of this prolific littoral marine fauna are protected either by shells or by exoskeletons, each individual that dies leaves its record behind in some hard part which falls to the bottom when the animal dies, or else soon comes to rest there, where it is buried by sand or mud. Not only are the remains of the animals which lived in the littoral zone of the sea preserved in the deposits forming there, but many derelicts, dead or alive, are washed in from the land and the rivers and we have a phenomenon observable in no other bionomic realm, namely, the commingling in one life district of the remains of organisms from all the other districts. During storms, terrestrial animals are drowned in the torrential floods, trees and other vegetation are carried away in the undermining of the banks, and these, together with the remains of fluviatile organisms and even with the living forms which cannot resist the strength of the current, are all carried out to sea to be dropped and there entombed with the remains of marine organisms. In tropical and semi-arid regions such mingling of terrestrial and marine forms is the common, not the unusual, thing. Darwin has called attention to many such cases in his Voyage of the Beagle, where he describes the great drought which occurred between the years 1827 and 1832 in Buenos Ayres, South America, when the birds and animals died by the thousand, the vegetation became withered and parched, and the dry winds swept over the desolate waste of land desiccated and dusty. The large rivers shrivelled, the small ones disappeared altogether; and where a little water still remained in the broader courses, it became highly saline, bringing death to the animals who drank. Herds of cattle rushed into the river, crazed by thirst, and there perished from the salt water and because they were too weak to climb up the banks again. Following this drought which lasted five years, came the rainy season and torrential floods. "Hence it is almost certain," Darwin concludes, "that some thousands of the skeletons were buried by the deposits of the very next year" (48, 127). Not only in semi-arid climates where torrential floods are active, but even in pluvial climates are terrestrial and fluviatile organisms carried out to the littoral zone of the sea, where they are buried in the delta deposits together with marine shells and tests. Thus, terrestrial vertebrate remains have been found in the deltas of the Ganges and Zambesi, the bones of recent antelope, buffalo, lion, hippopotamus and other mammals having been recorded; in the Po delta arthropods occur with lignites. Such terrestrial relics are by no means confined to deltas or river flood plains, but are found
