Biology | Definition, History, Concepts, Branches, & Facts

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border-right-sm border-left-sm open"> <div class="drawer d-flex flex-column open"> <div class="left-rail-section-content"> <div class="topic-left-rail-header text-truncate bg-gray-50 position-relative text-right d-flex align-items-center"> <div class="tlr-title px-20 py-15 text-left"> <em class="material-icons text-gray-400 d-lg-none" data-icon="toc"></em> <a class="font-serif font-weight-bold text-black link-blue" href="https://www.britannica.com/science/biology">biology</a> </div> <button aria-label="Close" class="js-sections-close-button btn-link btn-sm btn d-lg-none position-absolute top-0 p-10 right-0" > <em class="material-icons font-26" data-icon="close"></em> </button> </div> <div class="section-content pl-10 pr-20 pl-sm-50 pr-sm-60 pl-lg-5 pr-lg-10 pt-10 pt-lg-0 bg-gray-50 clear-catfish-ad"> <div class="toc mb-20"> <div class="font-serif font-14 font-weight-bold mx-15 mb-15 mt-20"> Table of Contents </div> <ul class="list-unstyled my-0" data-level="h1"><li data-target="#ref1"><div class="pl-25"><a class="link-gray-900 w-100" href="/science/biology">Introduction & Top Questions</a></div><div class="ml-40 toc-drawer sub-toc-drawer"></div></li><li data-target="#ref48813"><div class="d-flex align-items-center"><button class="h1-link-drawer-button btn btn-xs btn-circle d-flex rounded" type="button" aria-label="Toggle Heading"><em class="material-icons font-18" data-icon="keyboard_arrow_right"></em></button><a class="w-100 link-gray-900" href="/science/biology#ref48813">Basic concepts of biology</a></div><div class="ml-40 toc-drawer sub-toc-drawer"><ul class="list-unstyled" data-level="h2"><li data-target="#ref48814"><a class="w-100 link-gray-900" href="/science/biology#ref48814">Biological principles</a><ul class="list-unstyled" data-level="h3"><li data-target="#ref48815"><a class="w-100 link-gray-900" href="/science/biology#ref48815">Homeostasis</a></li></ul><ul class="list-unstyled" data-level="h3"><li data-target="#ref48816"><a class="w-100 link-gray-900" href="/science/biology#ref48816">Unity</a></li></ul><ul class="list-unstyled" data-level="h3"><li data-target="#ref48817"><a class="w-100 link-gray-900" href="/science/biology/Evolution">Evolution</a></li></ul><ul class="list-unstyled" data-level="h3"><li data-target="#ref48818"><a class="w-100 link-gray-900" href="/science/biology/Evolution#ref48818">Diversity</a></li></ul><ul class="list-unstyled" data-level="h3"><li data-target="#ref48819"><a class="w-100 link-gray-900" href="/science/biology/Evolution#ref48819">Behaviour and interrelationships</a></li></ul><ul class="list-unstyled" data-level="h3"><li data-target="#ref48820"><a class="w-100 link-gray-900" href="/science/biology/Evolution#ref48820">Continuity</a></li></ul></li></ul><ul class="list-unstyled" data-level="h2"><li data-target="#ref48821"><a class="w-100 link-gray-900" href="/science/biology/Evolution#ref48821">The study of structure</a><ul class="list-unstyled" data-level="h3"><li data-target="#ref48822"><a class="w-100 link-gray-900" href="/science/biology/Evolution#ref48822">Cells and their constituents</a></li></ul><ul class="list-unstyled" data-level="h3"><li data-target="#ref48823"><a class="w-100 link-gray-900" href="/science/biology/Evolution#ref48823">Tissues and organs</a></li></ul></li></ul></div></li><li data-target="#ref48825"><div class="d-flex align-items-center"><button class="h1-link-drawer-button btn btn-xs btn-circle d-flex rounded" type="button" aria-label="Toggle Heading"><em class="material-icons font-18" data-icon="keyboard_arrow_right"></em></button><a class="w-100 link-gray-900" href="/science/biology/The-history-of-biology">The history of biology</a></div><div class="ml-40 toc-drawer sub-toc-drawer"><ul class="list-unstyled" data-level="h2"><li data-target="#ref48826"><a class="w-100 link-gray-900" href="/science/biology/The-history-of-biology#ref48826">The early heritage</a><ul class="list-unstyled" data-level="h3"><li data-target="#ref48827" class="has-children"><a class="w-100 link-gray-900" href="/science/biology/The-history-of-biology#ref48827">Earliest biological records</a><ul class="list-unstyled" data-level="h4"><li data-target="#ref48828"><a class="w-100 link-gray-900" href="/science/biology/The-history-of-biology#ref48828">Biological practices among Assyrians and Babylonians</a></li></ul><ul class="list-unstyled" data-level="h4"><li data-target="#ref48829"><a class="w-100 link-gray-900" href="/science/biology/The-history-of-biology#ref48829">Biological knowledge of Egyptians, Chinese, and Indians</a></li></ul></li></ul><ul class="list-unstyled" data-level="h3"><li data-target="#ref48830" class="has-children"><a class="w-100 link-gray-900" href="/science/biology/The-history-of-biology#ref48830">The Greco-Roman world</a><ul class="list-unstyled" data-level="h4"><li