The laws of heredity are complicated because of the fact that the same property is influenced by a gene from each parent. Frequently these two genes dictate different behavior and then the result is a compromise, sometimes evenhanded, sometimes unbalanced. But of the two genes only one will find its way to the child of the next generation. The compromise is temporary and original properties may emerge again. Which one of any pair of chromosomes (or of the two assemblies of genes) carries on is a matter of chance. In the world of the cells as in the world of atoms it is chance that determines the future—not fate. Of all these facts we need be particularly interested in one. The units of inheritance are rather constant but not quite immutable. There is a small possibility that any gene may suffer a mutation. That is, it may turn into a new chemical, carrying a new code and new properties. A gene is an extremely finely and precisely constituted object. It must be so in order to carry all the racial past in so little material. A mutation due to chance will spoil this order in almost every instance. The great majority of mutations are detrimental. Many are lethal. It is an incredible fact that these random mutations, almost always harmful and never proceeding according to any plan, should have been responsible in the very long run for all the many beautiful and perfect living creatures that nature has produced (and this includes the human race). The thread leading from single cells to cell colonies, worms, fishes, vertebrates, mammals and human beings does certainly not seem to be the work of chance. Much less does it seem to be the work of a gamble taking one chance of a small improvement against a thousand chances of deformity or death. Nevertheless it is such a terrible game of chance which has produced both the human body and in some manner also the human spirit. Big numbers are strange things and when each member of a huge assembly must be given individual attention then the numbers are even harder to appreciate. Billions of contemporary lives in billions of distinct generations have led to the incredible outcome: the harmony of life produced by gambling. Radiation is surely disruptive. It does cause mutations. Since the genes appear to be single molecules, a single process of ionization or excitation is likely to result in a change. As has been said before there is doubt whether or not cancer and leukemia can be caused by exceedingly little radiation. There is little doubt, however, that mutations can be caused by any small amount of radiation. The less radiation the less the chance. But the chance will always be there. A very great increase in the natural rate of mutations could indeed have terrifying effects. We can be quite certain, however, that radiation from atomic tests will increase the chance of mutations by only a very small amount.