 In a pre Darwinian age, species essentialism made sense. Such essentialism, however, is out of step with contemporary evolutionary theory. Evolutionary theory provides its own methods for explaining variation within a species. It tells us that the boundaries between species are vague. And it tells us that a number of forces conspire against the existence of a trait in all and only the members of a species. From a biological perspective, species essentialism is no longer a plausible position. Nevertheless, as we shall see in Section 2.5, some philosophers have recently tried to revive species essentialism.