68 THE HABITAT OF THE EURYPTERIDA 



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 anima] 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 organ- 

 isms 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 ter- 

 restrial 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 thou- 

 sands 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 fluvi- 

 atile 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 



