Biologists are interested in every kind of living thing. When they study organisms in relation to atomic radiations, they enter the field of radiobiology, which really is not a science in itself but merely a branch of the larger interest in biology. Biologists find that atomic energy has significance both in the study of individual organisms and in studies of organisms in their natural communities and habitats. [Illustration: _Skin-diving biologist collecting giant clam from coral bottom of Bikini Lagoon in the Pacific Ocean._] Radioactivity introduced into any community may be “taken up” by the biological system, becoming subject to cycling in food chains or to accumulation in plant or animal tissues. The presence of radioactivity permits study of the workings of a system as large as an ocean, perhaps, or of one no larger than a tree. And in each case it thus may be possible to observe how the cycles of organic renewal are related to the larger systems of life on earth. The Single Environment The environment in which we live is recognizable as a single complex, composed of many subenvironments—land, oceans, atmosphere, and the space beyond our envelope of air. The deer in the forest, the lizard in the desert burrow, and the peavine in the meadow are different kinds of organisms living in situations that are seemingly unalike. Each creature is part of its environment and a contributor to it, but it also is part of the total biosphere.[1] All creatures are linked to each other, however remotely, in their dependence on limited environments that together form the whole of nature. [Illustration: _Gray shark photographed in another Pacific lagoon._] We know much about the life of the earth, but there is far more that we do not know. Understanding of the large cyclical forces has continued to elude us. We do not even yet grasp the small and seemingly random biological relations between individual organisms—relations involving predator and prey, for instance, and those among species and families—such as exist together in symbiotic[2] harmony and interdependence. Through centuries of observation we have gained a store of information. We are left, however, with a still unsatisfied curiosity about the reach and strength of the tenuous biological cords that bind together the lives of the deer, the lizard, and the peavine. The Need to Understand Life on earth evolved amid constant exposure to ionizing[3] radiation, from the earth itself and from space, known as background radiation. Therefore environmental studies must be conducted in relation to, and with understanding of, background radioactivity. [Illustration: _This Pacific Ocean coconut crab, member of a family that usually sticks to tide-covered beaches, depends on coconut trees for its food._]