Fig. 5. Biogeographic temperature. Four matrices of equal size (28 islands, 26 species) and equal fill (37.7%) are presented in increasing "temperature" sequence: (a) a maximally cold matrix, (b) actual data, (c), (d) matrices randomly filled under successively relaxed constraints [see Patterson and Atmar (1986)]. The probabilities that RANDOM1 and RANDOM0 would generate a matrix of equal or lower temperature than that observed in the Rocky Mountain small mammal biogeography are P < 10^-20 and P < 10^-40, respectively.