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any living biological entity, such as an animal, plant, fungus, or bacterium



any living being.

Unicellular and multicellular organisms are distinguished from nonliving matter by several basic vital properties: cellular organization (seeCELL); metabolism, by which proteins and nucleic acids regenerate the organism and maintain a constant internal environment (see, HOMEOSTASIS); movement in all its specific forms—muscular, cytoplasmic, ciliary, and flagellar (seeMOVEMENT); excitability; growth and development; reproduction; variability and heredity (seeVARIATION); and adaptability (seeADAPTATION). Prokaryotes are organisms that lack chromosomes and a typical cell nucleus; examples are bacteria, blue-green algae, Rickettsia, and members of the order Mycoplasmatales. Prokaryotes are simpler in structure and smaller in size than the smallest cellular organisms; for example, the diameter of an animal cell is more than 3 microns (μ), while that of a bacterial cell is usually less than 3 μ. One of the smallest bacteria consists of a total of 5 × 107 atoms once its water is removed.

In its interaction with the environment, an organism is coordinated on the cytoplasmic, cellular, tissular, organic, and organismic levels so as to act as a unified system. The formation of a complete organism in phylogeny consists in the differentiation and integration of cells, tissues, organs, and functions (seeDIFFERENTIATION, INTEGRATION). In unicellular organisms vital functions are effected by special organelles. Over the course of evolution, the development of multicellularity permitted the progressive morphophysiological complexification and differentiation of organisms. This complexification and differentiation is made possible by the structural and functional coordination of cells, tissues, and organs, which is achieved through nervous and humoral means.

The interdependence of organs over the course of animal evolution was comprehensively studied by A. N. Severtsov and students of his school. The phylogenetic aspects of the evolution and differentiation of tissues that arose from cells with common structures, functions, and developmental courses were studied by A. A. Zavarzin, N. G. Khlopin, and A. V. Rumiantsev and their students. The differentiation and integration of organs and functions were also studied by many other Russian and Soviet scientists, including I. I. Mechnikov, I. P. Pavlov, I. I. Shmal’gauzen, and V. A. Dogel’, as well as by foreign scientists, including E. Haeckel, A. Dohrn, and G. de Beer.

Modern biology, especially genetics, has elucidated the genetic connection between generations of organisms and the connections between phylogeny and ontogeny on every level of organization in an individual organism. (SeeHISTOGENESIS, , , MORPHOGENESIS, PHYLOGENY.)


Shmal’gauzen, I. I. Organizm kak tseloe v individual’nom i istoricheskom razvitii. Moscow-Leningrad, 1938.
Khlopin, N. G. Obshchebiologicheskie i eksperimental’nye osnovy gistologii. Moscow, 1946.
Severtsov, A. N. Morfologicheskie zakonomemosti evoliutsii: Sobr. soch., vol. 5. Moscow-Leningrad, 1949.
Zavarzin, A. A. Izbr. trudy, vols. 1–4. Moscow-Leningrad, 1950–53.
Shmal’gauzen, I. I. “Integratsiia biologicheskikh sistem i ikh samoreguliatsiia.” Biul. Moskovskogo obshchestva ispytatelei prirody; Otdel biologicheskii, 1961, vol. 66, fasc. 2.
Shmal’gauzen, I. I. Reguliatsiia formoobrazovaniia v individual’nom razvitii. Moscow, 1964.
Amlinskii, I. E. “Nekotorye problemy stanovleniia mnogokletochnosti.” In the collection Struktura i formy materii. Moscow, 1967.
Ryzhkov, V. L. “Mesto individa sredi biologicheskikh sistem.” In the collection Razvitie kontseptsii strukturnykh urovnei v biologii. Moscow, 1972.
De Beer, G. R. Embryos and Ancestors. Oxford, 1958.
Regulation and Control in Living Systems. Edited by H. Kalmus. New York, 1967.



An individual constituted to carry out all life functions.
References in classic literature ?
While, therefore, an epic like the "Odyssey" is an organism and dramatic in structure, a work such as the "Theogony" is a merely artificial collocation of facts, and, at best, a pageant.
He transforms the "Struggle for Existence"--the passive and involuntary condition--into the "Struggle for Power," which is active and creative, and much more in harmony with Darwin's own view, given above, concerning the importance of the organism itself.
The response of an organism to a given stimulus is very often dependent upon the past history of the organism, and not merely upon the stimulus and the HITHERTO DISCOVERABLE present state of the organism.
If we confine ourselves to facts which have been actually observed, we must say that past occurrences, in addition to the present stimulus and the present ascertainable condition of the organism, enter into the causation of the response.
Further, speaking broadly, the change in response is usually of a kind that is biologically advantageous to the organism.
1921; "Die mnemischen Empfindungen," Leipzig, l909), we will give the name of "mnemic phenomena" to those responses of an organism which, so far as hitherto observed facts are concerned, can only be brought under causal laws by including past occurrences in the history of the organism as part of the causes of the present response.
When an organism, either animal or plant, is subjected to a stimulus, producing in it some state of excitement, the removal of the stimulus allows it to return to a condition of equilibrium.
The first, or "Law of Engraphy," is as follows: "All simultaneous excitements in an organism form a connected simultaneous excitement-complex, which as such works engraphically, i.
Concerning the nature of an engram, Semon confesses that at present it is impossible to say more than that it must consist in some material alteration in the body of the organism ("Die mnemischen Empfindungen," p.
There is no such thing as dead, inert matter: it is all alive; all instinct with force, actual and potential; all sensitive to the same forces in its environment and susceptible to the contagion of higher and subtler ones residing in such superior organisms as it may be brought into relation with, as those of man when he is fashioning it into an instrument of his will.
I may add, that as some organisms will breed most freely under the most unnatural conditions (for instance, the rabbit and ferret kept in hutches), showing that their reproductive system has not been thus affected; so will some animals and plants withstand domestication or cultivation, and vary very slightly--perhaps hardly more than in a state of nature.
Another fact of which he became convinced, after reading many scientific books, was that the men who shared his views had no other construction to put on them, and that they gave no explanation of the questions which he felt he could not live without answering, but simply ignored their existence and attempted to explain other questions of no possible interest to him, such as the evolution of organisms, the materialistic theory of consciousness, and so forth.

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