Rivers as intermediaries between land and sea--Sea navigation merges into river navigation--Historical importance of seas and oceans influenced by their debouching streams--Lack of coast articulations supplied by rivers--River highways as basis of commercial preeminence--Importance of rivers in large countries--Rivers as highways of expansion--Determinants of routes in arid or semi-arid lands--Increasing historical importance of rivers from source to mouth--Value of location at hydrographic centers--Effect of current upon trade and expansion--Importance of mouth to upstream people--Prevention of monopoly of river mouths--Motive for canals in lower course--Watershed canals for extension of inland waterways--Rivers and railroads--Natural unity of every river system--In arid lands as common source of water supply--Tendency towards ethnic and cultural unity in a river valley--Identity of country with river valley--Rivers as boundaries of races and peoples--Rivers as political boundaries--Fluvial settlements and peoples--Boatman tribes or castes--River islands as protected sites--River and lake islands as robber strongholds--River peninsulas--River islands as sites of trading posts and colonies--Swamps as barriers and boundaries--Swamps as regions of survivals--Swamps as places of refuge--The spirit of the marshes--Economic and political importance of lakes--Lakes as nuclei of states--Lakes as fresh-water seas. CHAPTER XII. CONTINENTS AND THEIR PENINSULAS Insularity of the land-masses--Classification of land-masses according to size and location--Effect of the size of land-masses--Independence due to location versus independence due to size--Continental convergence and ethnic kinship--Africa's location--The Atlantic abyss--Geographical character of the Pacific--Pacific affinities of North America--The Atlantic face of America as the infant Orient of the world--The Atlantic abyss in the movements of peoples--Races and continents--Contrast of the northern and southern continents--Effects of continental structure upon historical development--Structure of North and South America--Cultural superiority of Pacific slope Indians--Coast articulations of continents--Importance of size in continental articulations--Peninsular conditions most favorable to historical development--The continental base of peninsulas--Continental base a zone of transition--Continental base the scene of invasion and war--Peninsular extremities as areas of isolation--Ethnic unity of peninsulas--Peninsulas as intermediaries. CHAPTER XIII. ISLAND PEOPLES