data-target="#ref48831"><a class="w-100 link-gray-900" href="/science/biology/The-history-of-biology#ref48831">Theories about humankind and the origin of life</a></li></ul><ul class="list-unstyled" data-level="h4"><li data-target="#ref48832"><a class="w-100 link-gray-900" href="/science/biology/Aristotelian-concepts">Aristotelian concepts</a></li></ul><ul class="list-unstyled" data-level="h4"><li data-target="#ref48833"><a class="w-100 link-gray-900" href="/science/biology/Aristotelian-concepts#ref48833">Botanical investigations</a></li></ul><ul class="list-unstyled" data-level="h4"><li data-target="#ref48834"><a class="w-100 link-gray-900" href="/science/biology/Aristotelian-concepts#ref48834">Post-Grecian biological studies</a></li></ul></li></ul><ul class="list-unstyled" data-level="h3"><li data-target="#ref48835" class="has-children"><a class="w-100 link-gray-900" href="/science/biology/The-Arab-world-and-the-European-Middle-Ages">The Arab world and the European Middle Ages</a><ul class="list-unstyled" data-level="h4"><li data-target="#ref48836"><a class="w-100 link-gray-900" href="/science/biology/The-Arab-world-and-the-European-Middle-Ages#ref48836">Arab domination of biology</a></li></ul><ul class="list-unstyled" data-level="h4"><li data-target="#ref48837"><a class="w-100 link-gray-900" href="/science/biology/The-Arab-world-and-the-European-Middle-Ages#ref48837">Development of botany and zoology</a></li></ul><ul class="list-unstyled" data-level="h4"><li data-target="#ref48838"><a class="w-100 link-gray-900" href="/science/biology/The-Arab-world-and-the-European-Middle-Ages#ref48838">Revitalization of anatomy</a></li></ul></li></ul><ul class="list-unstyled" data-level="h3"><li data-target="#ref48839" class="has-children"><a class="w-100 link-gray-900" href="/science/biology/The-Arab-world-and-the-European-Middle-Ages#ref48839">The Renaissance</a><ul class="list-unstyled" data-level="h4"><li data-target="#ref48840"><a class="w-100 link-gray-900" href="/science/biology/The-Arab-world-and-the-European-Middle-Ages#ref48840">Resurgence of biology</a></li></ul><ul class="list-unstyled" data-level="h4"><li data-target="#ref48841"><a class="w-100 link-gray-900" href="/science/biology/The-Arab-world-and-the-European-Middle-Ages#ref48841">Advances in botany</a></li></ul><ul class="list-unstyled" data-level="h4"><li data-target="#ref48842"><a class="w-100 link-gray-900" href="/science/biology/The-Arab-world-and-the-European-Middle-Ages#ref48842">Advances in anatomy</a></li></ul></li></ul></li></ul><ul class="list-unstyled" data-level="h2"><li data-target="#ref48843"><a class="w-100 link-gray-900" href="/science/biology/Advances-to-the-20th-century">Advances to the 20th century</a><ul class="list-unstyled" data-level="h3"><li data-target="#ref48844"><a class="w-100 link-gray-900" href="/science/biology/Advances-to-the-20th-century#ref48844">The discovery of the circulation of blood</a></li></ul><ul class="list-unstyled" data-level="h3"><li data-target="#ref48845"><a class="w-100 link-gray-900" href="/science/biology/Advances-to-the-20th-century#ref48845">The establishment of scientific societies</a></li></ul><ul class="list-unstyled" data-level="h3"><li data-target="#ref48846" class="has-children"><a class="w-100 link-gray-900" href="/science/biology/Advances-to-the-20th-century#ref48846">The development of the microscope</a><ul class="list-unstyled" data-level="h4"><li data-target="#ref48847"><a class="w-100 link-gray-900" href="/science/biology/Advances-to-the-20th-century#ref48847">Malpighi’s animal and plant studies</a></li></ul><ul class="list-unstyled" data-level="h4"><li data-target="#ref48848"><a class="w-100 link-gray-900" href="/science/biology/Advances-to-the-20th-century#ref48848">The discovery of “animalcules”</a></li></ul><ul class="list-unstyled" data-level="h4"><li data-target="#ref48849"><a class="w-100 link-gray-900" href="/science/biology/Advances-to-the-20th-century#ref48849">Swammerdam’s innovative techniques</a></li></ul><ul class="list-unstyled" data-level="h4"><li data-target="#ref48850"><a class="w-100 link-gray-900" href="/science/biology/Advances-to-the-20th-century#ref48850">Grew’s anatomical studies of plants</a></li></ul><ul class="list-unstyled" data-level="h4"><li data-target="#ref48851"><a class="w-100 link-gray-900" href="/science/biology/The-discovery-of-cells">The discovery of cells</a></li></ul></li></ul><ul class="list-unstyled" data-level="h3"><li data-target="#ref48852" class="has-children"><a class="w-100 link-gray-900" href="/science/biology/The-discovery-of-cells#ref48852">The development of taxonomic principles</a><ul class="list-unstyled" data-level="h4"><li data-target="#ref48853"><a class="w-100 link-gray-900" href="/science/biology/The-discovery-of-cells#ref48853">The use of structure for classifying organisms</a></li></ul><ul class="list-unstyled" data-level="h4"><li data-target="#ref48854"><a class="w-100 