16 National Geographic Magazine. 



until it comes to " maturity," that is to the greatest variety or 

 differentiation of form. At a still later date the widening of the 

 valleys consumes the intervening hills, and the form becomes 

 tamer, until in " old age " it returns to the simple plain surface 

 of " youth." Young mountains possess structural lakes and are 

 drained largely by longitudinal valleys ; old mountains have no 

 such lakes and have transverse drainage, formed as the growing- 

 headwaters of external streams lead out much water that form- 

 erly followed the longitudinal valleys. Young rivers may have 

 falls on tilted beds, but such are short lived. Falls on horizontal 

 beds are common and survive on the headwater branches of even 

 mature rivers. All falls disappear in old rivers, provided they 

 are not resuscitated by some accident in the normal, simple cycle 

 of river life. The phases of growth are as distinct as in organic 

 forms. As this idea has grown in my mind from reading the 

 authors above named, geography has gained a new interest. The 

 different parts of the world are brought into natural relations 

 with one another ; the interest that change, growth and life had 

 before given to the biologic sciences only, now extends to the 

 study of inorganic forms. It matters not that geographic growth 

 is destructive ; it involves a systematic change of form from the 

 early youth to the distant old age of a given structure, and that 

 is enough. It matters not that the change is too slow for us to 

 see its progress in any single structure. We do not believe that 

 an oak grows from an acorn from seeing the full growth accom- 

 plished while waiting for the evidence of the fact, but because 

 partly by analogy with plants of quicker development, partly by 

 the sight of oaks of different ages, we are convinced of a change 

 that we seldom wait to see. It is the same with geographic forms. 

 We find evidence of the wasting of great mountains in the wast- 

 ing of little mounds of sand ; and we may by searching find exam- 

 ples of young, mature and old mountains, that follow as well 

 marked a sequence as that formed by small, full grown and 

 decaying oaks. If the relative positions of the members in the 

 sequence is not manifest at first, we have the mental pleasure of 

 searching for their true arrangement. The face of nature thus 

 becomes alive and full of expression, and the conception of its 

 change becomes so real that one almost expects to see the change 

 in successive visits to one place. 



Now consider the deductive application of this principle. 

 Having recognized the sequence of forms developed during the 



