CHAPTER 18.. Division of Nicaragua Into Three Zones.--Journey From Juigalpa to

Division of Nicaragua into three zones.--Journey from Juigalpa to lake of Nicaragua.--Voyage on lake.--Fresh-water shells and insects.--Similarity of fresh-water productions all over the world. --Distribution of European land and fresh-water shells.--Discussion of the reasons why fresh-water productions have varied less than those of the land and of the sea.