link-gray-900" href="/science/biology/The-discovery-of-cells#ref48854">Reorganization of groups of organisms</a></li></ul></li></ul><ul class="list-unstyled" data-level="h3"><li data-target="#ref48855"><a class="w-100 link-gray-900" href="/science/biology/The-discovery-of-cells#ref48855">The development of comparative biological studies</a></li></ul><ul class="list-unstyled" data-level="h3"><li data-target="#ref48856" class="has-children"><a class="w-100 link-gray-900" href="/science/biology/The-study-of-the-origin-of-life">The study of the origin of life</a><ul class="list-unstyled" data-level="h4"><li data-target="#ref48857"><a class="w-100 link-gray-900" href="/science/biology/The-study-of-the-origin-of-life#ref48857">Spontaneous generation</a></li></ul><ul class="list-unstyled" data-level="h4"><li data-target="#ref48858"><a class="w-100 link-gray-900" href="/science/biology/The-study-of-the-origin-of-life#ref48858">The death of spontaneous generation</a></li></ul><ul class="list-unstyled" data-level="h4"><li data-target="#ref48859"><a class="w-100 link-gray-900" href="/science/biology/The-study-of-the-origin-of-life#ref48859">The origin of primordial life</a></li></ul></li></ul><ul class="list-unstyled" data-level="h3"><li data-target="#ref48860"><a class="w-100 link-gray-900" href="/science/biology/The-study-of-the-origin-of-life#ref48860">Biological expeditions</a></li></ul><ul class="list-unstyled" data-level="h3"><li data-target="#ref48861"><a class="w-100 link-gray-900" href="/science/biology/The-study-of-the-origin-of-life#ref48861">The development of cell theory</a></li></ul><ul class="list-unstyled" data-level="h3"><li data-target="#ref48862"><a class="w-100 link-gray-900" href="/science/biology/The-theory-of-evolution">The theory of evolution</a></li></ul><ul class="list-unstyled" data-level="h3"><li data-target="#ref48863" class="has-children"><a class="w-100 link-gray-900" href="/science/biology/The-theory-of-evolution#ref48863">The study of the reproduction and development of organisms</a><ul class="list-unstyled" data-level="h4"><li data-target="#ref48864"><a class="w-100 link-gray-900" href="/science/biology/The-theory-of-evolution#ref48864">Preformation versus epigenesis</a></li></ul><ul class="list-unstyled" data-level="h4"><li data-target="#ref48865"><a class="w-100 link-gray-900" href="/science/biology/The-theory-of-evolution#ref48865">The fertilization process</a></li></ul></li></ul><ul class="list-unstyled" data-level="h3"><li data-target="#ref48866" class="has-children"><a class="w-100 link-gray-900" href="/science/biology/The-theory-of-evolution#ref48866">The study of heredity</a><ul class="list-unstyled" data-level="h4"><li data-target="#ref48867"><a class="w-100 link-gray-900" href="/science/biology/The-theory-of-evolution#ref48867">Pre-Mendelian theories of heredity</a></li></ul><ul class="list-unstyled" data-level="h4"><li data-target="#ref48868"><a class="w-100 link-gray-900" href="/science/biology/The-theory-of-evolution#ref48868">Mendelian laws of heredity</a></li></ul><ul class="list-unstyled" data-level="h4"><li data-target="#ref48869"><a class="w-100 link-gray-900" href="/science/biology/The-theory-of-evolution#ref48869">Elucidation of the hereditary mechanism</a></li></ul></li></ul></li></ul><ul class="list-unstyled" data-level="h2"><li data-target="#ref48870"><a class="w-100 link-gray-900" href="/science/biology/Biology-in-the-20th-and-21st-centuries">Biology in the 20th and 21st centuries</a><ul class="list-unstyled" data-level="h3"><li data-target="#ref48871"><a class="w-100 link-gray-900" href="/science/biology/Biology-in-the-20th-and-21st-centuries#ref48871">Important conceptual and technological developments</a></li></ul><ul class="list-unstyled" data-level="h3"><li data-target="#ref48872"><a class="w-100 link-gray-900" href="/science/biology/Biology-in-the-20th-and-21st-centuries#ref48872">Intradisciplinary and interdisciplinary work</a></li></ul><ul class="list-unstyled" data-level="h3"><li data-target="#ref48874"><a class="w-100 link-gray-900" href="/science/biology/Biology-in-the-20th-and-21st-centuries#ref48874">Changing social and scientific values</a></li></ul><ul class="list-unstyled" data-level="h3"><li data-target="#ref48875"><a class="w-100 link-gray-900" href="/science/biology/Biology-in-the-20th-and-21st-centuries#ref48875">Coping with problems of the future</a></li></ul></li></ul></div></li></ul> <a class="toc-extra-link link-gray-900" href="https://www.britannica.com/science/biology/additional-info">References & Edit History</a> <a class="toc-extra-link link-gray-900" href="/facts/biology">Related Topics</a> </div> <div class="tlr-media-slider pb-10 mb-30"> <a class="section-header link-gray-900 font-serif font-14 font-weight-bold mb-10 mx-10" href="https://www.britannica.com/science/biology/images-videos">Images & Videos</a> <div class="slider js-slider position-relative d-inline-flex