ABSTRACT :
Our notion of a natural kind may well have had its psychological origin in our experience of the ordered variety of living species. And yet, by a reversal that is not unusual in philosophy, biological species now seem to have lost the sta tus for which they themselves provided the model. For several biologists and philosophers have claimed in recent years that species are not classes or kinds at all, but dispersed individuals. Others have resisted this reconceptualization, while a third ca mp have looked for an intermediate alternative. Although many arguments on both sides of the dispute make it look like pallid nit-picking, there are important philosophical issues at stake: the nature of biology in relation to physics, the place of tempor ality in science, and the unity of science. It may seem that treating species as individuals commits us to a view of biological science as irreducibly historical, and therefore alien from the rest of science. It turns out, however, that the opposite is th e case: treating species as individuals works in favour rather than against the unity of science.