There has been neither time nor opportunity, for example, to gather radiation-effects data on more than a few hundred of the 1,500,000 kinds of living organisms inhabiting the earth. Nor is it possible to predict the extent to which life can adjust itself to environmental changes resulting from scarcely perceptible alterations of natural radiological balances. Also undetermined is the relation between environmental changes and the biological exchanges making up the often mentioned, but insufficiently understood, “balance of nature”. The case of carbon-14 is an example of a permanent man-made modification of the environment. From the early ages of the earth, carbon-14 has been created in the upper atmosphere by the transmutation of nitrogen in cosmic-ray reactions. Carbon itself is an almost universal component of living matter, and the ratio between stable carbon and radioactive carbon is believed to have been unchanged for thousands of years. It is this circumstance that permits the use of carbon-14 as a tool for “dating”, or determining the ages of, fossil remains, prehistoric artifacts, and geologic formations. But carbon-14 also is produced in nuclear fusion, and the testing of thermonuclear devices after 1952 produced an estimated increase of 4% in the amount of carbon-14 on earth. This is enough to disturb the natural equilibrium. Since the half-life[12] of carbon-14 is some 5800 years, the addition will be a factor of environmental consideration for scores of human generations. Nuclear tests, although not the only sources of man-made radioactivity, have been until now the most significant ones and the only sources touching large areas of the earth. The total product of nuclear testing is small in relation to the natural burden of radioactivity, raising the level of radiation to which all life is subject by a factor of one-tenth or less. But it is the unknown element, the degree to which fallout radioactivity may introduce new influences into the environment, that gives concern.[13] [Illustration: _One of the last cows of the herd exposed to fallout by the world’s first atomic detonation in New Mexico in July 1945, photographed in 1964. The calf is her 15th to be born in 15 years. The cow, believed about 20 years old, has been under observation by scientists, who found she suffered little apparent effect, although the fallout caused some hair to turn gray (see light patches on back). Other cows in the herd died natural deaths._] When a nuclear device is detonated, the release of energy is due to the fission of uranium or plutonium atoms or to the fusion of hydrogen atoms. At the instant of fission, some 75 radionuclides, or fission products, are created.