align-items-center mw-100 "> <div class="slider-container js-slider-container overflow-hidden d-flex 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Fields such as <a href=\"https:\/\/www.britannica.com\/science\/genetics\">genetics<\/a> and <a href=\"https:\/\/www.britannica.com\/science\/evolution-scientific-theory\">evolution<\/a> give insight into the past and can help shape the future, and research in <a href=\"https:\/\/www.britannica.com\/science\/ecology\">ecology<\/a> and <a href=\"https:\/\/www.britannica.com\/science\/conservation-ecology\">conservation<\/a> inform how we can protect this planet\u2019s precious <a href=\"https:\/\/www.britannica.com\/science\/biodiversity\">biodiversity<\/a>." } } , { "@type" : "Question", "name" : "<div>Where do biology graduates work?<\/div><em><\/em>", "acceptedAnswer" : { "@type" : "Answer", "text" : "Biology graduates can hold a wide range of jobs, some of which may require additional education. 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Advances in <a href="https://www.britannica.com/science/medicine" class="md-crosslink " data-show-preview="true">medicine</a>, <a href="https://www.britannica.com/topic/agriculture" class="md-crosslink " data-show-preview="true">agriculture</a>, <a href="https://www.britannica.com/technology/biotechnology" class="md-crosslink " data-show-preview="true">biotechnology</a>, and many other areas of biology have brought improvements in the quality of life. Fields such as <a href="https://www.britannica.com/science/genetics" class="md-crosslink " data-show-preview="true">genetics</a> and <a href="https://www.britannica.com/science/evolution-scientific-theory" class="md-crosslink " data-show-preview="true">evolution</a> give insight into the past and can help shape the future, and research in <a href="https://www.britannica.com/science/ecology" class="md-crosslink " data-show-preview="true">ecology</a> and <a href="https://www.britannica.com/science/conservation-ecology" class="md-crosslink " data-show-preview="true">conservation</a> inform how we can protect this planet’s precious <a href="https://www.britannica.com/science/biodiversity" class="md-crosslink " data-show-preview="true">biodiversity</a>.</p></div></div></div><div class="top-question bg-gray-50 rounded" data-value="3"><h3 class="accordion--question font-14 font-weight-normal cursor-pointer rounded"><div class="pe-none d-flex justify-content-between align-items-center"><div>Where do biology graduates work?</div><em class="material-icons" data-icon="expand_more"></em></div></h3><div class="accordion--answer hidden p-15 pt-5 font-16 mb-5"><div class="accordion--answer-copy"><p>Biology graduates can hold a wide range of jobs, some of which may require additional education. A person with a degree in biology could work in <a href="https://www.britannica.com/topic/agriculture" class="md-crosslink " data-show-preview="true">agriculture</a>, health care, <a href="https://www.britannica.com/technology/biotechnology" class="md-crosslink " data-show-preview="true">biotechnology</a>, education, environmental conservation, research, <a href="https://www.britannica.com/science/forensic-science" class="md-crosslink " data-show-preview="true">forensic science</a>, policy, science communication, and many other areas. </p></div></div></div></div></div><span class="marker before-article"></span><section data-level="1" id="ref1"><span class="marker PREMOD1 mod-inline"></span><p class="topic-paragraph"><strong><span id="ref498648"></span>biology</strong>, study of living things and their vital processes. The <a href="https://www.britannica.com/science/field-physics" class="md-crosslink autoxref " data-show-preview="true">field</a> deals with all the physicochemical aspects of <a href="https://www.britannica.com/science/life" class="md-crosslink " data-show-preview="true">life</a>. The modern tendency toward cross-disciplinary research and the unification of scientific knowledge and investigation from different fields has resulted in significant overlap of the field of biology with other scientific <a class="md-dictionary-link md-dictionary-tt-off mw" data-term="disciplines" href="https://www.merriam-webster.com/dictionary/disciplines" data-type="MW">disciplines</a>. Modern principles of other fields—<a href="https://www.britannica.com/science/chemistry" class="md-crosslink " data-show-preview="true">chemistry</a>, <a href="https://www.britannica.com/science/medicine" class="md-crosslink " data-show-preview="true">medicine</a>, and <a href="https://www.britannica.com/science/physics-science" class="md-crosslink " data-show-preview="true">physics</a>, for example—are <a class="md-dictionary-link md-dictionary-tt-off mw" data-term="integrated" href="https://www.merriam-webster.com/dictionary/integrated" data-type="MW">integrated</a> with those of biology in areas such as <a href="https://www.britannica.com/science/biochemistry" class="md-crosslink " data-show-preview="true">biochemistry</a>, biomedicine, and <a href="https://www.britannica.com/science/biophysics" class="md-crosslink " data-show-preview="true">biophysics</a>.