   The question of nominalism versus realism regarding species points toward a final aspect of Darwin’s theory with which many of those otherwise sympathetic to him disagreed, his gradualism. For apart from the question of whether his views entailed ‘nominalism’ about natural kinds, they do seem to reflect a belief that the evolutionary process must be a slow and gradual one. It is perhaps here that we see the most lasting impact of Darwin’s careful study of Charles Lyell’s Principles of Geology while on H.M.S. Beagle. I stress slow and gradual, for it is clear that one could have a slow but non-gradual evolutionary process (perhaps the long periods of evolutionary stasis punctuated by geologically rapid periods of speciation postulated by Eldridge and Gould’s ‘punctuated equilibrium model’ is such); and one could have a rapid but gradual one (for example the process George Gaylord Simpson labeled ‘adaptive radiation’, where a population migrates to a location with a variety of unexploited niches, and rapidly evolves to exploit them). Darwin stresses over and over again that he conceives of natural selection ‘adding up infinitely small variations’, and that he imagines the process of speciation to take place over a very long period of time.