Jordan's rule

Etymology
Named after American scientist David Starr Jordan (1851–1931).

Proper noun

 * 1) An ecogeographic rule describing the inverse relationship between water temperature and meristic characteristics in various species of fish, including the fact that fin ray, vertebrae, or scale numbers increase with decreasing temperature.
 * 2) An ecogeographic rule stating that, given any species in any region, the nearest related species is not likely to be found in the same region nor in a remote region, but in a neighbouring district separated from the first by a barrier of some sort.