 Richard Boyd (1999) considers biological species to be paradigmatic natural kind clusters, where the clustering is due to a homeostatic mechanism. Homeostatic property clusters (HPCs) occur when mechanisms exist that cause the properties to cluster by ensuring that deviations from the cluster have a low chance of persisting; the presence of some of the properties in the cluster favours the presence of the others. A homeostatic mechanism thereby achieves self-regulation, maintaining a stable range of properties. In the case of species, the homeostatic mechanisms may be intrinsic (e.g. gene exchange within a population or developmental factors) or extrinsic (e.g. a species’s evolutionary niche provides common selective factors). Since individuals inherit their basic characteristics from their parents, and very few people are taller than 2.5m, we can expect the next generation to contain few people of that height. Height can, however, be affected by environmental factors; a mutation can cause offpring to have genes different from their parents that might cause greater than usual height, and malfunctioning of the pituitary gland can cause an excess of growth hormone. So there can be individual cases of excessive height. But they will remain rare because such gigantism and acromegaly are accompanied by serious complicating disorders and often result in premature death. Consequently, these individuals are less likely to reproduce than others. Even greater extremes of height (e.g. 4m) are just not possible, since the same developmental factors would prevent the individual from being viable at an earlier stage in life.