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Individuality

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INDIVIDUALITY. The individuality of organisms is a difficult problem of scientific philosophy. The common view is that each obviously separate organic entity is an individual—man, cow, beetle, oak-tree, cabbage. But difficulties arise when we take lower organisms into account, and look deeper into life history. Many animals such as polyps form colonies whose mem bers are attached to a common living stem. All gradations occur between single polyps, those forming temporary colonies by bud ding, and those with permanent colonies. In permanent colonies, all members may be alike, or there may be division of labour among different types. In extreme cases the colony behaves as a single unit (Portuguese man-o'-war and other Siphonophora), the members being entirely subordinate. (See COELENTERATA.) The units of the colony are here themselves compound, con sisting of many cells : but a parallel series occurs with single celled units. The Protozoa (q.v.) are typically single, free-living cells. A number are colonial, and a few colonial forms (e.g., Zoo tizamniu n) exhibit division of labour, one or two (e.g., Volvox) being co-ordinated to move as wholes. The same type of problem, but in exaggerated form, arises in regard to symbiotic organisms. (See SYMBIOSIS.) What we call a termite is an obligatory partner ship between an insect, the termite proper, and protozoan inhabi tants of its intestine which digest wood for the partnership. A lichen (q.v.) is a compound of a fungus and a green alga.

Equal difficulties arise with regard to reproduction and regen eration. All organisms are part of a continuous stream of life, the offspring taking origin from a piece of living substance de tached from its parent or parents. The individual can therefore never be sharply delimited from the stream of life, any more than an eddy from the river. In higher animals each fertilized egg normally gives rise to a single organism; but exceptionally it divides, wholly or partially, during early stages. When it divides completely, so-called "identical" twins result ; common-sense calls these two individuals. But all gradations are found between this condition, slight union as in the Siamese twins, intimate union, and various states of partial doubling. Similar partial and total duplications can be artificially produced in lower organisms. (See

organisms, single, colonies and life