  This criticism is certainly probative against those who believe that the boundary between species is morally relevant because they hold a “biblical or Aristotelian view of species as fixed and immutable rather than being slices of a dynamic, ever-changing process” (Rollin 2003, 15), but the proponent of the Unnaturalness Argument need not hold such a view. For example, the groups constituting one’s family and one’s fellow citizens change over time, and yet we owe special duties to our compatriots and to our kin in virtue of our special relationships to them. So, fixed boundaries are not necessary for moral relevance. And the mere fact that bacteria naturally transfer genes across species boundaries does not mean that the activity is natural when it is performed by humans.