  By the same token, however, Darwinism has evolved.  As one example of this truth, think for a moment of contemporary debates about the nature of selection. The problems people had with natural selection in the 19th century continue to be problematic, but there are a variety of problems that were either not discussed, or discussed very differently, in the 19th century. Can, and does, natural selection work at levels other than the level of Darwin’s focus, individual organisms; is there a non-vacuous way to formulate the theory abstractly; how are we to understand the relationships between the concepts of fitness, selection and adaptation?  How strong are the constraints on the selection process, and what sorts of constraints are there?  Are there other motors of evolutionary change besides selection, and if so, how important are they?  In particular, how important is ‘drift’, and how are we to differentiate it from selection?