</p><span class="marker MOD1 mod-inline"></span><span class="marker PREMOD2 mod-inline"></span><p class="topic-paragraph">Biology is subdivided into separate branches for convenience of study, though all the subdivisions are interrelated by basic principles. Thus, while it is custom to separate the study of <a href="https://www.britannica.com/plant/plant" class="md-crosslink " data-show-preview="true">plants</a> (<a href="https://www.britannica.com/science/botany" class="md-crosslink " data-show-preview="true">botany</a>) from that of <a href="https://www.britannica.com/animal/animal" class="md-crosslink " data-show-preview="true">animals</a> (<a href="https://www.britannica.com/science/zoology" class="md-crosslink " data-show-preview="true">zoology</a>), and the study of the structure of organisms (<a href="https://www.britannica.com/science/morphology-biology" class="md-crosslink " data-show-preview="true">morphology</a>) from that of function (<a href="https://www.britannica.com/science/physiology" class="md-crosslink " data-show-preview="true">physiology</a>), all living things share in common certain biological phenomena—for example, various means of <a href="https://www.britannica.com/science/reproduction-biology" class="md-crosslink " data-show-preview="true">reproduction</a>, <a href="https://www.britannica.com/science/cell-division" class="md-crosslink " data-show-preview="true">cell division</a>, and the transmission of genetic material.</p><span class="marker MOD2 mod-inline"></span><span class="marker PREMOD3 mod-inline"></span><p class="topic-paragraph">Biology is often approached on the basis of levels that deal with fundamental units of life. At the level of <span id="ref498649"></span><a href="https://www.britannica.com/science/molecular-biology" class="md-crosslink " data-show-preview="true">molecular biology</a>, for example, life is regarded as a <a class="md-dictionary-link md-dictionary-tt-off mw" data-term="manifestation" href="https://www.merriam-webster.com/dictionary/manifestation" data-type="MW">manifestation</a> of chemical and <a href="https://www.britannica.com/science/energy" class="md-crosslink autoxref " data-show-preview="true">energy</a> transformations that occur among the many chemical <a class="md-dictionary-link md-dictionary-tt-off mw" data-term="constituents" href="https://www.merriam-webster.com/dictionary/constituents" data-type="MW">constituents</a> that compose an organism. As a result of the <a href="https://www.britannica.com/science/biological-development" class="md-crosslink autoxref " data-show-preview="true">development</a> of increasingly powerful and precise laboratory instruments and techniques, it is possible to understand and define with high precision and accuracy not only the ultimate physiochemical organization (ultrastructure) of the molecules in living matter but also the way living matter reproduces at the molecular level. Especially crucial to those advances was the rise of <a href="https://www.britannica.com/science/genomics" class="md-crosslink " data-show-preview="true">genomics</a> in the late 20th and early 21st centuries.</p><span class="marker MOD3 mod-inline"></span><span class="marker PREMOD4 mod-inline"></span><p class="topic-paragraph"><span id="ref498650"></span><a href="https://www.britannica.com/science/cytology" class="md-crosslink " data-show-preview="true">Cell biology</a> is the study of cells—the fundamental units of structure and function in living organisms. <a href="https://www.britannica.com/science/cell-biology" class="md-crosslink " data-show-preview="true">Cells</a> were first observed in the 17th century, when the <a class="md-dictionary-link md-dictionary-tt-off mw" data-term="compound" href="https://www.merriam-webster.com/dictionary/compound" data-type="MW">compound</a> <a href="https://www.britannica.com/technology/microscope" class="md-crosslink " data-show-preview="true">microscope</a> was invented. Before that time, the individual organism was studied as a whole in a field known as <span id="ref498651"></span>organismic biology; that area of research remains an important component of the biological sciences. <span id="ref498652"></span><a href="https://www.britannica.com/science/population-biology-and-anthropology" class="md-crosslink " data-show-preview="true">Population</a> biology deals with groups or populations of organisms that inhabit a given area or region. Included at that level are studies of the roles that specific kinds of plants and animals play in the complex and self-perpetuating interrelationships that exist between the living and the nonliving world, as well as studies of the built-in controls that maintain those relationships