argued that “probably all the organic beings which have ever lived on this earth have descended from some one primordial form.”4 Darwin thought that this primordial form gradually developed into new forms of life, which in turn gradually developed into other forms of life, eventually producing, after many millions of generations, all the complex life we see in the present. Biology textbooks today often depict this idea just as Darwin did, with a great branching tree. The bottom of the trunk of Darwin’s