Hydrology has a more or less intimate relation to all the geophysical sciences. There are other sciences which are terrestrial, although not counted as geophysical, where the interrelations with hydrology are especially marked. I refer in particular to the biological sciences. Direct interrelations of animal life to hydrology are not wanting. Examples are furnished by the relation of the migration of fishes to freshwater floods of streams and by the life‐habits of many oysters, which seek the transition‐zone between salt and fresh water for elements vital to their existence and for protection from their enemies, and suffer if the ordinary course of the hydrological cycle is disturbed. Darwin directed attention to the agronomic importance of earthworms, but it remained for Lawes, Gilbert, and Warington (The amount and composition of drainage‐waters collected at Rothamsted, J. R. Agr. Soc. Eng., v. 27, 275, 1881) fifty years ago to point out that in some of the best agricultural soils earthworm perforations apparently provide the principal means of entrance of rain‐water to and escape of air from the soil, thus permitting the underground phase of the hydrological cycle to be enacted and moisture‐supply for vegetation