Human population density

For humans, population density is the number of people per unit of area (which may include or exclude cultivated or potentially productive area). Commonly this may be calculated for a county, city, country, another territory, or the entire world.

The world population is 6.6 billion, and Earth's area is 510 million square kilometers (200 million square miles). Therefore the world-wide human population density is 6.6 billion / 510 million = 13 per km² (33 per sq mi), or 43 per km² (112 per sq mi) if only the Earth's land area of 150 million km² (58 million sq mi) is taken into account. This density rises when the population grows. It also includes all continental and island land area, including Antarctica. Considering that over half of the earth's land mass consists of areas inhospitable to human inhabitation, such as deserts and high mountains, and that population tends to cluster around seaports and fresh water sources, this number by itself does not give the most accurate measurement of human population density.

Several of the most densely-populated territories in the world are city-states, microstates, micronations, or dependencies. These territories share a relatively small area and a high urbanization level, with an economically specialized city population drawing also on rural resources outside the area, illustrating the difference between high population density and overpopulation.

Cities with high population densities are, by some, considered to be overpopulated, though the extent to which this is the case depends on factors like quality of housing and infrastructure or access to resources. Most of the most densely-populated cities are in southern and eastern Asia, though Cairo and Lagos in Africa also fall into this category.

City population is however, heavily dependent on the definition of "urban area" used: densities are typically higher for the central municipality itself, than when more recently-developed and administratively unincorporated suburban communities are included, as in the concepts of agglomeration or metropolitan area, the latter including sometimes neighbouring cities.

Types of Web caches

Web caches can be deployed in a variety of ways. User agent caches, such as those in web browsers, are private caches, operating on behalf of a single user. Intermediaries can also implement shared caches that serve more than one person.

Proxy caches, also known as forward proxy caches, are usually deployed by internet service providers, schools and corporations to save bandwidth. Interception proxy caches (sometimes called "transparent caches") are a variant that doesn't require clients to be explicitly configured to use them.

Gateway caches, sometimes known as reverse proxy caches, surrogate caches, or web accelerators, operate on behalf of the origin server, and to clients are indistinguishable from it. A number of gateway caches can work together to implement a Content Delivery Network.

Intermediaries that cache often perform other duties, such as user authentication and content filtering. Multiple caches can also be coordinated using peering protocols like Internet Cache Protocol and HTCP.

Cytokinesis

Cytokinesis is the process whereby the cytoplasm of a single cell is divided to spawn two daughter cells. It usually initiates during the late stages of mitosis, and sometimes meiosis, splitting a binucleate cell in two, to ensure that chromosome number is maintained from one generation to the next. In animal cells, one notable exception to the normal process of cytokinesis is oogenesis (the creation of an ovum in the ovarian follicle of the ovary), where the ovum takes almost all the cytoplasm and organelles, leaving very little for the resulting polar bodies, which then die. In plant cells, a dividing structure known as the cell plate forms across the centre of the cytoplasm and a new cell wall forms between the two daughter cells.

During normal proliferative divisions, animal cell cytokinesis begins shortly after the onset of sister chromatid separation in the anaphase of mitosis. A contractile ring, made of non-muscle myosin II and actin filaments, assembles equatorially (in the middle of the cell) at the cell cortex (adjacent to the cell membrane). Myosin II uses the free energy released when ATP is hydrolysed to move along these actin filaments, constricting the cell membrane to form a cleavage furrow. Continued hydrolysis causes this cleavage furrow to ingress (move inwards), a striking process that is clearly visible down a light microscope. Ingression continues until a so-called midbody structure (composed of electron-dense, proteinaceous material) is formed and the process of abscission then physically cleaves this midbody into two. Abscission depends on septin filaments beneath the cleavage furrow, which provide a structural basis to ensure completion of cytokinesis. After cytokinesis, non-kinetochore microtubules reorganize and disappear into a new cytoskeleton as the cell cycle returns to interphase (see also cell cycle).

Organic Society

The 14th century Islamic scholar Ibn Khaldun concluded that societies are living organisms that experience cyclic birth, growth, maturity, decline, and ultimately death due to universal causes several centuries before the Western civilisation developed the science of sociology. Nonetheless, theories of social and cultural evolution were common in modern European thought. Prior to the 18th century, Europeans predominantly believed that societies on Earth were in a state of decline. European society held up the world of antiquity as a standard to aspire to, and Ancient Greece and Ancient Rome produced levels of technical accomplishment which Europeans of the Middle Ages sought to emulate. At the same time, Christianity taught that people lived in a debased world fundamentally inferior to the Garden of Eden and Heaven. During The Age of Enlightenment, however, European self-confidence grew and the notion of progress became increasingly popular. It was during this period that what would later become known as "sociological and cultural evolution" would have its roots.

Historical significance of writing systems

Historians draw a distinction between prehistory and history, with history defined by the advent of writing. The cave paintings and petroglyphs of prehistoric peoples can be considered precursors of writing, but are not considered writing because they did not represent language directly.

Writing systems always develop and change based on the needs of the people who use them. Sometimes the shape, orientation and meaning of individual signs also changes over time. By tracing the development of a script it is possible to learn about the needs of the people who used the script as well as how it changed over time.

Breeding back

Breeding back can be described as either a natural or a human attempt to assemble or re-assemble the genes of an extinct subspecies or of a domesticated breed, which may still be present in the larger gene pool of the overall species or other interbreedable species.

In Domestic animals breeding back has occurred with the Utonagan and the Northern Inuit dogs in an attempt to recreate the 'wolf-look' without actually cross breeding with wolves. Other selectively bred examples of breeding back include that of the aurochs, the extinct forerunner of domestic cattle. The product of these attempts is called the Heck cattle. Another prominent breeding back effort is the Quagga Project to bring back the extinct subspecies of the Plains Zebra called Quagga. The Heck horse, a phenotypic copy of the tarpan has also been produced, although it lacks the upright manes.

Breeding back is controversial, especially claims that an extinct animal has been recreated. Phenotypical reconstruction (similar appearance) does not assure behavioral similarity. For some of the species that are being bred back, questions remain about the ecological niche, hardiness, and disease resistance of the original species. For instance, the aurochs died out almost 400 years ago and the records kept cannot definitively answer some of these questions.

Side-sword

A side-sword was a type of war sword used by infantry during the Renaissance of Europe. This sword was a direct descendant of the arming sword. Quite popular between the 16th and 17th centuries, they were ideal for handling the mix of armored and unarmored opponents of that time. Early versions look very much like an arming sword with an ornate hilt and ricasso. A new technique of placing ones finger on the ricasso to improve the grip (a practice that would continue in the rapier) led to the production of hilts with a guard for the finger. The term is a recently-coined calque of the Italian spada da lato and will not be found in any actual sources from the 16th or 17th centuries.

This sword design eventually led to the development of the civilian rapier, but it was not replaced by it, and the Side-sword continued to be used during the rapier's lifetime. As it could be used for both cutting and thrusting, the term cut and thrust sword is sometimes used interchangeably with side-sword. Also of note is that as rapiers became more popular, attempts were made to hybridize the blade, sacrificing the effectiveness found in each unique weapon design. These are still considered side-swords and are sometimes labeled sword rapier or cutting rapier by modern collectors. See European dueling sword for further history.

Also of note, side-swords used in conjunction with bucklers became so popular that it caused the term swashbuckler to be coined. This word stems from the new fighting style of the side-sword and buckler which was filled with much "swashing and making a noise on the buckler".