the length of this sea boundary materially affects this function.  Area and coastline are not dead mathematical quantities, but like organs of one body stand in close reciprocal activity, and can be understood only in the light of their persistent mutual relations.  The division of the area of a land by the length of its coastline yields a quotient which to the anthropo-geographer is not a dry figure, but an index to the possible relations between seaboard and interior.  A comparison of some of these ratios will illustrate this fact.

Germany’s shoreline, traced in contour without including details, measures 787 miles; this is just one-fifth that of Italy and two-fifths that of France, so that it is short.  But since Germany’s area is nearly twice Italy’s and a little larger than that of France, it has 267 square miles of territory for every mile of coast, while Italy has only 28 square miles, and France 106.  Germany has towns that are 434 miles from the nearest seaboard, but in Italy the most inland point is only 148 miles from the Mediterranean.[441] If we turn now to the United States and adopt Mendenhall’s estimate of its general or contour coastline as 5,705 miles, we find that our country has 530 square miles of area dependent for its outlet upon each mile of seaboard.  This means that our coast has a heavy task imposed upon it, and that its commercial and political importance is correspondingly enhanced; that the extension of our Gulf of Mexico littoral by the purchase of Florida and the annexation of Texas were measures of self-preservation, and that the unbroken contour and mountain-walled face of our Pacific littoral is a serious national handicap.

[Sidenote:  Criticism of this formula.]

But this method is open to the legitimate and fundamental criticism that, starting from the conception of a coast as a mere line instead of a zone, it ignores all those features which belong to every littoral as a strip of the earth’s surface—­location, geologic structure, relief, area, accessibility to the sea in front and to the land behind, all which vary from one part of the world’s seaboard to another, and serve to differentiate the human history of every littoral.  Moreover, of all parts of the earth’s surface, the coast as the hem of the sea and land, combining the characters of each, is most complex.  It is the coast as a human habitat that primarily concerns anthropo-geography.  A careful analysis of the multifarious influences modifying one another in this mingled environment of land and water reveals an intricate interplay of geographic forces, varying from inland basin to marginal sea, from marginal sea to open ocean, and changing from one historical period to another—­an interplay so mercurial that it could find only a most inadequate expression in the rigid mathematical formula of Carl Ritter.

[Sidenote:  Accessibility of coasts from hinterland.]