naturally. Those broadly based levels—<a href="https://www.britannica.com/science/molecule" class="md-crosslink " data-show-preview="true">molecules</a>, cells, whole organisms, and populations—may be further subdivided for study, giving rise to specializations such as <a href="https://www.britannica.com/science/morphology-biology" class="md-crosslink " data-show-preview="true">morphology</a>, <a href="https://www.britannica.com/science/taxonomy" class="md-crosslink " data-show-preview="true">taxonomy</a>, biophysics, biochemistry, <a href="https://www.britannica.com/science/genetics" class="md-crosslink " data-show-preview="true">genetics</a>, <a href="https://www.britannica.com/science/epigenetics" class="md-crosslink " data-show-preview="true">epigenetics</a>, and <a href="https://www.britannica.com/science/ecology" class="md-crosslink " data-show-preview="true">ecology</a>. A field of biology may be especially concerned with the investigation of one kind of living thing—for example, the study of <a href="https://www.britannica.com/animal/bird-animal" class="md-crosslink " data-show-preview="true">birds</a> in <a href="https://www.britannica.com/science/ornithology" class="md-crosslink " data-show-preview="true">ornithology</a>, the study of <a href="https://www.britannica.com/animal/fish" class="md-crosslink " data-show-preview="true">fishes</a> in <a href="https://www.britannica.com/science/ichthyology" class="md-crosslink " data-show-preview="true">ichthyology</a>, or the study of microorganisms in <a href="https://www.britannica.com/science/microbiology" class="md-crosslink " data-show-preview="true">microbiology</a>.</p><span class="marker MOD4 mod-inline"></span></section> <span class="marker h2"></span><section data-level="1" id="ref48813"><h2 class="h1">Basic concepts of biology</h2> <section data-level="2" id="ref48814"><h2 class="h2">Biological principles</h2> <section data-level="3" id="ref48815"><h2 class="h3"><span id="ref498653"></span><a href="https://www.britannica.com/science/homeostasis" class="md-crosslink " data-show-preview="true">Homeostasis</a></h2> <span class="marker PREMOD5 mod-inline"></span><p class="topic-paragraph">The concept of <a href="https://www.britannica.com/science/homeostasis" class="md-crosslink " data-show-preview="true">homeostasis</a>—that living things maintain a constant internal environment—was first suggested in the 19th century by French physiologist <span id="ref498654"></span><a href="https://www.britannica.com/biography/Claude-Bernard" class="md-crosslink " data-show-preview="true">Claude Bernard</a>, who stated that “all the vital mechanisms, varied as they are, have only one object: that of preserving constant the conditions of life.”</p><a class="link-module shadow-sm d-block qa-quiz-module" href="/quiz/biology-bonanza" data-link-module-iframe-link=""> <img loading="lazy" src="https://cdn.britannica.com/12/167112-131-ABF5C65B/greylag-Flock-Greylag-geese-winter-migration-goose.jpg" alt="greylag. Flock of Greylag geese during their winter migration at Bosque del Apache National Refugee, New Mexico. greylag goose (Anser anser)" class="rounded-sm mr-15" width="70" /> <div class="line-clamp clamp-5"> <div class="module-title bg-green">Britannica Quiz</div> <div class="font-weight-semi-bold mt-5">Biology Bonanza</div> </div> </a><span class="marker MOD5 mod-inline"></span> <span class="marker PREMOD6 mod-inline"></span><p class="topic-paragraph">As originally conceived by Bernard, <a class="md-dictionary-link md-dictionary-tt-off mw" data-term="homeostasis" href="https://www.merriam-webster.com/dictionary/homeostasis" data-type="MW">homeostasis</a> applied to the struggle of a single organism to survive. The concept was later extended to include any biological system from the <a href="https://www.britannica.com/science/cell-biology" class="md-crosslink autoxref " data-show-preview="true">cell</a> to the entire <a href="https://www.britannica.com/science/biosphere" class="md-crosslink " data-show-preview="true">biosphere</a>, all the areas of <a href="https://www.britannica.com/place/Earth" class="md-crosslink " data-show-preview="true">Earth</a> inhabited by living things.