 In such cases, the boundaries of the natural kind are vague. Cluster kinds nevertheless do form a kind (Criterion 4 above). Members will have at least some properties in common (Criterion 1). Since the emergence of cluster kinds is not entirely random and a causal-historical account can be provided in virtue of stable homeostatic mechanisms, cluster kinds are sufficiently robust to support induction (Criterion 2) and perhaps even laws of nature in some cases (Criterion 3). Conversely, because the boundaries between natural kinds are vague, a cluster kind realist will reject the claim that natural kinds are categorically distinct (Criterion 6). She may be neutral on whether they always form hierarchies (Criterion 5). That said, not all clusters need exhibit the flexibility and vagueness of these biological, homeostatic cases. In other cases the clustering may be directly due to the laws of nature. Thus the laws of physics rule out fractional nuclear charge, with the consequence that the chemical elements are kinds that have clearly defined boundaries and nuclear charge is a necessary and sufficient condition of kind membership.