The water a factor in man's mobility--Oceans and seas the factor of
union in universal history--Origin of navigation--Primitive
forms--Relation of river to marine navigation--Retarded and advanced
navigation--Geographic conditions in Polynesia--Mediterranean versus
Atlantic seamanship--Three geographic stages of maritime
development--Enclosed seas as areas of ethnic and cultural
assimilation--Assimilation facilitated by ethnic kinship--Importance of
zonal and continental location of enclosed seas--Thalassic character of
the Indian Ocean--Limitations of small area in enclosed seas--Successive
maritime periods in history--Contrasted historical rôles of northern and
southern hemispheres--Size of the ocean--Neutrality of the seas--_Mare
clausum and Mare liberum_.