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BIOLOGY (Gr. Qios, life)
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See also:BIOLOGY (Gr. Qios, See also:life) . The biological sciences are those which See also:deal with the phenomena manifested by living See also:matter; and though it is customary and convenient to See also:group apart such of these phenomena as are termed See also:mental, and such of them as are exhibited by men in society, under the heads of See also:psychology and See also:sociology, yet it must be allowed that no natural boundary separates the subject matter of the latter sciences from that of biology. Psychology is inseparably linked with See also:physiology; and the phases of social life exhibited by animals other than See also:man, which sometimes curiously fore-See also:shadow human policy, fall strictly within the See also:province of the biologist. On the other See also:hand, the biological sciences are sharply marked off from the abiological, or those which treat of the phenomena manifested by not-living matter, in so far as the properties of living matter distinguish it absolutely from all other kinds of things, and as the See also:present See also:state of knowledge furnishes us with no See also:link between the living and the not-living. These distinctive properties of living matter are i. Its chemical composition—containing, as it invariably does, one or more forms of a complex See also:compound of See also:carbon, See also:hydrogen, The See also:pro- See also:oxygen and See also:nitrogen, the so-called protein or See also:albumin parties of (which has never yet been obtained except as a pro- living duct of living bodies), See also:united with a large proportion matter. of See also:water, and forming the See also:chief constituent of a sub-stance which, in its See also:primary unmodified state, is known as See also:protoplasm. 2. Its universal disintegration and See also:waste by oxidation; and its concomitant reintegration by the intussusception of new matter. A See also:process of waste resulting from the decomposition of the molecules of the protoplasm, in virtue of which they break up into more highly oxidated products, which cease to See also:form any See also:part of the living See also:body, is a See also:constant concomitant of life. There is See also:reason to believe that carbonic See also:acid is always one of these waste products, while the others contain the See also:remainder of the carbon, the nitrogen, the hydrogen and the other elements which may enter into the See also:composition of the protoplasm. The new matter taken in to make See also:good this constant loss is either a ready-formed protoplasmic material, supplied by some other living being, or it consists of the elements of protoplasm, united together in simpler combinations, which consequently have to be built up into protoplasm by the agency of the living matter itself. In either See also:case, the addition of molecules to those which already existed takes See also:place, not at the See also:surface of the living See also:mass, but by interposition between the existing molecules of the latter. If the processes of disintegration and of reconstruction which characterize life See also:balance one another, the See also:size of the mass of living matter remains stationary, while, if the reconstructive process is the more rapid, the living body grows. But the increase of size which constitutes growth is the result of a process of molecular intussusception, and therefore differs altogether from the process of growth by See also:accretion, which may be observed in crystals and is effected purely by the See also:external addition of new matter—so that, in the well-known See also:aphorism of See also:Linnaeus, the word " grow " as applied to stones signifies a totally different process from what is called " growth " in See also:plants and animals. 3. Its tendency to undergo cyclical changes. In the See also:ordinary course of nature, all living matter proceeds from pre-existing living matter, a portion of the latter being detached and acquiring an See also:independent existence. The new form takes on the characters of that from which it arose; exhibits the same See also:power of propagating itself by means of an off-shoot; and, sooner or later, like its predecessor, ceases to live, and is resolved into more highly oxidated compounds of its elements. Thus an individual living body is not only constantly changing its substance, but its size and form are undergoing continual modifications, the end of which is the See also:death and decay of that individual; the continuation of the See also:kind being secured by the detachment of portions which tend to run through the same See also:cycle of forms as the See also:parent. No forms of matter which are either not living, or have not been derived from living matter, exhibit these three properties, nor any approach to the remarkable phenomena defined under the second and third heads. But in addition to these distinctive characters, living matter has some other peculiarities, the chief of which are the dependence of all its activities upon moisture and upon See also:heat, within a limited range of temperature, and the fact that it usually possesses a certain structure or organization. As has been said, a large proportion of water enters into the composition of all living matter; a certain amount of drying arrests vital activity, and the See also:complete See also:abstraction Life See also:con- of this water is absolutely incompatible with either by moisture. actual or potential life. But many of the simpler forms of life may undergo See also:desiccation to such an extent as to See also:arrest their vital manifestations and convert them into the semblance of not-living matter, and yet remain potentially alive. That is to say, on being duly moistened they return to life again. And this revivification may take place after months, or even years, of arrested life. The properties of living matter are intimately related to temperature. Not only does exposure to heat sufficient to coagulate protein matter destroy life, by demolishing Life eon, the molecular structure upon which life depends; but ditioned all vital activity, all phenomena of nutritive growth, by See also:tern-See also:movement and See also:reproduction are possible only be- perature. tween certain limits of temperature. These limits may be set down as from a little above the freezing point of water to a little below the boiling point. It is to be noted, however, that these limits apply to the living matter itself, and many of the apparent