- 20 - other isotopes such as Ra226 | mere and their decay products. A question commonly asked is what role have ionizing radiations from natural sources played in the evolutionary process. Radiation causes mutation; the mutant gene is the raw material of evolution; is radiation thereby an important factor in evolution? This line of reasoning also can be extended to inquiring if an increase in radiation in the environment from fallout may speed up the evolutionary process. The answers to these questions are given by Crow (1959) who states that "...it is likely that jonizing radiation has played only a minor role in the recent evo- lutionary history of most organisms." For Drosophila the background radiation would have to be increased more than 10,000 times to account for the natural mutation rate. Crow also shows evidence that the reason for the slow rate of evolution in some groups is not from insufficient genetic variability. He observes also that the effects produced by mutations are of all sorts and are mostly harmful. The reason that mutations are mostly harmful is that genes duplicate complicated structures for which there may be Only one or a few ways in which to build the structure correctly, but many ways in which to make it incorrectly.