   The core of Darwin’s theory is the concept of natural selection. Perhaps because of his use of the term selection, this core element of his theory apparently baffled nearly everyone. Could it be, as Lyell, Herschel and Darwin’s great American defender Asa Gray would ask, an ‘intermediate cause’, i.e. a causal principle instituted and sustained by God?  Or is it, in its very nature, the antithesis of such a principle, as his old geology teacher Sedgwick believed?  Could it possibly create species, or is it, by its nature, a negative force, eliminating what has already been created by other means?  In one of his copies of On the Origin of Species, Alfred Russell Wallace crosses out ‘natural selection’ and writes ‘survival of the fittest’ next to it. Wallace always felt that ‘selection’ inappropriately imported anthropomorphic notions of Nature choosing purposefully between variants into natural history. And, in a devastating review, Fleeming Jenkin happily accepted the principle of natural selection but challenged its power to modify an ancestral species into descendent species, and thus limited its scope to the production of varieties.  A number of reviewers, even some sympathetic ones, questioned the possibility of extending the theory to account for the evolution of those characteristics that differentiate humans from their nearest relatives.