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 formerly 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 accomplished 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 wasting of little mounds of sand; and we may by searching find examples 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
