580 The American Naturalist. [July, 
islands. But it is certain that forest development in the future 
will follow no such clearly defined courses as in the past; 
the wonderful complexity of the geographical botany of the 
present has forever sealed the possibility of another distinctive 
tree-group attaining such a world-wide prominence as either 
the Conifers or the Diclinae or the Palms. These three must 
stand alone asa unique monument to the struggle for existence 
in the primaeval Mesozoic forest. For even as the conditions 
of that age made possible a remarkably homogeneous plant 
world, even so the great tension system of the earth’s present 
vegetation makes diversity, to an equally or more remarkable 
degree, the key-note of future development. 