exceptions are due to cases in which the living matter is enclosed in protective wrappings capable of resisting heat and See also:cold. In many See also:low organisms, such as the spores of bacteria, the thick, non-conducting See also:wall may preserve the living protoplasm from subjection to external temperatures below freezing point, or above boiling point, but all the See also:evidence goes to show that applications of such cold or heat, if prolonged or arranged so as to penetrate to the living matter, destroy life. In warm-blooded animals, such as birds and mammals, protective mechanisms for the regulation of temperature enable them to endure exposure to extreme heat or cold, but in such cases the actually living cells do not appreciably rise or fall in temperature. A variation of a very few degrees in the See also:blood itself produces death. See also:Recent investigations point to the conclusion that the immediate cause of the arrest of vitality, in the first place, and of its destruction, in the second, is the coagulation of certain substances in the protoplasm, and that the latter contains various coagulable matters, which solidify at different temperatures. And' it remains to be seen, how far the death of any form of living matter, at a given temperature, depends on the destruction of its fundamental substance at that heat, and how far death is brought about by the coagulation of merely See also:accessory compounds. It may be safely said of all those living things which are large enough to enable us to See also:trust the evidence of microscopes, that they are heterogeneous optically, and that their different parts, and especially the surface layer, as Life and contrasted with the interior, differ physically Lio and orn.ganizachemically; while, in most living things, See also:mere heterogeneity is exchanged for a definite structure, whereby the body is distinguished into visibly different parts, which possess different See also:powers or functions. Living things which present this visible structure are said to be organized; and so widely does organization obtain among living beings, that organized and living are not . unfrequently used as if they were terms of co-extensive applicability. This, however, is not exactly accurate, if it be thereby implied that all living things have a visible organization, as there are numerous forms of living matter of which it cannot properly be said that they possess either a definite structure or permanently specialized See also:organs: though, doubtless, the simplest particle of living matter must possess a highly complex molecular structure, which is far beyond the reach of See also:vision. The broad distinctions which, as a matter of fact, exist between every known form of living substance and every other component of the material See also:world, justify the separation of the biological sciences from all others. But it must not be supposed that the See also:differences between living and not-living matter are such as to justify the See also:assumption that the forces at See also:work in the one are different from those which are to be met with in the other. Considered apart from the phenomena of consciousness, the phenomena of life are all dependent upon the working of the same See also:physical and chemical forces as those which are active in the See also:rest of the world. It may be convenient to use the terms " vitality " and " vital force " to denote the causes of certain See also:great See also:groups of natural operations, as we employ the names of " See also:electricity " and " See also:electrical force " to denote others; but it ceases to be proper to do so, if such a name implies the absurd assumption that " electricity " and " vitality " are entities playing the part of efficient causes of electrical or vital phenomena. A mass of living protoplasm is simply a molecular See also:machine of great complexity, the See also:total results of the working of which, or its vital phenomena, depend—on the one hand, upon its construction, and, on the other, upon the See also:energy supplied to it; and to speak of " vitality " as anything but the name of a See also:series of operations is as if one should talk of the " horologity " of a See also:clock. Living matter, or protoplasm and the products of its See also:meta-morphosis, may be regarded under four aspects: e18sslnca- r. It has a certain external and See also:internal form, the See also:don of the latter being more usually called structure; phenomena 2. It occupies a certain position in space and in ante. See also:time; 3. It is the subject of the operation of certain forces in virtue of which it undergoes internal changes, modifies external See also:objects, and is modified by them; and 4. Its form, place and powers are the effects of certain causes. In See also:correspondence with these four aspects of its subject, biology is logically divisible into four chief subdivisions—I. See also:MORPHOLOGY; II. See also:DISTRIBUTION; III. PHYSIOLOGY; IV. See also:AETIOLOGY. Various accidental circumstances, however, have brought it about that the actual distribution of scientific work does not correspond with the logical subdivisions of biology. The difference in technical methods and the See also:historical See also:evolution of teaching posts (for in all civilized countries the progress of biological knowledge has been very closely associated with the existence of institutions for the See also:diffusion of knowledge and for professional See also:education) have been the chief contributory causes to this See also:practical confusion. Details of the morphology of plants will be found in the articles See also:relating to the chief groups of plants, those of animals in the corresponding articles on groups of animals, while the See also:classification of animals adopted in this work will be found in the See also:article See also:ZOOLOGY. Distribution is treated of under ZOOLOGICAL DISTRIBUTION, See also:PLANKTON, PALAEONTOLOGY and PLANTS: Distribution. PHYSIOLOGY and its allied articles deal with the subject generally and in relation to man, while the See also:special physiology of plants is dealt with in a See also:section of the article PLANTS. Aetiology is treated of under the heading EVOLUTION. But practical See also:necessity has given rise to the existence of many other divisions; see See also:CYTOLOGY, for the structure of cells; See also:EMBRYOLOGY, for the development of individual organisms; See also:HEREDITY and REPRODUCTION, for the relations between parents and offspring. (T. H. H.; P. C. 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