A little over a century ago, Charles Darwin published The Origin of Species; the event has recently been widely celebrated. Darwin's work was of significance not only to biologists, but to men of almost every other discipline, for it made understandable the processes of change by which the world of life today has evolved from one that was much dif ferent in ages past. Many scientists in the early nineteenth century be lieved that such a change must have taken place, but until the mechanism of the process was explained, the fact of change was not generally ac cepted. This mechanism Darwin supplied in his theory of natural selec tion. It was very simple. Random changes occur from generation to generation. Those that tend to the survival of the individual possessing them are transmitted by inheritance to his offspring. The descendants of other individuals tend to be eliminated. Through the accumulation of such changes, the character of a species slowly alters, and this in time has led to the great diversity of the organic world. Various objections were raised at first to Darwin's theory. Critics of it have sometimes questioned, for example, whether all traits that living things display are of sufficient importance to be significant for survival; but it has often been shown either that these traits are of value, or that they are correlated with valuable ones. After a hundred years, the theory has now become generally accepted as the cause of evolutionary change; it has gained further interpretation and confirmation from genetic studies. Its almost universal support by biologists today was an outstand ing feature of the host of publications that appeared in the recent centen nial celebrations. Natural selection, indeed, has now become so firmly entrenched as the cornerstone of evolutionary theory that one cannot raise against it even a minor objection without the danger of being called a believer in vitalism, orthogenesis, or some other objectionable hetero doxy. In the minds of a few biologists, however, there are a number of points in which Darwin's theory still seems less than completely adequate to explain all the facts. It is important, I think, that we continue to dis