habitats of these birds. In addition, at least half of the surviving spec…
As their name suggests, honeyeaters are often found near flowering plants feeding on nectar. All of the Honeyeaters have slender, pointed bills with a long, brushlike tongue that is used to sip the nectar. However, there are many variations of the bill shape, depending on the specific diet of each species. Species with longer bills are usually feeding from tubular flowers, while those with shorter…
Codes of honor are all-pervasive in human societies, but the modern study of honor as an academic formulation originated in the Mediterranean region, and especially in the work of anthropologists working in Spain and Greece. Julio Caro Baroja, J. G Peristiany, and Julian Pitt-Rivers wrote some of the seminal and most influential works on the concept of honor. They placed the honor complex in the M…
Ancient and modern Arabs, as well as other ethnic groups of Muslim and Mediterranean peoples, adopted ideas of honor that reinforce the ties of an individual to his or her tribal clan or extended family. One type of honor, sharaf, applies to men and is attained through maintenance of a family's reputation, hospitality, generosity, chivalry, bravery, piety, and, sometimes, nobility or politi…
The hoopoe (Upupa epops) is the only species in its family, the Upupidae. This species breeds in northwestern Africa, on Madagascar, throughout the Middle East, and in southern Europe and southern Asia. Its usual habitats are open forests, savannas, grasslands, and some types of cultivated lands and parks. Some populations of hoopoes are sedentary, while others are migratory. Hoopoes have a body l…
A soil horizon is a horizontal layer of soil with physical or chemical characteristics that separate it from layers above and below. More simply, each horizon contains chemicals, such as rust-like iron oxides, or soil particles that differ from adjacent layers. Soil scientists generally name these horizons (from top to bottom) "O," "A," "B," "C,�…
Hormones are biochemical messengers that regulate physiological events in living organisms. More than 100 hormones have been identified in humans. Hormones are secreted by endocrine (ductless) glands such as the hypothalamus, the pituitary gland, the pineal gland, the thyroid, the parathyroid, the thymus, the adrenals, the pancreas, the ovaries, and the testes. Hormones are secreted directly into …
Hornbills are medium- to large-sized, large-billed, long-tailed birds of tropical forests, savannas, and grasslands, comprising the family Bucerotidae. The 45 species of hornbills are distributed widely through the tropical regions of Africa and Asia. Most hornbills live in forests, and nest in holes in trees, while the species of open habitats nest in cavities in hollow trees or in holes in cliff…
The horse chestnut and buckeyes (Aesculus spp.) are various species of angiosperm trees in the family Hippocastaneae. There are about 20 species of trees and shrubs in this family, occurring widely in temperate, angiosperm forests of Europe, Asia, and North America. The horse chestnut and buckeyes have seasonally deciduous, oppositely arranged, palmately compound leaves, which means that the five …
Horsehair or gordian worms are unusual invertebrates in the phylum Nematomorpha. These very long, thin creatures have a superficial resemblance to animated horse hairs, hence their common name. Often, horsehair worms occur in seemingly inextricable tangles of two or more individuals, especially during the breeding season, which is generally in the springtime. The second common name of these animal…
Horses are members of the family Equidae, which includes the wild asses of Africa and Asia and the zebras of African plains and mountains. The origins of horse-like mammals have been traced back some 55 million years to a small dog-sized, plant-eating animal known as Hyracotherium. More recently, during the Pliocene and Miocene periods (which ended some 1.5-2 million years ago) horses and their re…
Often referred to as a living fossil, the horseshoe crab has changed very little in over 400 million years. Related to spiders, this animal is easily identified by the large greenish brown, helmet-like dorsal plate, called either the cephalothorax or prosoma. A separate plate covers its abdomen. A long tail spine, referred to as the caudal spine or telson, extends from its abdomen. Measured from t…
Horsetails are a group of relatively primitive, vascular plants in the genus Equisetum, family Equisetaceae, subdivision Sphenophytina. The sphenophytes have an ancient evolutionary lineage occurring as far back as the Devonian period. These plants were most abundant and diverse in species about 300 million years ago, during the late Devonian and early Carboniferous periods. Fossils from that time…
The word horticulture comes from Latin and refers to the cultivation of gardens. There are three main branches of the science of growing plants: forestry, agronomy, and horticulture. Forestry is concerned with the cultivation of stands of trees for their commercial and ecological uses. Agronomy involves the large-scale cultivation of crops, such as wheat, cotton, fruits, and vegetables. Horticultu…
Hot spots are a common term for plumes of magma welling up through the crust (Earth's outermost layer of rock) far from the edges of plates. To understand what hot spots are and why they are important, some understanding of the theory of plate tectonics is necessary. This widely accepted theory proposed by Alfred Wegener in 1912 states that the crust is composed of huge plates of rock that …
A hovercraft is a vehicle that can be used to journey over water and land. Unlike a boat, which floats on the water, a hovercraft is suspended above the water on a cushion of air. This also allows a hovercraft to move over land and float over small depressions such as a ditch or over waves. A powerful and specially designed fan creates the air cushion that is part of the hovercraft. For this reaso…
Floating in orbit approximately 380 miles (612 km) above the earth, the 12.5-ton Hubble Space Telescope has peered farther into the Universe than any telescope before it. The Hubble, which was launched in 1990, has produced images with unprecedented resolution at visible, near-ultraviolet, and near-infrared wavelengths since its originally faulty optics were corrected in 1993. Although ground-base…
Human capital refers to the knowledge, skills, and capabilities of individuals that generate economic output. Human capital averages about two-thirds of the total value of the capital of most economies, which includes land, machinery, and other physical assets as well as the skills and talents of people. The value of human capital is often apparent after physical destruction, as during World War I…