</p><span class="marker MOD6 mod-inline"></span></section> <section data-level="3" id="ref48816"><h2 class="h3">Unity</h2> <span class="marker PREMOD7 mod-inline"></span><div class="assemblies"><div class="w-100"><figure class="md-assembly m-0 mb-md-0 card card-borderless print-false" data-assembly-id="106985" data-asm-type="image"><div class="md-assembly-wrapper card-media" data-type="image"><a href="https://cdn.britannica.com/85/78585-050-DF72434D/cells-animal-plant-ways-nucleus-difference-organelles.jpg" class="gtm-assembly-link position-relative d-flex align-items-center justify-content-center media-overlay-link card-media" data-href="/media/1/66054/106985"><picture><source media="(min-width: 680px)" srcset="https://cdn.britannica.com/85/78585-050-DF72434D/cells-animal-plant-ways-nucleus-difference-organelles.jpg"><img src="https://cdn.britannica.com/85/78585-050-DF72434D/cells-animal-plant-ways-nucleus-difference-organelles.jpg?w=300" alt="cells" data-width="1600" data-height="1600" loading="eager"></picture><button class="magnifying-glass btn btn-circle position-absolute shadow btn-white top-10 right-10" aria-label="Zoom in"><em class="material-icons link-blue" data-icon="zoom_in"></em></button></a></div><figcaption class="card-body"><div class="md-assembly-caption text-muted font-14 font-serif line-clamp"><span><a class="gtm-assembly-link md-assembly-title font-weight-bold d-inline font-sans-serif mr-5 media-overlay-link" href="https://cdn.britannica.com/85/78585-050-DF72434D/cells-animal-plant-ways-nucleus-difference-organelles.jpg" data-href="/media/1/66054/106985">cells</a><span>Animal cells and plant cells contain membrane-bound organelles, including a distinct nucleus. In contrast, bacterial cells do not contain organelles.</span><button class="js-more-btn d-none btn btn-unstyled font-12 bg-white js-content" aria-label="Toggle more/less fact data"><span class="link-blue">(more)</span></button></span></div></figcaption></figure></div></div><p class="topic-paragraph">All living organisms, regardless of their uniqueness, have certain biological, chemical, and physical characteristics in common. All, for example, are composed of basic units known as <span id="ref498655"></span><a href="https://www.britannica.com/science/cell-theory" class="md-crosslink " data-show-preview="true">cells</a> and of the same chemical substances, which, when analyzed, exhibit noteworthy similarities, even in such <a class="md-dictionary-link md-dictionary-tt-off mw" data-term="disparate" href="https://www.merriam-webster.com/dictionary/disparate" data-type="MW">disparate</a> organisms as <a href="https://www.britannica.com/science/bacteria" class="md-crosslink " data-show-preview="true">bacteria</a> and <a href="https://www.britannica.com/topic/human-being" class="md-crosslink " data-show-preview="true">humans</a>. Furthermore, since the action of any organism is determined by the manner in which its cells interact and since all cells interact in much the same way, the basic functioning of all organisms is also similar.</p><div class="module-spacing"> <DIV class="marketing-INLINE_SUBSCRIPTION marketing-content" data-marketing-id="INLINE_SUBSCRIPTION"><style> .student-promo-banner-wrapper { container-type: inline-size; margin-bottom: 15px; } @container (min-width: 475px) { .student-promo-banner { flex-direction: row; } .student-promo-banner-img-wrapper { margin-bottom: 0; margin-right: 10px; justify-content: flex-start; } .student-promo-banner-text-wrapper { text-align: left; margin-bottom: 0px; margin-left: 10px; } .student-promo-banner-button-wrapper { margin-right: 0; } }</style> <div class="student-promo-banner-wrapper"> <div class="student-promo-banner d-flex flex-column align-items-center bg-blue rounded p-20"> <div class="student-promo-banner-img-wrapper mb-20 mr-0 d-flex justify-content-center"> <img class="rounded" style="max-width: 100px; min-width: 80px" src="https://cdn.britannica.com/marketing/BlueThistle.webp" /> </div> <div class="student-promo-banner-text-wrapper ml-0 mb-10 text-center text-white"> <div class="h2 mb-10">Get Unlimited Access</div> <div class="h4 font-weight-semi-bold">Try Britannica Premium for free and discover more.</div> </div> <div class="student-promo-banner-button-wrapper d-flex justify-content-center align-items-center ml-auto mr-auto"> <a class="btn btn-m btn-orange" href="https://premium.britannica.com/premium-membership/?utm_source=premium&utm_medium=inline-cta&utm_campaign=august-2024">Subscribe</a> </div> </div> </div> </DIV></div><span class="marker MOD7 mod-inline"></span> <span class="marker PREMOD8 mod-inline"></span><p class="topic-paragraph">There is not only unity of basic living substance and functioning but also unity of origin of all living things. According to a theory proposed in 1855 by German pathologist <span id="ref498656"></span><a href="https://www.britannica.com/biography/Rudolf-Virchow" class="md-crosslink " data-show-preview="true">Rudolf Virchow</a>, “all living cells arise from pre-existing living cells.” That theory appears to be true for all living things at the present time under existing environmental conditions. If, however, life originated on Earth more than once in the past, the fact that all organisms have a sameness of basic structure, <a class="md-dictionary-link md-dictionary-tt-off mw" data-term="composition" href="https://www.merriam-webster.com/dictionary/composition" data-type="MW">composition</a>, and function would seem to indicate that only one original type succeeded.