Human chorionic gonadotropin (HCG) is a glyco-protein hormone produced by the extraembryonic tissue of the early human embryo. After fertilization, the human zygote undergoes cleavage followed by the formation of a blastocyst. The blastocyst is a hollow sphere constructed of an inner cell mass, which becomes the embryo proper, and a trophoblast, which is embryonic tissue that will contribute to th…
An oocyte is an unfertilized egg. Oocytes and spermatozoa are called gametes, and represent different cells that fuse their genes to form a new cell, the fertilized egg. The fertilized cell is called a zygote, and it rapidly divides into several totipotent cells (cells capable of developing into any cell type) called blastomers. Totipotent cells can be considered the opposite of differentiated cel…
Human ecology is the study of the reciprocal inter-actions of humans with their environment. Key aspects of human ecology are demographics, resource use, environmental influences on health and society, and environmental impacts of human activities. All of these subjects are intimately linked, because increasing populations of humans require more resources, the exploitation and use of which cause i…
The history of how the human species, Homo sapiens sapiens, evolved is reconstructed by evidence gathered by paleontologists, anthropologists, archeologists, anatomists, biochemists, behavioral scientists, and many other professions. The evidence comes from the record left by fossils and by extrapolation from modern primates and human hunter-gatherer tribes. Fossils are evidence of past life. In p…
The United States Human Genome Project (HGP) is an initiative formally launched in 1990 by the National Institutes of Health (NIH) and the U.S. Department of Energy (DOE) to better understand all aspects related to human genetic material, or deoxyribonucleic acid (DNA). DNA represents a genetic alphabet and the specific sequences that are part of DNA called genes code for various proteins by virtu…
The idea of human rights posits that human beings, regardless of extrinsic differences in circumstance (nationality, class, religion) or physical condition (race, gender, age), possess a basic and absolute dignity that must be respected by governments and other people. Sometimes these rights claims have been grounded in systems of positive law, sometimes in conceptions of human nature or divine cr…
The dominant Chinese conception of humanism is the Confucian theory of ren. The term ren has been translated in various ways, including as "benevolence," "goodness," "virtue," "humanity," "humanness," and "being authoritative." These different translations indicate the complexity of this Confucian theory. Ren as th…
A common misconception of African humanism is that it is a set of values brought into, instead of emerging from, communities on the African continent. This prejudice is due primarily to the influence of modern European humanism, which is premised upon a secular naturalism as the only model of humanism. The modern European humanist tradition, which treats Christianity as the model of all religion,…
The introduction of the term humanism is commonly attributed to the German pedagogical theorist F. J. Niethammer's 1808 book, which promoted reading of the ancient classics among secondary students as a counterweight to scientific and technological training. The word soon enjoyed wide currency in many European languages, in part because the much earlier Italian term umanista was already use…
While reason tells us that it is obvious to anyone, irrespective of his or her background, that humans are bipedal, featherless creatures, other characteristics we attribute to humans are not always so obvious. Also, while we all appear to assume that humans are creatures who have minds, in contrast to other creatures, it is not quite obvious what is meant by "mind" or what happens a…
Confucius's study of ancient Chinese classics led him to believe that during the reigns of the "sage kings" Yao and Shun, China had been well governed and harmony had prevailed throughout their realms. This was accomplished not through harsh punishments or excessive regulations, but by the moral force of their personalities and their attention to social rituals. They are extol…
Studies of European views of man and of the dignity of man have been central to the history of ideas, and books continue to be published discussing Western or European views of man. Meanwhile, women, lower-class men, and people of color have delved into the scholarship to determine if a thinker's text intended man (homme in French) to be generic as in the Hebrew adam, Greek anthropos, or La…
The otherwise distinctive presentation of the male body with shortened extremities, diminutive hands, enlarged feet, elongated torso, protruding belly, and protracted phallus on the otherwise untitled Baluban sculpture Male Figure (probably nineteenth century), might at first glance appear as a "primitive" distortion of the human image. However, prolonged observation proffers the ind…
Humidity is a measure of the quantity of water vapor in the air. There are different methods for determining this quantity and those methods are reflected in a variety of humidity indexes and readings. The humidity reading in general use by most meteorologists is relative humidity. The relative humidity of air describes the saturation of air with water vapor. Given in terms of percent humidity (e.…
Humor is such an integral part of the human psyche that philosophers and other intellectuals have long been fascinated with its origins in and its effects on the human brain. Several early theorists have provided subject matter for continuing observation and debate. The Greek word chumoi means "juices," and the ancient Greeks used the word, from which we get the English humor (as wel…
Humus is an amorphous, dark brown, organic material that is formed by the incomplete decomposition of biomass. Strictly speaking, humus is composed of organic residues that are sufficiently fragmented and decomposed by microbial and other decomposition processes that the original source of the biotic materials is no longer recognizable. Humus is mostly composed of a very complex mixture of large o…
A hybrid is an offspring between two different species, or the offspring between two parents of the same species that differ in one or more heritable characteristics. An example of the first kind of hybrid is a mule, a cross between a female horse (Equus caballus) and a male donkey (E. asinus). An example of the second kind is the offspring from a cross between true-breeding redand white-flowered …
Hydra are solitary animals of the phylum Coelenterata that measure from just a few millimeters in size to more than 3.5 ft (1 m) in length. They are all thin animals that rarely measure more than 0.4 in (1 cm) in diameter. Most are cylindrical in shape, with a broadened basal disk that serves to attach the animal to some firm substrate. Most species are sessile but some can, if conditions require,…