</p><span class="marker MOD8 mod-inline"></span> <span class="marker PREMOD9 mod-inline"></span><p class="topic-paragraph">A common origin of life would explain why in humans or bacteria—and in all forms of life in between—the same chemical substance, deoxyribonucleic acid (<span id="ref498657"></span><a href="https://www.britannica.com/science/DNA" class="md-crosslink " data-show-preview="true">DNA</a>), in the form of <a href="https://www.britannica.com/science/gene" class="md-crosslink " data-show-preview="true">genes</a> accounts for the ability of all living matter to replicate itself exactly and to transmit genetic information from parent to offspring. Furthermore, the mechanisms for that transmittal follow a pattern that is the same in all organisms.</p><div class="one-good-fact-module"> </div><span class="marker MOD9 mod-inline"></span> <span class="marker PREMOD10 mod-inline"></span><p class="topic-paragraph">Whenever a change in a <a href="https://www.britannica.com/science/gene" class="md-crosslink autoxref " data-show-preview="true">gene</a> (a <span id="ref498658"></span><a href="https://www.britannica.com/science/mutation-genetics" class="md-crosslink " data-show-preview="true">mutation</a>) occurs, there is a change of some kind in the organism that contains the gene. It is this universal phenomenon that gives rise to the differences (<span id="ref498660"></span><a href="https://www.britannica.com/science/variation-biology" class="md-crosslink " data-show-preview="true">variations</a>) in <span id="ref498659"></span><a href="https://www.britannica.com/science/population-genetics" class="md-crosslink ">populations</a> of organisms from which nature selects for survival those that are best able to <a class="md-dictionary-link md-dictionary-tt-off eb" data-term="cope" href="https://www.britannica.com/dictionary/cope" data-type="EB">cope</a> with changing conditions in the <a href="https://www.britannica.com/science/environment" class="md-crosslink autoxref " data-show-preview="true">environment</a>.</p><span class="marker MOD10 mod-inline"></span></section> </section></section><span class="marker end-of-content"></span><span class="marker after-article"></span></div> <div id="chatbot-root"></div> </div> </div> </div> <div class="ai-dialog-placeholder"></div> </div> </div> <aside class="col-md-da-320"></aside> </div> </div> </div> </div> </article> </div> </div> </div> </div> </main> <div id="md-footer"></div> <noscript><iframe src="//www.googletagmanager.com/ns.html?id=GTM-5W6NC8" height="0" width="0" style="display:none;visibility:hidden"></iframe></noscript> <script type="text/javascript" id="_informizely_script_tag"> var IzWidget = IzWidget || {}; (function (d) { var scriptElement = d.createElement('script'); scriptElement.type = 'text/javascript'; scriptElement.async = true; scriptElement.src = "https://insitez.blob.core.windows.net/site/f780f33e-a610-4ac2-af81-3eb184037547.js"; var node = d.getElementById('_informizely_script_tag'); node.parentNode.insertBefore(scriptElement, node); } )(document); </script>  <script> window.ap3c = window.ap3c || {}; var ap3c = window.ap3c; ap3c.cmd = ap3c.cmd || []; ap3c.cmd.push(function() { ap3c.init('ZO4siT4cLwnykPnzZWJtd3Byb2Q', 'https://engage.email.britannica.com/'); ap3c.track({v: 0}); }); ap3c.activity = function(act) { ap3c.act = (ap3c.act || []); ap3c.act.push(act); }; var s, t; s = document.createElement('script'); s.type = 'text/javascript'; s.src = "https://engage.email.britannica.com/app.js"; t = document.getElementsByTagName('script')[0]; t.parentNode.insertBefore(s, t); </script> <script class="marketing-page-info" type="application/json"> {"pageType":"Topic","templateName":"DESKTOP","pageNumber":1,"pagesTotal":10,"pageId":66054,"pageLength":895,"initialLoad":true,"lastPageOfScroll":false} </script> <script class="marketing-content-info" type="application/json"> [] </script> <script src="https://cdn.britannica.com/mendel-resources/3-133/js/libs/jquery-3.5.0.min.js?v=3.133.9"></script> <script type="text/javascript" data-type="Init Mendel Code Splitting"> (function() { $.ajax({ dataType: 'script', cache: true, url: 'https://cdn.britannica.com/mendel-resources/3-133/dist/topic-page.js?v=3.133.9' }); })(); </script> <script class="analytics-metadata" type="application/json"> {"leg":"D","adLeg":"C","userType":"ANONYMOUS","pageType":"Topic","pageSubtype":null,"articleTemplateType":"PAGINATED","gisted":false,"pageNumber":1,"hasSummarizeButton":false,"hasAskButton":true} </script> <script type="text/javascript"> EBStat={accountId:-1,hostnameOverride:'webstats.eb.com',domain:'www.britannica.com', json:''}; </script> <script type="text/javascript"> ( function() { $.ajax( { dataType: 'script', cache: true, url: '//www.britannica.com/webstats/mendelstats.js?v=1' } ) .done( function() { try {writeStat(null,EBStat);} catch(err){} } ); })(); </script> <div id="bc-fixed-dialogue"></div> </body> </html>

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Biology | Definition, History, Concepts, Branches, & Facts | Britannica